COURSE TITLE:Fundamentals and Control of D.C. Generators and
Motors
DUTY TITLE: Operation of D.C. Motors & Generators
POS #:1800
TASK:Operation of a D.C. Motor
PURPOSE:To Understand the Concept, Control, and Troubleshooting
Techniques of the Various Types of D.C. Motors.
TASKS:
1801
Demonstrate knowledge of basic direct current circuits.
1802
Explain the theory of operation of a direct current motor.
1803
Operate and test a direct current motor.
1804
Operate and test a direct current shunt motor.
1805
Perform calculations for horsepower, speed and torque for direct
current motors.
1806
Measure performance and efficiency of a direct current
motor.
1807
Demonstrate knowledge of technical terms and units used in a
basic direct current circuit.
1808
Demonstrate knowledge of the basic operations of variable speed
control for direct current motors.
REVISION: 2019
ENGLISH LANGUAGE ARTS
CC.1.2.11-12.J Acquire and use accurately general academic and
domain-specific words and phrases, sufficient for reading, writing,
speaking, and listening at the college and career readiness level;
demonstrate independence in gathering vocabulary knowledge when
considering a word or phrase important to comprehension or
expression
CC.1.3.11-12.I Determine or clarify the meaning of unknown and
multiple-meaning words and phrases based on grade level reading and
content, choosing flexibly from a range of strategies and
tools.
MATH
CC.2.1.HS.F.4 Use units as a way to understand problems and to
guide the solution of multi-step problems.
CC.2.1.HS.F.6 Extend the knowledge of arithmetic operations and
apply to complex numbers.
READING IN SCIENCE & TECHNOLOGY
CC.3.5.11-12.B. Determine the central ideas or conclusions of a
text; summarize complex concepts, processes, or information
presented in a text by paraphrasing them in simpler but still
accurate terms.
CC.3.5.11-12.C. Follow precisely a complex multistep procedure
when carrying out experiments, taking measurements, or performing
technical tasks; analyze the specific results based on explanations
in the text.
WRITING IN SCIENCE & TECHNOLOGY
CC.3.6.11-12.E. Use technology, including the Internet, to
produce, publish, and update individual or shared writing products
in response to ongoing feedback, including new arguments or
information.
CC.3.6.11-12.F. Conduct short as well as more sustained research
projects to answer a question (including a self generated question)
or solve a problem; narrow or broaden the inquiry when appropriate;
synthesize multiple sources on the subject, demonstrating
understanding of the subject under investigation
*ACADEMIC STANDARDS *
READING, WRITING, SPEAKING & LISTENING
1.1.11.A Locate various texts, assigned for independent projects
before reading.
1.1.11.D Identify strategies that were most effective in
learning
1.1.11.E Establish a reading vocabulary by using new words
1.1.11.F Understanding the meaning of, and apply key vocabulary
across the various subject areas
1.4.11.D Maintain a written record of activities
1.6.11.A Listen to others, ask questions, and take notes
MATH
2.2.11.A Develop and use computation concepts
2.2.11.B Use estimation for problems that don’t need exact
answers
2.2.11.C Constructing and applying mathematical models
2.2.11.D Describe and explain errors that may occur in
estimates
2.2.11.E Recognize that the degree of precision need in
calculating
2.3.11.A Selecting and using the right units and tools to
measure precise measurements
2.5.11.A Using appropriate mathematical concepts for multi-step
problems
2.5.11.B Use symbols, terminology, mathematical rules, Etc.
2.5.11.C Presenting mathematical procedures and results
SCIENCE
3.1.12.A Apply concepts of systems, subsystems feedback and
control to solve complex technological problems
3.1.12.B Apply concepts of models as a method predict and
understand science and technology
3.1.12.C Assess and apply patterns in science and technology
3.1.12D Analyze scale as a way of relating concepts and ideas to
one another by some measure
3.1.12.E Evaluate change in nature, physical systems and
man-made systems
3.2.12.A Evaluate the nature of scientific and technological
knowledge
3.2.12.B Evaluate experimental information for
appropriateness
3.2.12.C Apply elements of scientific inquiry to solve multi –
step problems
3.2.12.D Analyze the technological design process to solve
problems
3.4.12.A Apply concepts about the structure and properties of
matter
3.4.12.B Apply energy sources and conversions and their
relationship to heat and temperature
3.4.12.C Apply the principles of motion and force
3.8.12.A Synthesize the interactions and constraints of
science
3.8.12.B Use of ingenuity and technological resources to solve
specific societal needs and improve the quality of life
3.8.12.C Evaluate the consequences and impacts of scientific and
technological solutions
ECOLOGY STANDARDS
4.2.10.A Explain that renewable and non-renewable resources
supply energy and material.
4.2.10.B Evaluate factors affecting availability of natural
resources.
4.2.10.C Analyze the use of renewable and non-renewable
resources.
4.2.12.B Analyze factors affecting the availability of renewable
and non-renewable resources.
4.3.10.A Describe environmental health issues.
4.3.10.B Explain how multiple variables determine the effects of
pollution on environmental health, natural processes and human
practices.
4.3.12.C Analyze the need for a healthy environment.
4.8.12.A Explain how technology has influenced the
sustainability of natural resources over time.
CAREER & EDUCATION
13.1.11.A Relate careers to individual interest, abilities, and
aptitudes
13.2.11.E Demonstrate in the career acquisition process the
essential knowledge needed
13.3.11.A Evaluate personal attitudes that support career
advancement
ASSESSMENT ANCHORS
M11.A.3.1.1 Simplify expressions using the order of
operations
M11.A.2.1.3 Use proportional relationships in problem solving
settings
M11.A.1.2 Apply any number theory concepts to show relationships
between real numbers in problem solving
STUDENT
The student will be able to identify, connect and control the
output of all three types of D.C. motors.
(Series-Shunt-Compound)
TERMINAL PERFORMANCE OBJECTIVE
Given all the electrical tools and materials required, the
student will be able to identify, connect and control the output of
all three types of D.C. motors. (Series-Shunt-Compound)
SAFETY
· Always wear safety glasses when working in the shop.
· Always check with the instructor before turning power on.
· Always use tools in the correct manner.
· Keep work area clean and free of debris.
· Make sure guard is over motor couplings.
· Never wire a project without the correct wiring diagram.
RELATED INFORMATION
1. Attend lecture by instructor.
2. Obtain handout.
3. Review chapters in textbook.
4. Define vocabulary words.
5. Complete all questions in this packet.
6. Complete all projects in this packet.
7. Complete K-W-L Literacy Assignment by Picking an Article From
the
“Electrical Contractor” Magazine Located in the Theory Room. You
can pick any article you feel is important to the electrical
trade.
EQUIPMENT & SUPPLIES
1. Safety glasses 11. Various light bulbs
2. Hammer 12. Light fixture bank
3. Screw driver 13. Alligator clips
4. Awl 14. D.C. motor module
5. Wire strippers 15. D.C. generator
6. Side cutters 16. Prime mover (A.C. or D.C. motor)
7. Cable rippers 17. Couplings
8. Lineman pliers 18. Power supply
9. Needle nose pliers 19. THHN wire
10. Multimeter 20. In line meters
VOCABULARY
CC.1.3.11-12.I Determine or clarify the meaning of unknown and
multiple-meaning words and phrases based on grade level reading and
content, choosing flexibly from a range of strategies and tool
CC.3.5.11-12.D. Determine the meaning of symbols, key terms, and
other domain-specific words and phrases as they are used in a
specific scientific or technical context relevant to grades 11–12
texts and topics.
· Stator:
· Riser:
· C.E.M.F:
· Rheostat:
· Shunt field:
· Laminations:
· Neutral plane:
· Armature loop:
· Permanent magnet:
· E.M.F:
· Electromagnet:
· Brushless DC motors:
· Compound motor:
· Constant-speed motors:
· Counter-electromotive force (CEMF) (back-EMF):
· Cumulative-compound motors:
· Differential-compound motors:
· Field-loss relay:
· Motor:
· Permanent magnet motors:
· Printed-circuit motor:
· Series motor:
· Servomotors:
· Shunt motor:
· Speed regulation:
· Torque:
OUTLINE
DC Motor Principles
Compare DC motors with DC generators, pointing out how similar
they are. Explain how the DC motor functions
Torque
Describe what torque is and describe how it is formed by the
magnetic .eld of the pole pieces and the magnetic field of the loop
or armature. Discuss the factors that determine how much torque is
produced.
Increasing the Number of Loops
Discuss the advantage of armatures constructed with many turns
of wire per loop and many loops.
The Commutator
Compare the use of the commutator in a DC generator and its use
in a DC motor. Emphasize that in the DC motor, it forces the
direction of current flow to remain constant through sections of
the rotating armature.
Shunt Motors
Describe the construction of the shunt motor and discuss why it
is such a good motor to use when constant speed is required.
Counter-Electromotive Force
Explain what CEMF is and what three factors it is proportional
to.
Speed-Torque Characteristics
Discuss what happens when a DC motor is started without a load.
Then discuss what happens when a load is added to the motor. Be
sure to talk about the level of torque needed to overcome the
various types of losses.
Speed Regulation
Have students write the speed regulation rule into their notes.
Be sure that students understand why a lower armature resistance
will result in better speed regulation.
Series Motors
Describe the DC series motor and compare the characteristics of
the series motor with those of the DC shunt motor. Be sure students
note the relationship of torque to current in the series motor.
Series Motor Speed Characteristics
Emphasize the fact that series motors have no speed limitations
and, therefore, need to be coupled with a load at all times.
Explain what can happen if a series motor is run without a load.
Give examples of where series motors are used.
Compound Motors
Explain that just like with the compound generator, the compound
motor is a combination of the shunt and series motors. Discuss the
benefits gained by utilizing the best of each motor, which makes
the compound motor the most widely used in industry. Go through the
steps for setting up a compound motor, and emphasize how important
it is to not set up a differential-compound motor. Demonstrate how
to check for rotation direction on a display motor.
Terminal Identification for DC Motors
Referring to Figure 30-12, and displaying a DC motor, guide
students through the various terminal identifications.
Determining the Direction of Rotation of a DC Motor
Explain how reversing the connections of the armature leads or
field leads changes the rotation direction of the motor. Also,
discuss how this is done differently in a compound motor.
Speed Control
Discuss how reducing the armature current, resulting in less
torque, causes the speed to reduce. Then, describe ways of reducing
the armature. Also, discuss why some ways of reducing the armature
are not the best.
The Field-Loss Relay
Explain how the field loss relay is used to protect the compound
motor and the load. Also, discuss the use of two separate shunt
fields in compound motors. Have students note that most large DC
motors have voltage applied to the shunt field at all times to
prevent moisture build-up.
Horsepower
Review the basic units of power and have students add these to
their notes, along with the horsepower formula.
Work out the two example problems on the board.
Brushless DC Motors
Describe this motor and compare it to previously studied motors.
Explain the use of the converter in supplying power to the
brushless motor, and discuss the difference between sine waves and
trapezoidal waves. Explain the use of multiple stator poles to
achieve a low speed and high torque output.
Inside-Out Motors
Describe and display a rotor wound inside. Make sure students
note that motors with a large amount of inertia exhibit superior
speed regulation.
Differences between Brush-Type and Brushless Motors
Discuss the advantages and disadvantages of each type of
motor.
Converters
Elaborate on the use of converters.
Permanent Magnet Motors
Explain the make-up of a permanent magnet motor and discuss why
it is more efficient than a field wound motor.
Operating Characteristics
Discuss how PM motors function, and compare them to separately
excited shunt motors.
DC Servomotors
Describe what servomotors are and how they are used.
DC ServoDisc® Motors
Describe this printed-circuit motor and compare it to a
servomotor, emphasizing the difference in the armature.
Explain how the torque and tangential forces are produced.
Characteristics of ServoDisc® Motors
Explain why the absence of iron in the armature of the disc
motor eliminates the cogging effect that the PM DC motors
experience. Discuss pulse-width modulation and how it is used.
The Right-Hand Motor Rule
Describe how to use the right hand to determine the direction of
flow of thrust, field flux, and current through the armature, just
as you did previously with the left-hand generator rule. Compare
this to that left-hand rule, and make sure students don’t get the
two confused.
Unit Round Up
Go over the summary together, checking for understanding, as you
have students elaborate on each point in the summary.
Have students complete the review questions on their own, and go
over these during your next class session.
ANSWER TO PRACTICAL APPLICATIONS
The motor has probably been connected differential compound
instead of cumulative compound. To test the motor:
1. Uncouple the motor from the load
2. Disconnect the shunt field winding (F1 and F2)
3. Momentarily apply power (bump the motor) and check for
direction of rotation. The motor is now a series motor so the
application of power must be short. If the motor turns in the
incorrect direction, change S1 and
S2.
4. Reconnect the shunt field winding.
5. Apply power to the motor. If it turns in the incorrect
direction change F1 and F2. When a compound motor
is operated at no load, the shunt field should determine the
direction of rotation.
6. When the motor turns in the same direction for both tests it
is connected cumulative compound.
PROCEDURE
CC.2.1.HS.F.4 Use units as a way to understand problems and to
guide the solution of multi-step problems.
CC.3.5.11-12.C. Follow precisely a complex multistep procedure
when carrying out experiments, taking measurements, or performing
technical tasks; analyze the specific results based on explanations
in the text.
1.6.11A Listen to others, ask questions, and take notes
3.4.12.B Apply energy sources and conversions and their
relationship to heat and temperature
EXPERIMENT # 1 D.C. SERIES MOTOR
1. Connect D.C. series motor to work station. (Diagram “A”)
2. Connect D.C. shunt motor to the generator (Prime mover).
3. Connect variable D.C. power supply to the motor so you can
variate the speed of the motor.
4. Connect a D.C. ammeter in series with the armature and the
field.
5. Turn on power and notice that you can vary the speed of the
motor by increasing or decreasing the D.C. power supply.
6. Adjust the speed of the motor by 10 levels to the highest
speed, and record all of the readings (Voltage and Current).
7. Explain why the current and voltages are increasing with the
RPM increase:
8. Reverse the motor by either switching the field or the
armature leads. (Diagram “B”)
9. Install the aluminum coupling on the shaft of the D.C.
motor.
10. Install the prony brake on the work station make sure the
coupling fits inside the belt.
(TO AVOID OVERHEATING OF THE COUPLING PUT A SMALL AMOUNT OF
WATER INSIDE THE COUPLING.)
11. Put a small load on the motor using the prony brake and
repeat step #6, record your readings:
12. Explain why the readings are different:
13. Put the input voltage at a constant speed and increase the
load slightly. Record your readings:
14. Explain why the load affected the current of the motor, and
what area it affected. (Field current or Armature current).
DIAGRAM “A” DIAGRAM “B”
EXPERIMENT # 2 D.C. SHUNT MOTOR
PROCEDURE
1. Connect the D.C. shunt motor in the self excited form.
(Diagram “A”)
2. Connect the power supply to the motor.
3. Install ammeters in series with the fields and the
armature.
4. Turn on power and notice that you can vary the speed of the
motor by increasing or decreasing the D.C. power supply.
5. Adjust the speed of the motor by 10 levels to the highest
speed, and record all of the readings (Voltage and Current).
6. Explain what the currents and voltages are doing with the RPM
increase:
7. Reverse the motor by either switching the field or the
armature leads.
8. Install the aluminum coupling on the shaft of the D.C.
motor.
9. Install the prony brake on the work station make sure the
coupling fits inside the belt. (TO AVOID OVERHEATING OF THE
COUPLING PUT A SMALL AMOUNT OF WATER INSIDE THE COUPLING.)
10. Put a small load on the motor using the prony brake and
repeat step #5, record your readings:
11. Explain why the readings are different:
12. Put the input voltage at a constant speed and increase the
load slightly. Record your readings:
13. Explain why the load affected the current of the motor, and
what area it affected. (Field current or Armature current).
14. Connect the motor as a separately excited control. (Diagram
“B”).
15. Repeat step # 5, was there any difference in the readings
using this type of control?
16. What are some advantages and disadvantages of using a
separately excited type of control?
DIAGRAM “A”
DIAGRAM “B”
EXPERIMENT # 3 D.C. COMPOUND MOTOR
PROCEDURE
1. Connect the D.C. compound motor in the self excited form.
(Diagram “A”)
2. Connect the power supply to the motor.
3. Install ammeters in series with the fields and the
armature.
4. Turn on power and notice that you can vary the speed of the
motor by increasing or decreasing the D.C. power supply.
5. Adjust the speed of the motor by 10 levels to the highest
speed, and record all of the readings (Voltage and Current).
6. Explain what the currents and voltages are doing with the RPM
increase:
7. Reverse the motor by either switching the field or the
armature leads.
8. Install the aluminum coupling on the shaft of the D.C.
motor.
9. Install the prony brake on the work station make sure the
coupling fits inside the belt. (TO AVOID OVERHEATING OF THE
COUPLING PUT A SMALL AMOUNT OF WATER INSIDE THE COUPLING.)
10. Put a small load on the motor using the prony brake and
repeat step #5, record your readings:
11. Explain why the readings are different:
12. Put the input voltage at a constant speed and increase the
load slightly. Record your readings:
13. Explain why the load affected the current of the motor, and
what area it affected. (Field current or Armature current).
14. Hook the motor up as a separately excited control. (Diagram
“B”).
15. Repeat step # 5, was there any difference in the readings
using this type of control?
16. What are some advantages and disadvantages of using a
separately excited type of control on this unit?
DIAGRAM “A”
DIAGRAM “B”
CC.3.5.11-12.B. Determine the central ideas or conclusions of a
text; summarize complex concepts, processes, or information
presented in a text by paraphrasing them in simpler but still
accurate terms.
CC.3.6.11-12.E. Use technology, including the Internet, to
produce, publish, and update individual or shared writing products
in response to ongoing feedback, including new arguments or
information.
3.1.12.B Apply concepts of models as a method predict and
understand science and technology
ANSWER THE FOLLOWING QUESTIONS
1. Why would the operation of a single coil armature be
“jerky”?
2. Name the three parts of a D.C. motor:
3. How is a megger used to test a D.C. motor?
4. What first item to receive power in a D.C. motor to start
it?
5. What is the last item to have power removed from the D.C.
motor to stop it?
6. Explain the left hand rule?
7. What are the two ways to change direction of a D.C.
motor?
8. What would happen if the fields were lost while the motor was
running at full RPM? (EXPLAIN!)
WRITE A SUMMARY ON THE ADVANTAGES AND DISADVANTAGES OF USING
D.C. MOTORS, GIVE EXAMPLES.
REFERENCE PAGES
Nikola Tesla (Serbian Cyrillic: Никола Тесла) (10 July 1856 – 7
January 1943) was an inventor, physicist, mechanical engineer, and
electrical engineer. Born in Smiljan, Croatian Krajina, Military
Frontier, he was an ethnic Serb subject of the Austrian Empire and
later became an American citizen. Tesla is best known for his many
revolutionary contributions to the discipline of electricity and
magnetism in the late 19th and early 20th century. Tesla's patents
and theoretical work formed the basis of modern alternating current
electric power (AC) systems, including the polyphase power
distribution systems and the AC motor, with which he helped usher
in the Second Industrial Revolution. Contemporary biographers of
Tesla have deemed him "the man who invented the twentieth century"
and "the patron saint of modern electricity."
After his demonstration of wireless communication (radio) in
1893 and after being the victor in the "War of Currents", he was
widely respected as America's greatest electrical engineer. Much of
his early work pioneered modern electrical engineering and many of
his discoveries were of groundbreaking importance. During this
period, in the United States, Tesla's fame rivaled that of any
other inventor or scientist in history or popular culture, but due
to his eccentric personality and unbelievable and sometimes bizarre
claims about possible scientific and technological developments,
Tesla was ultimately ostracized and regarded as a "mad scientist".
Never having put much focus on his finances, Tesla died
impoverished at the age of 86.
The SI unit measuring magnetic flux density or magnetic
induction (commonly known as the magnetic field ), the tesla, was
named in his honor (at the Conférence Générale des Poids et
Mesures, Paris, 1960).
Aside from his work on electromagnetism and engineering, Tesla
is said to have contributed in varying degrees to the establishment
of robotics, remote control, radar and computer science, and to the
expansion of ballistics, nuclear physics, and theoretical physics.
In 1943, the Supreme Court of the United States credited him as
being the inventor of the radio. Many of his achievements have been
used, with some controversy, to support various pseudoscience’s,
UFO theories, and early new age occultism. Tesla is honored in
Serbia and Croatia, as well as in Czech Republic (he was awarded
the highest order of the White Lion by Czechoslovakia) and in
unofficial ways in his adopted home, the United States.
NICOLA TESLA
(10 July 1856 – 7 January 1943)
History and development
The principle of conversion of electrical energy into mechanical
energy by electromagnetic means was demonstrated by the British
scientist Michael Faraday in 1821 and consisted of a free-hanging
wire dipping into a pool of mercury. A permanent magnet was placed
in the middle of the pool of mercury. When a current was passed
through the wire, the wire rotated around the magnet, showing that
the current gave rise to a circular magnetic field around the wire.
This motor is often demonstrated in school physics classes, but
brine (salt water) is sometimes used in place of the toxic mercury.
This is the simplest form of a class of electric motors called
homopolar motors. A later refinement is the Barlow's Wheel. These
were demonstration devices, unsuited to practical applications due
to limited power.
The first commutator-type direct-current electric motor capable
of a practical application was invented by the British scientist
William Sturgeon in 1832. Following Sturgeon's work, a
commutator-type direct-current electric motor made with the
intention of commercial use was built by the American Thomas
Davenport and patented in 1837. Although several of these motors
were built and used to operate equipment such as a printing press,
due to the high cost of primary battery power, the motors were
commercially unsuccessful and Davenport went bankrupt. Several
inventors followed Sturgeon in the development of DC motors but all
encountered the same cost issues with primary battery power. No
electricity distribution had been developed at the time. Like
Sturgeon's motor, there was no practical commercial market for
these motors.
The modern DC motor was invented by accident in 1873, when
Zénobe Gramme connected the dynamo he had invented to a second
similar unit, driving it as a motor. The Gramme machine was the
first electric motor that was successful in the industry.
In 1888 Nikola Tesla invented the first practicable AC motor and
with it the polyphase power transmission system. Tesla continued
his work on the AC motor in the years to follow at the Westinghouse
company.
DC motors
A DC motor is designed to run on DC electric power. Two examples
of pure DC designs are Michael Faraday's homopolar motor (which is
uncommon), and the ball bearing motor, which is (so far) a novelty.
By far the most common DC motor types are the brushed and brushless
types, which use internal and external commutation respectively to
create an oscillating AC current from the DC source -- so they are
not purely DC machines in a strict sense.
Brushed DC motors
The classic DC motor design generates an oscillating current in
a wound rotor with a split ring commutator, and either a wound or
permanent magnet stator. A rotor consists of a coil wound around a
rotor which is then powered by any type of battery.
Brushless DC motors
Many of the limitations of the classic commutator DC motor are
due to the need for brushes to press against the commutator. This
creates friction. At higher speeds, brushes have increasing
difficulty in maintaining contact. Brushes may bounce off the
irregularities in the commutator surface, creating sparks. This
limits the maximum speed of the machine. The current density per
unit area of the brushes limits the output of the motor. The
imperfect electric contact also causes electrical noise. Brushes
eventually wear out and require replacement, and the commutator
itself is subject to wear and maintenance. The commutator assembly
on a large machine is a costly element, requiring precision
assembly of many parts.
These problems are eliminated in the brushless motor. In this
motor, the mechanical "rotating switch" or commutator/brushgear
assembly is replaced by an external electronic switch synchronised
to the rotor's position. Brushless motors are typically 85-90%
efficient, whereas DC motors with brushgear are typically 75-80%
efficient.
Midway between ordinary DC motors and stepper motors lies the
realm of the brushless DC motor. Built in a fashion very similar to
stepper motors, these often use a permanent magnet external rotor,
three phases of driving coils, one or more Hall effect sensors to
sense the position of the rotor, and the associated drive
electronics. The coils are activated, one phase after the other, by
the drive electronics as cued by the signals from the Hall effect
sensors. In effect, they act as three-phase synchronous motors
containing their own variable-frequency drive electronics. A
specialized class of brushless DC motor controllers utilize EMF
feedback through the main phase connections instead of Hall effect
sensors to determine position and velocity. These motors are used
extensively in electric radio-controlled vehicles. When configured
with the magnets on the outside, these are referred to by modelists
as outrunner motors.
Brushless DC motors are commonly used where precise speed
control is necessary, computer disk drives or in video cassette
recorders the spindles within CD, CD-ROM (etc.) drives, and
mechanisms within office products such as fans, laser printers and
photocopiers. They have several advantages over conventional
motors:
Compared to AC fans using shaded-pole motors, they are very
efficient, running much cooler than the equivalent AC motors. This
cool operation leads to much-improved life of the fan's
bearings.
Without a commutator to wear out, the life of a DC brushless
motor can be significantly longer compared to a DC motor using
brushes and a commutator. Commutation also tends to cause a great
deal of electrical and RF noise; without a commutator or brushes, a
brushless motor may be used in electrically sensitive devices like
audio equipment or computers.
The same Hall effect sensors that provide the commutation can
also provide a convenient tachometer signal for closed-loop control
(servo-controlled) applications. In fans, the tachometer signal can
be used to derive a "fan OK" signal.
The motor can be easily synchronized to an internal or external
clock, leading to precise speed control.
Brushless motors have no chance of sparking, unlike brushed
motors, making them better suited to environments with volatile
chemicals and fuels.
Brushless motors are usually used in small equipment such as
computers and are generally used to get rid of unwanted heat.
They are also very quiet motors which is an advantage if being
used in equipment that is affected by vibrations.
Modern DC brushless motors range in power from a fraction of a
watt to many kilowatts. Larger brushless motors up to about 100 kW
rating are used in electric vehicles. They also find significant
use in high-performance electric model aircraft.
Universal motors
A variant of the wound field DC motor is the universal motor.
The name derives from the fact that it may use AC or DC supply
current, although in practice they are nearly always used with AC
supplies. The principle is that in a wound field DC motor the
current in both the field and the armature (and hence the resultant
magnetic fields) will alternate (reverse polarity) at the same
time, and hence the mechanical force generated is always in the
same direction. In practice, the motor must be specially designed
to cope with the AC current (impedance must be taken into account,
as must the pulsating force), and the resultant motor is generally
less efficient than an equivalent pure DC motor. Operating at
normal power line frequencies, the maximum output of universal
motors is limited and motors exceeding one kilowatt are rare. But
universal motors also form the basis of the traditional railway
traction motor in electric railways. In this application, to keep
their electrical efficiency high, they were operated from very low
frequency AC supplies, with 25 Hz and 16 2/3 hertz operation being
common. Because they are universal motors, locomotives using this
design were also commonly capable of operating from a third rail
powered by DC.
The advantage of the universal motor is that AC supplies may be
used on motors which have the typical characteristics of DC motors,
specifically high starting torque and very compact design if high
running speeds are used. The negative aspect is the maintenance and
short life problems caused by the commutator. As a result such
motors are usually used in AC devices such as food mixers and power
tools which are used only intermittently. Continuous speed control
of a universal motor running on AC is very easily accomplished
using a thyristor circuit, while stepped speed control can be
accomplished using multiple taps on the field coil. Household
blenders that advertise many speeds frequently combine a field coil
with several taps and a diode that can be inserted in series with
the motor (causing the motor to run on half-wave rectified AC).
Universal motors can rotate at relatively high revolutions per
minute (rpm). This makes them useful for appliances such as
blenders, vacuum cleaners, and hair dryers where high-speed
operation is desired. Many vacuum cleaner and weed trimmer motors
exceed 10,000 rpm, Dremel and other similar miniature grinders will
often exceed 30,000 rpm. Motor damage may occur due to overspeed
(rpm in excess of design specifications) if the unit is operated
with no significant load. On larger motors, sudden loss of load is
to be avoided, and the possibility of such an occurrence is
incorporated into the motor's protection and control schemes.
Often, a small fan blade attached to the armature acts as an
artificial load to limit the motor speed to a safe value, as well
as provide cooling airflow to the armature and field windings.
With the very low cost of semiconductor rectifiers, some
applications that would have previously used a universal motor now
use a pure DC motor, sometimes with a permanent magnet field.
AC motors
In 1882, Nikola Tesla identified the rotating magnetic field
principle, and pioneered the use of a rotary field of force to
operate machines. He exploited the principle to design a unique
two-phase induction motor in 1883. In 1885, Galileo Ferraris
independently researched the concept. In 1888, Ferraris published
his research in a paper to the Royal Academy of Sciences in
Turin.
Introduction of Tesla's motor from 1888 onwards initiated what
is sometimes referred to as the Second Industrial Revolution,
making possible the efficient generation and long distance
distribution of electrical energy using the alternating current
transmission system, also of Tesla's invention (1888).[1] Before
the invention of the rotating magnetic field, motors operated by
continually passing a conductor through a stationary magnetic field
(as in homopolar motors).
Tesla had suggested that the commutators from a machine could be
removed and the device could operate on a rotary field of force.
Professor Poeschel, his teacher, stated that would be akin to
building a perpetual motion machine.[2] Tesla would later attain
U.S. Patent 0,416,194 , Electric Motor (December 1889), which
resembles the motor seen in many of Tesla's photos. This classic
alternating current electro-magnetic motor was an induction
motor.
Michail Osipovich Dolivo-Dobrovolsky later invented a
three-phase "cage-rotor" in 1890. This type of motor is now used
for the vast majority of commercial applications.
Components
1. A typical AC motor consists of two parts:
2. An outside stationary stator having coils supplied with AC
current to produce a rotating magnetic field, and;
An inside rotor attached to the output shaft that is given a
torque by the rotating field.
Slip ring
The slip ring or wound rotor motor is an induction machine where
the rotor comprises a set of coils that are terminated in slip
rings to which external impedances can be connected. The stator is
the same as is used with a standard squirrel cage motor!
By changing the impedance connected to the rotor circuit, the
speed/current and speed/torque curves can be altered.
The slip ring motor is used primarily to start a high inertia
load or a load that requires a very high starting torque across the
full speed range. By correctly selecting the resistors used in the
secondary resistance or slip ring starter, the motor is able to
produce maximum torque at a relatively low current from zero speed
to full speed. A secondary use of the slip ring motor is to provide
a means of speed control. Because the torque curve of the motor is
effectively modified by the resistance connected to the rotor
circuit, the speed of the motor can be altered. Increasing the
value of resistance on the rotor circuit will move the speed of
maximum torque down. If the resistance connected to the rotor is
increased beyond the point where the maximum torque occurs at zero
speed, the torque will be further reduced.
When used with a load that has a torque curve that increases
with speed, the motor will operate at the speed where the torque
developed by the motor is equal to the load torque. Reducing the
load will cause the motor to speed up, and increasing the load will
cause the motor to slow down until the load and motor torque are
equal. Operated in this manner, the slip losses are dissipated in
the secondary resistors and can be very significant. The speed
regulation is also very poor.
Maintenance and Troubleshooting of Electric Motors
Reliance Electric
Motor Maintenance - SCHEDULED ROUTINE CARE
IntroductionThe key to minimizing motor problems is scheduled
routine inspection and service. The frequency of routine service
varies widely between applications.
Including the motors in the maintenance schedule for the driven
machine or general plant equipment is usually sufficient. A motor
may require additional or more frequent attention if a breakdown
would cause health or safety problems, severe loss of production,
damage to expensive equipment or other serious losses.
Written records indicating date, items inspected, service
performed and motor condition are important to an effective routine
maintenance program. From such records, specific problems in each
application can be identified and solved routinely to avoid
breakdowns and production losses.
The routine inspection and servicing can generally be done
without disconnecting or disassembling the motor. It involves the
following factors:
Dirt and Corrosion
1. Wipe, brush, vacuum or blow accumulated dirt from the frame
and air passages of the motor. Dirty motors run hot when thick dirt
insulates the frame and clogged passages reduce cooling air flow.
Heat reduces insulation life and eventually causes motor
failure.
2. Feel for air being discharged from the cooling air ports. If
the flow is weak or unsteady, internal air passages are probably
clogged. Remove the motor from service and clean.
3. Check for signs of corrosion. Serious corrosion may indicate
internal deterioration and/or a need for external repainting.
Schedule the removal of the motor from service for complete
inspection and possible rebuilding.
4. In wet or corrosive environments, open the conduit box and
check for deteriorating insulation or corroded terminals. Repair as
needed.
LubricationLubricate the bearings only when scheduled or if they
are noisy or running hot. Do NOT over-lubricate. Excessive grease
and oil creates dirt and can damage bearings.
Heat, Noise and VibrationFeel the motor frame and bearings for
excessive heat or vibration. Listen for abnormal noise. All
indicate a possible system failure. Promptly identify and eliminate
the source of the heat, noise or vibration.
Winding InsulationWhen records indicate a tendency toward
periodic winding failures in the application, check the condition
of the insulation with an insulation resistance test. Such testing
is especially important for motors operated in wet or corrosive
atmospheres or in high ambient temperatures.
Brushes and Commutators (DC Motors)
1. Observe the brushes while the motor is running. The brushes
must ride on the commutator smoothly with little or no sparking and
no brush noise (chatter).
2. Stop the motor. Be certain that:
· The brushes move freely in the holder and the spring tension
on each brush is about equal.
· Every brush has a polished surface over the entire working
face indicating good seating.
· The commutator is clean, smooth and has a polished brown
surface where the brushes ride. NOTE: Always put each brush back
into its original holder. Interchanging brushes decreases
commutation ability.
· There is no grooving of the commutator (small grooves around
the circumference of the commutator). If there is grooving, remove
the motor from service immediately as this is a symptomatic
indication of a very serious problem.
3. Replace the brushes if there is any chance they will not last
until the next inspection date.
4. If accumulating, clean foreign material from the grooves
between the commutator bars and from the brush holders and
posts.
5. Brush sparking, chatter, excessive wear or chipping, and a
dirty or rough commutator indicate motor problems requiring prompt
service. Figure 1. Typical DC Motor Brushes And Commutator
Brushes and Collector Rings (Synchronous Motors)
1. Black spots on the collector rings must be removed by rubbing
lightly with fine sandpaper. If not removed, these spots cause
pitting that requires regrinding the rings.
Figure 2. Rotary Converter Armature Showing Commutator And Slip
Rings.
2. An imprint of the brush, signs of arcing or uneven wear
indicate the need to remove the motor from service and repair or
replace the rings.
3. Check the collector ring brushes as described under "Brushes
and Commutators". They do not, however, wear as rapidly as
commutator brushes.
BEARING LUBRICATION
IntroductionModern motor designs usually provide a generous
supply of lubricant in tight bearing housings. Lubrication on a
scheduled basis, in conformance with the manufacturer's
recommendations, provides optimum bearing life.
Thoroughly clean the lubrication equipment and fittings before
lubricating. Dirt introduced into the bearings during lubrication
probably causes more bearing failures than the lack of
lubrication.
Too much grease can overpack bearings and cause them to run hot,
shortening their life.
Excessive lubricant can find its way inside the motor where it
collects dirt and causes insulation deterioration.
Many small motors are built with permanently lubricated
bearings. They cannot and should not be lubricated.
Oiling Sleeve BearingsAs a general rule, fractional horsepower
motors with a wick lubrication system should be oiled every 2000
hours of operation or at least annually. Dirty, wet or corrosive
locations or heavy loading may require oiling at three-month
intervals or more often. Roughly 30 drops of oil for a 3-inch
diameter frame to 100 drops for a 9-inch diameter frame is
sufficient. Use a 150 SUS viscosity turbine oil or SAE 10
automotive oil.
Some larger motors are equipped with oil reservoirs and usually
a sight gage to check proper level.(Figure 3) As long as the oil is
clean and light in color, the only requirement is to fill the
cavity to the proper level with the oil recommended by the
manufacturer. Do not overfill the cavity. If the oil is discolored,
dirty or contains water, remove the drain plug. Flush the bearing
with fresh oil until it comes out clean. Coat the plug threads with
a sealing compound, replace the plug and fill the cavity to the
proper level.
When motors are disassembled, wash the housing with a solvent.
Discard used felt packing. Replace badly worn bearings. Coat the
shaft and bearing surfaces with oil and reassemble.
Figure 3. Cross Section of the Bearing System of a Large
Motor
Greasing Ball and Roller BearingsPractically all Reliance ball
bearing motors in current production are equipped with the
exclusive PLS/Positive Lubrication System. PLS is a patented
open-bearing system that provides long, reliable bearing and motor
life regardless of mounting position. Its special internal passages
uniformly distribute new grease pumped into the housing during
regreasing through the open bearings and forces old grease out
through the drain hole. The close running tolerance between shaft
and inner bearing cap minimizes entry of contaminants into the
housing and grease migration into the motor. The unique V-groove
outer slinger seals the opening between the shaft and end bracket
while the motor is running or is at rest yet allows relief of
grease along the shaft if the drain hole is plugged. (Figure 4)
The frequency of routine greasing increases with motor size and
severity of the application as indicated in Table 1. Actual
schedules must be selected by the user for the specific
conditions.
During scheduled greasing, remove both the inlet and drain
plugs. Pump grease into the housing using a standard grease gun and
light pressure until clean grease comes out of the drain hole.
If the bearings are hot or noisy even after correction of
bearing overloads (see "Troubleshooting") remove the motor from
service. Wash the housing and bearings with a good solvent. Replace
bearings that show signs of damage or wear. Repack the bearings,
assemble the motor and fill the grease cavity.
Whenever motors are disassembled for service, check the bearing
housing. Wipe out any old grease. If there are any signs of grease
contamination or breakdown, clean and repack the bearing system as
described in the preceding paragraph.
Figure 4. Cross Section of PLS Bearing System (Positive
Lubrication System)
HEAT, NOISE AND VIBRATION
HeatExcessive heat is both a cause of motor failure and a sign
of other motor problems.
The primary damage caused by excess heat is to increase the
aging rate of the insulation. Heat beyond the insulation's rating
shortens winding life. After overheating, a motor may run
satisfactorily but its useful life will be shorter. For maximum
motor life, the cause of overheating should be identified and
eliminated.
As indicated in the Troubleshooting Sections, overheating
results from a variety of different motor problems. They can be
grouped as follows:
· WRONG MOTOR: It may be too small or have the wrong starting
torque characteristics for the load. This may be the result of poor
initial selection or changes in the load requirements.
· POOR COOLING: Accumulated dirt or poor motor location may
prevent the free flow of cooling air around the motor. In other
cases, the motor may draw heated air from another source. Internal
dirt or damage can prevent proper air flow through all sections of
the motor. Dirt on the frame may prevent transfer of internal heat
to the cooler ambient air.
· OVERLOADED DRIVEN MACHINE: Excess loads or jams in the driven
machine force the motor to supply higher torque, draw more current
and overheat.
Table 1. Motor Operating Conditions
· Light Duty: Motors operate infrequently (1 hour/day or less)
as in portable floor sanders, valves, door openers.
· Standard Duty: Motors operate in normal applications (1 or 2
work shifts). Examples include air conditioning units, conveyors,
refrigeration apparatus, laundry machinery, woodworking and textile
machines, water pumps, machine tools, garage compressors.
· Heavy Duty: Motors subjected to above normal operation and
vibration (running 24 hours/day, 365 days/year). Such operations as
in steel mill service, coal and mining machinery, motor-generator
sets, fans, pumps.
· Severe Duty: Extremely harsh, dirty motor applications. Severe
vibration and high ambient conditions often exist.
· EXCESSIVE FRICTION: Misalignment, poor bearings and other
problems in the driven machine, power transmission system or motor
increase the torque required to drive the loads, raising motor
operating temperature.
· ELECTRICAL OVERLOADS: An electrical failure of a winding or
connection in the motor can cause other Windings or the entire
motor to overheat.
Noise and VibrationNoise indicates motor problems but ordinarily
does not cause damage. Noise, however, is usually accompanied by
vibration.
Vibration can cause damage in several ways. It tends to shake
windings loose and mechanically damages insulation by cracking,
flaking or abrading the material. Embrittlement of lead wires from
excessive movement and brush sparking at commutators or current
collector rings also results from vibration. Finally, vibration can
speed bearing failure by causing balls to "brinnell," sleeve
bearings to be pounded out of shape or the housings to loosen in
the shells.
Whenever noise or vibration are found in an operating motor, the
source should be quickly isolated and corrected. What seems to be
an obvious source of the noise or vibration may be a symptom of a
hidden problem. Therefore, a thorough investigation is often
required.
Noise and vibrations can be caused by a misaligned motor shaft
or can be transmitted to the motor from the driven machine or power
transmission system. They can also be the result of either
electrical or mechanical unbalance in the motor.
After checking the motor shaft alignment, disconnect the motor
from the driven load. If the motor then operates smoothly, look for
the source of noise or vibration in the driven equipment.
If the disconnected motor still vibrates, remove power from the
motor. If the vibration stops, look for an electrical unbalance. If
it continues as the motor coasts without power, look for a
mechanical unbalance.
Electrical unbalance occurs when the magnetic attraction between
stator and rotor is uneven around the periphery of the motor. This
causes the shaft to deflect as it rotates creating a mechanical
unbalance. Electrical unbalance usually indicates an electrical
failure such as an open stator or rotor winding, an open bar or
ring in squirrel cage motors or shorted field coils in synchronous
motors. An uneven air gap, usually from badly worn sleeve bearings,
also produces electrical unbalance.
The chief causes of mechanical unbalance include a distorted
mounting, bent shaft, poorly balanced rotor, loose parts on the
rotor or bad bearings. Noise can also come from the fan hitting the
frame, shroud, or foreign objects inside the shroud. If the
bearings are bad, as indicated by excessive bearing noise,
determine why the bearings failed. (See Troubleshooting Problems D
and L.)
Brush chatter is a motor noise that can be caused by vibration
or other problems unrelated to vibration. See Troubleshooting
Problem M for details.
WINDlNGS
Care of Windings and InsulationExcept for expensive, high
horsepower motors, routine inspections generally do not involve
opening the motor to inspect the windings. Therefore, long motor
life requires selection of the proper enclosure to protect the
windings from excessive dirt, abrasives, moisture, oil and
chemicals.
When the need is indicated by severe operating conditions or a
history of winding failures, routine testing can identify
deteriorating insulation. Such motors can be removed from service
and repaired before unexpected failures stop production.
Whenever a motor is opened for repair, service the windings as
follows:
1. Accumulated dirt prevents proper cooling and may absorb
moisture and other contaminants that damage the insulation. Vacuum
the dirt from the windings and internal air passages. Do not use
high pressure air because this can damage windings by driving the
dirt into the insulation.
2. Abrasive dust drawn through the motor can abrade coil noses,
removing insulation. If such abrasion is found, the winding should
be revarnished or replaced.
3. Moisture reduces the dielectric strength of insulation which
results in shorts. If the inside of the motor is damp, dry the
motor per information in "Cleaning and Drying Windings".
4. Wipe any oil and grease from inside the motor. Use care with
solvents that can attack the insulation.
5. If the insulation appears brittle, overheated or cracked, the
motor should be revarnished or, with severe conditions,
rewound.
6. Loose coils and leads can move with changing magnetic fields
or vibration, causing the insulation to wear, crack or fray.
Revarnishing and retying leads may correct minor problems. If the
loose coil situation is severe, the motor must be rewound.
7. Check the lead-to-coil connections for signs of overheating
or corrosion. These connections are often exposed on large motors
but taped on small motors. Repair as needed.
8. Check wound rotor windings as described for stator windings.
Because rotor windings must withstand centrifugal forces, tightness
is even more important. In addition, check for loose pole pieces or
other loose parts that create unbalance problems.
9. The cast rotor rods and end rings of squirrel cage motors
rarely need attention. However, open or broken rods create
electrical unbalance that increases with the number of rods broken.
An open end ring causes severe vibration and noise.
Testing WindingsRoutine field testing of windings can identify
deteriorating insulation permitting scheduled repair or replacement
of the motor before its failure disrupts operations. Such testing
is good practice especially for applications with severe operating
conditions or a history of winding failures and for expensive, high
horsepower motors and locations where failures can cause health and
safety problems or high economic loss.
The easiest field test that prevents the most failures is the
ground-insulation, or &127megger," test. It applies DC voltage,
usually 500 or 1000 volts, to the motor and measures the resistance
of the insulation.
NEMA standards require a minimum resistance to ground at 40
degrees C ambient of 1 megohm per kv of rating plus 1 megohm.
Medium size motors in good condition will generally have
megohmmeter readings in excess of 50 megohms. Low readings may
indicate a seriously reduced insulation condition caused by
contamination from moisture, oil or conductive dirt or
deterioration from age or excessive heat.
One megger reading for a motor means little. A curve recording
resistance, with the motor cold and hot, and date indicates the
rate of deterioration. This curve provides the information needed
to decide if the motor can be safely left in service until the next
scheduled inspection time.
The megger test indicates ground insulation condition. It does
not, however, measure turn-to-turn insulation condition and may not
pick up localized weaknesses. Moreover, operating voltage peaks may
stress the insulation more severely than megger voltage. For
example, the DC output of a 500-volt megger is below the normal
625-volt peak each half cycle of an AC motor operating on a
440-volt system. Experience and conditions may indicate the need
for additional routine testing.
A test used to prove existence of a safety margin above
operating voltage is the AC high potential ground test. It applies
a high AC voltage (typically, 65% of a voltage times twice the
operating voltage plus 1000 volts) between windings and frame.
Although this test does detect poor insulation condition, the
high voltage can arc to ground, burning insulation and frame, and
can also actually cause failure during the test. It should never be
applied to a motor with a low megger reading.
DC rather than AC high potential tests are becoming popular
because the test equipment is smaller and the low test current is
less dangerous to people and does not create damage of its own.
Cleaning and Drying WindingsMotors which have been flooded or
which have low megger readings because of contamination by
moisture, oil or conductive dust should be thoroughly cleaned and
dried. The methods depend upon available equipment.
A hot water hose and detergents are commonly used to remove
dirt, oil, dust or salt concentrations from rotors, stators and
connection boxes. After cleaning, the windings must be dried,
commonly in a forced-draft oven. Time to obtain acceptable megger
readings varies from a couple hours to a few days.
BRUSH AND COMMUTATOR CARESome maintenance people with many
relatively trouble-free AC squirrel cage motors forget that brushes
and commutators require more frequent routine inspection and
service. The result can be unnecessary failures between scheduled
maintenance.
As indicated in Troubleshooting Problem M on Page 27, many
factors are involved in brush and commutator problems. All
generally involve brush sparking usually accompanied by chatter and
often excessive wear or chipping. Sparking may result from poor
commutator conditions or it may cause them.
The degree of sparking should be determined by careful visual
inspection. The illustrations shown inFigure 5 are a useful guide.
It is very important that you gauge the degree number as accurately
as possible. The solution to the problem may well depend upon the
accuracy of your answer since many motor, load, environmental and
application conditions can cause sparking.
It is also imperative that a remedy be determined as quickly as
possible. Sparking generally feeds upon itself and becomes worse
with time until serious damage results.
Some of the causes are obvious and some are not. Some are
constant and others intermittent. Therefore, eliminating brush
sparking, especially when it is a chronic or recurring problem,
requires a thorough review of the motor and operating conditions.
Always recheck for sparking after correcting one problem to see
that it solved the total problem. Also remember that, after
grinding the commutator and properly reseating the brushes,
sparking will occur until the polished, brown surface reforms on
the commutator.
NOTE: Small sparks are yellow in color, and the large sparks are
white in color. The white sparks, or blue-white sparks, are most
detrimental to commutation (both brush and commutator).
Figure 5. Degrees of Generator and Motor Sparking
First consider external conditions that affect commutation.
Frequent motor overloads, vibration and high humidity cause
sparking. Extremely low humidity allows brushes to wear through the
needed polished brown commutator surface film. Oil, paint, acid and
other chemical vapors in the atmosphere contaminate brushes and the
commutator surface.
Look for obvious brush and brush holder deficiencies:
1. Be sure brushes are properly seated, move freely in the
holders and are not too short.
2. The brush spring pressure must be equal on all brushes.
3. Be sure spring pressure is not too light or too high. Large
motors with adjustable springs should be set at about 3 to 4 pounds
per square inch of brush surface in contact with the
commutators.
4. Remove dust that can cause a short between brush holders and
frame.
5. Check lead connections to the brush holders. Loose
connections cause overheating.
Look for obvious commutator problems:
1. Any condition other than a polished, brown surface under the
brushes indicates a problem. Severe sparking causes a rough
blackened surface. An oil film, paint spray, chemical contamination
and other abnormal conditions can cause a blackened or discolored
surface and sparking. Streaking or grooving under only some brushes
or flat and burned spots can result from a load mismatch and cause
motor electrical problems. Grooved commutators should be removed
from service. A brassy appearance shows excessive wear on the
surface resulting from low humidity or wrong brush grade.
2. High mica or high or low commutator bars make the brushes
jump, causing sparking.
3. Carbon dust, copper foil or other conductive dust in the
slots between commutator bars causes shorting and sometimes
sparking between bars.
If correcting any obvious deficiencies does not eliminate
sparking or noise, look to the less obvious possibilities:
1. If brushes were changed before the problem became apparent,
check the grade of brushes. Weak brushes may chip. Soft, low
abrasive brushes may allow a thick film to form. High friction or
high abrasion brushes wear away the brown film, producing a brassy
surface. If the problem appears only under one or more of the
brushes, two different grades of brushes may have been installed.
Generally, use only the brushes recommended by the motor
manufacturer or a qualified brush expert.
2. The brush holder may have been reset improperly. If the boxes
are more than 1/8" from the commutator, the brushes can jump or
chip. Setting the brush holder off neutral causes sparking.
Normally the brushes must be equally spaced around the commutator
and must be parallel to the bars so all make contact with each bar
at the same time.
3. An eccentric commutator causes sparking and may cause
vibration. Normally, concentricity should be within .001" on high
speed, .002" on medium speed and .004" on slow speed motors.
4. Various electrical failures in the motor windings or
connections manifest themselves in sparking and poor commutation.
Look for shorts or opens in the armature circuit and for grounds,
shorts or opens in the field winding circuits. A weak interpole
circuit or large air gap also generate brush sparking.
DC MOTOR BRUSH HOLDERS AND
THE PERFORMANCE OF CARBON BRUSHES
Introduction
A DC Motor carbon brush is an electrical contact which makes a
connection with a moving surface. Optimal performance on motors,
generators and other types of moving contact applications will be
attained only when the carbon brush, the brushholder and the
contact surface are properly designed and maintained. All three
components are critical factors in a complex electro-mechanical
system. The DC Motor brushholder, as the name suggests, holds the
brush so that the brush can perform properly. Holders provide
stable support in the proper position in relation to the contact
surface and often provide the means for application of the contact
force on the brush.For many decades brushholders had received
little attention. New rotating equipment was supplied with copies
of the same old brushholder designs. Typically, when performance
problems occurred the focus had been on the brush as this was the
part exhibiting rapid wear. In the early 1980’s Helwig Carbon led
the industry towards the consideration of brushholders and
particularly spring pressure as a common cause of many brush
problems. Further, recent holder developments and the coordination
of the designs of constant pressure holders with Red Top brushes
have resulted in significant advancements in performance and
life.The purpose of this paper is to review the critical areas of
consideration for brushholders in relation to the proper
functioning of brushes. The most important factors are 1) maximum
stability of the carbon in the holder, 2) proper positioning of the
brush on the contact surface, and 3) minimum resistance through the
brush and holder portion of the electrical circuit.
Holder Size Dimensions
The fit of the carbon portion of the brush in the holder is
critical for stable electrical contact. If there is inadequate
space between the holder walls and the thickness and width of the
brush, there is potential for binding of the brush in the holder
particularly with increased temperature and contamination. On the
other hand, an excess amount of space between the holder and the
carbon will result in an unstable electrical contact as the brush
face can move tangentially or axially within the holder. The holder
and brush tolerances on the thickness and width therefore must be
well coordinated. Brushes are machined undersize per NEMA
tolerances or per drawing specifications while brushholders are
made oversize. As a general guideline for brushholders, industrial
sizes typically should be held oversize to a tolerance of
+.002/+.008". Smaller frame units with a brush thickness less than
.500" and greater than .125" should have holders with a tolerance
of +.001/+.005". Micro size units with brushes of thickness .125"
or less should have holders held to a tolerance of
+.001/+.003".Over a long period of usage the thickness dimension on
a holder can become worn from brush movement or distorted from
heat. Therefore, it is important to periodically measure the
thickness and width dimensions on the top and bottom of the holders
to ensure they are within tolerance and that the brush will have
adequate support for a stable electrical contact. When motor and
generator brushholders are subjected to high temperatures, it may
be necessary to provide extra compensation for thermal expansion
depending on the temperature rise and the degree of heat
dissipation. In these cases it is easier to reduce the brush
thickness and width dimensions slightly to avoid sticking in the
holder rather than adjusting holder dimensions. Metal graphite
brushes with over 50% metal content by weight are manufactured with
an increased undersize tolerance per NEMA standards as they usually
carry higher current, generate more heat, and have a higher
coefficient of thermal expansion than non-metal grades.Brush and
holder length can also have a significant effect on the stability
and performance of the brush. Most often the length is limited due
to the space available within the frame. There are, however, also
practical length limitations due to the excess resistance of a long
piece of carbon. As the carbon length is increased the resistance
of the current path from the shunt to the contact surface is
increased. At the same time the amount of contact area between the
carbon and the longer holder is increased and the corresponding
contact resistance is decreased. This then creates the potential
for distorted current flow directly between the holder and the
carbon rather than through the shunting. On the other hand, short
brush and holder designs are more susceptible to instability at the
contact surface. There is potential for a higher degree of brush
tilt in the holder since the length of support is less in relation
to the brush thickness.In addition to dimensional concerns the
insides of the holder must be smooth and free of all obstructions
including burrs. If a used brush has any straight scratches down
the sides of the carbon then there are protrusions inside the brush
box, which will restrict the brush from making proper electrical
contact. Rough handling of brushholders can cause distortion of the
metal and effect the critical inside dimensions of the brush
cavity. Holders made from metal stampings are particularly
susceptible to irregularities on the inside dimensions and on
squareness. Broaching is generally accepted as the best
manufacturing method for assurance of consistent inside dimensions
and a smooth finish.Return to top of page.
Holder Position
The holder position will determine the location of the brush on
the moving contact surface. For slip ring applications the holders
are usually located around the top portion of the ring for ease of
access. In this position the weight of the brush contributes to the
contact force. If holders are mounted on the underside of a contact
surface then additional spring force may be necessary to compensate
for the weight of the brush. On DC machines with commutators proper
positioning of the holders in relation to the field poles is
critical. The brushes should be equally spaced around the
commutator. This spacing can be checked by wrapping a paper tape
around the commutator, marking the location of the same edge of
each brush, and then measuring the distance between marks on the
paper. The brushes must also contact the commutator within the
neutral zone where voltage levels are near zero. When the holder
position allows the brush to make contact outside the neutral zone
there will be higher bar to bar voltages under the brush,
circulating currents, bar edge burning, and damage from arcing.
Holder Angle
The most common angle for holder mounting is 0 degrees, i.e.
perpendicular to the contact surface. Most slip rings and reversing
commutator applications make use of this so-called radial mount.
The advantages are ease of holder installation, maximum spring
force transferred to the contact surface, and fair stability of
brush contact upon reversal of direction. Any brush face movement
within the holder will result in a change in the contact surface.
The most stable surface contact will occur when the top and bottom
of the brush are always held to the same side of the holder
regardless of the direction of rotation. Angle holder mountings
were developed to increase this stability and the effective area of
the brush contact. However stability will occur only when the
correct angles are used in relation to the direction of
rotation.When the entering edge is the short side of the brush or a
trailing position the face angle should be 20 degrees or less. At
greater angles the action of the rotation and the spring force
wedges the brush into the bottom corner of the holder and causes
high friction and an unstable contact. Normally trailing brushes
also have a shallow top bevel. When the entering edge is the long
side of the brush or a leading position the face angle should be 25
degrees or more. At angles of 20 degrees and less the action of the
rotation pulls the bottom of the brush to the opposite side of the
holder from the top of the brush. Leading brushes should have a top
bevel of 20 to 30 degrees. A stable contact can be maintained in
either or both directions of rotation with brush face angles
between 20 and 25 degrees. The potential disadvantage of holder
angles is the loss of effective downward force of the spring. A
portion of the spring force is dissipated in holding the brush
stable to one side for the holder. The loss in downward contact
force for various angles are as follows:
The spring force should be increased to compensate for the loss
of effective downward force from the action of the brush angle in
holding the brush to the side of the holder. If a brush has bevels
of 20 degrees on the top and 30 degrees on the bottom then the
spring force should be increased 6.0% + 13.4% or about 20% to
maintain the proper level of effective downward contact force at
the brush face.In the special case of post mounted double holders
commonly used on slip rings, the best design would allow both
brushes to make contact at zero degrees or perpendicular to the
ring. Any angle will result in one brush in the pair operating with
less contact stability.Return to top of page.
Holder Mounting Height
The vertical position of the holders above the contact surface
is very important in assuring proper brush support throughout the
wearable length of the rush and for proper positioning on the
contact surface. When a brushholder is mounted too high above the
contact surface or when the surface has been turned down to a
significantly smaller diameter, there will not be adequate support
for the carbon as the brush wears to a short length. This will
contribute to increased electrical wear due to the instability of
the contact. The holder mounting height should be proportional to
the size of the unit. On the large frame sizes the holders should
be mounted a maximum of .125" above the contact surface. In a few
cases units operated with intentional runout of the contact surface
which must be taken into consideration. The small micro frame sizes
should have a holder mounting height of approximately .032". During
holder mounting a flexible mounting pad of the appropriate
thickness can be placed on the contact surface to ensure consistent
height and spacing. This pad also helps protect the commutator from
damage during mounting.There are several common problems related to
excess height of the holder. When a commutator has been turned down
several times angled brushes will make contact in a different
position. With steep bottom bevels and significant decreases in
diameter the location of the brush contact could even move outside
the neutral zone. There will be a significant increase in wear
unless the holder is moved closer to the commutator or the neutral
is adjusted.Although single post mounted holders can be rotated to
move the holder closer to the commutator, the position of brush
contact will change. As above it is very likely that adjustment of
the neutral position will be required to avoid edge arcing.On
V-shaped toe-to-toe holders which are mounted too high above the
commutator the brushes can interfere at the toes. This will result
in one or both brushes not making contact with the commutator. It
is especially important that these old style holders are mounted
sufficiently close to the commutator to avoid this problem.Return
to top of page.
Spring Force
Many inventive methods have been used for the application of the
contact force on brushes. These included clock type springs,
torsion bars, lever springs, helical coil springs, and constant
force negator springs. As noted in the graph shown below the brush
wear rate will change as the spring pressure changes. This is one
of the most important concepts in understanding brush performance.
There has always been a problem with an accelerating rate of wear
as the brush gets shorter due to the declining spring force and the
dramatic increase in electrical wear. The most consistent brush
performance will be attained when the spring force is virtually
constant at the correct level throughout the wear length of the
brush.
The use of the proper constant force springs can be a
significant advantage with consistent minimal wear rate of the
brushes, reduced wear of the contact surface, less carbon dust, and
much lower overall maintenance costs on the unit.Testing and
application experience have resulted in the following recommended
ranges of spring pressure:
Electrical Connections
The primary function of the brush involves conducting current.
In many cases the brush holder is also a part of this electrical
circuit. Therefore it is necessary that all electrical connections
are of minimal resistance to provide the best path for current flow
from the main lead connection to the contact surface. Corrosion,
contamination, or electrolytic action over a period of time can
cause dramatic increases in resistance which then requires
cleaning. Careless installation of the brushes or the holders can
lead to loose connections. Any high resistance in the brush circuit
will result in excess heat or an undesirable path of current flow
and unequal loading of the brushes. On fractional horsepower
cartridge style brushholders with captive coil spring type brushes
the current should flow from the clip connector at the bottom of
the holder up the brass insert to the cap on the end of the brush
and then down through the shunt to the carbon. The brushes fail
very quickly if the round or eared cap on the end of the brush does
not make proper contact with the brass holder insert. When this
condition exists current will flow directly from the brass insert
to the spring or to the carbon. In either case there will be
extreme heat, loss of brush contact, commutator wear, and
eventually motor failure.Another problem with larger frame sizes
can occur when the holder mounting is part of the electric circuit.
If the holder mounting surface becomes dirty, corroded, or even
painted over then current will again need to follow another path
and thereby cause problems.
Tesla Model S
NAME:
LEVEL: DATE:
CHECK LIST FOR D.C. GENERATOR PACKET
STEPS/TASKS
1) The student completed all vocabulary associated with this
learning guide to 100% accuracy.
25
2) The student completed all written work associated with this
learning guide to 100% accuracy.
25
3) The student completed the written assessment to 80%
accuracy.
25
4) The student recorded all project results.
50
5) The student completed experiment #1
25
6) The student completed experiment # 2
25
7) The student completed experiment # 3
25
8) The student completed the required graph of experiment
results.
25
9) The student completed the summary of results of this learning
guide.
25
Total Points
250
* ALL STEPS/TASKS MUST MEET THE STANDARDS IN ORDER TO ACHIEVE
MASTERY.*
COMMENTS:
INSTRUCTOR SIGNATURE:
DATE:
NAME:
DATE:
DC MOTOR ASSESSMENT POST TEST
True/False
Indicate whether the sentence or statement is true or false.
____1.Counter-electromotive force (CEMF) and back-EMF are names
for the voltage induced into the armature of a DC motor.
____2.It is all right to operate a series motor with no load
connected.
Multiple Choice
Identify the letter of the choice that best completes the
statement or answers the question.
____3.A device that converts electrical input into mechanical
output is called
a.
alternator
b.
generator
c.
motor
d.
inverter
____4.When the field winding of a DC motor is connected in
parallel with the armature, the motor is called a _____ motor.
a.
series
b.
shunt
____5.The DC motor, known as a “constant speed motor,” is the
_____ motor.
a.
series
b.
shunt
____6.The speed regulation of a DC motor is proportional to
the
a.
applied voltage
b.
load torque
c.
armature resistance
d.
field resistance
____7.A compound motor has _____ field windings.
a.
series
b.
shunt
c.
series and shunt
d.
combined
____8.The cumulative compound motor connection means that the
fields of the series and shunt field windings
a.
add
b.
subtract
c.
multiply
d.
divide
____9.The compound motor connection that is rarely used is
the
a.
cumulative
b.
differential
____10.In a compound DC motor control circuit, the field loss
relay will disconnect the _____ if power is lost to the _____.
a.
rotor, armature
b.
armature, shunt field
c.
shunt field, armature
d.
armature, rotor
____11.The motor that does not contain a wound armature,
commutator, or brushes is the _____ motor.
a.
servo
b.
stepping
c.
selsyn
d.
brushless DC
____12.Permanent magnet motors have _____ efficiency than wound
field motors.
a.
higher
b.
lower
____13.Small permanent magnet motors that have lightweight
rotors are often used as
a.
stepping motors
b.
automobile starters
c.
servomotors
d.
ceiling fans
____14.The servomotor which has a fiberglass and copper rotor is
the _____ motor.
a.
copper
b.
copper-glass
c.
nonferrous
d.
ServoDisc®
____15.Because the rotor of a ServoDisc® motor is made from
copper laminated onto a fiberglass disk, the motor is also called a
_____ motor.
a.
copper
b.
laminated
c.
printed circuit
d.
nonferrous
____16.Some DC servomotors have square wave input voltage. To
vary the motor speed, the on and off time ratio of the square wave
is varied. This is called _____ modulation.
a.
phase
b.
pulse-width
c.
frequency
d.
amplitude
____17.A square wave voltage varies between 0 V and 12 V. The
positive pulses are 50 s wide and there is a 50 s gap between them.
What is the DC average of the waveform?
a.
3 V
b.
4 V
c.
6 V
d.
9 V
____18.A square wave voltage varies between 0 V and 12 V. The
positive pulses are 25 s wide and there is a 75 s gap between them.
What is the DC average of the waveform?
a.
3 V
b.
4 V
c.
6 V
d.
9 V
____19.A square wave voltage varies between 0 V and 12 V. The
positive pulses are 75 s wide and there is a 25 s gap between them.
What is the DC average of the waveform?
a.
3 V
b.
4 V
c.
6 V
d.
9 V
Completion
Complete each sentence or statement.
20.A force that tends to cause rotation is known as
_______________.
21.Torque is a force that tends to cause _______________.
Residential & Industrial Electricity
K-W-L WORKSHEET
NAME:
LEVEL: DATE:
ARTICLE TITLE:
TIME START:
TIME FINISH:
K What do I already KNOW
about this topic?
W What do I WANT to know
about this topic?
L What did I LEARN after
reading ABOUT this
topic?
I checked the following before reading:
· Headlines and Subheadings
· Italic, Bold, and Underlined words
· Pictures, Tables, and Graphs
· Questions or other key information
I made predictions AFTER previewing the article.
Comments:
· Instructor Signature:
NAME:
DATE:
DC MOTOR ASSESSMENT PRE TEST
True/False
Indicate whether the sentence or statement is true or false.
____1.Counter-electromotive force (CEMF) and back-EMF are names
for the voltage induced into the armature of a DC motor.
____2.It is all right to operate a series motor with no load
connected.
Multiple Choice
Identify the letter of the choice that best completes the
statement or answers the question.
____3.A device that converts electrical input into mechanical
output is called
a.
alternator
b.
generator
c.
motor
d.
inverter
____4.When the field winding of a DC motor is connected in
parallel with the armature, the motor is called a _____ motor.
a.
series
b.
shunt
____5.The DC motor, known as a “constant speed motor,” is the
_____ motor.
a.
series
b.
shunt
____6.The speed regulation of a DC motor is proportional to
the
a.
applied voltage
b.
load torque
c.
armature resistance
d.
field resistance
____7.A compound motor has _____ field windings.
a.
series
b.
shunt
c.
series and shunt
d.
combined
____8.The cumulative compound motor connection means that the
fields of the series and shunt field windings
a.
add
b.
subtract
c.
multiply
d.
divide
____9.The compound motor connection that is rarely used is
the
a.
cumulative
b.
differential
____10.In a compound DC motor control circuit, the field loss
relay will disconnect the _____ if power is lost to the _____.
a.
rotor, armature
b.
armature, shunt field
c.
shunt field, armature
d.
armature, rotor
____11.The motor that does not contain a wound armature,
commutator, or brushes is the _____ motor.
a.
servo
b.
stepping
c.
selsyn
d.
brushless DC
____12.Permanent magnet motors have _____ efficiency than wound
field motors.
a.
higher
b.
lower
____13.Small permanent magnet motors that have lightweight
rotors are often used as
a.
stepping motors
b.
automobile starters
c.
servomotors
d.
ceiling fans
____14.The servomotor which has a fiberglass and copper rotor is
the _____ motor.
a.
copper
b.
copper-glass
c.
nonferrous
d.
ServoDisc®
____15.Because the rotor of a ServoDisc® motor is made from
copper laminated onto a fiberglass disk, the motor is also called a
_____ motor.
a.
copper
b.
laminated
c.
printed circuit
d.
nonferrous
____16.Some DC servomotors have square wave input voltage. To
vary the motor speed, the on and off time ratio of the square wave
is varied. This is called _____ modulation.
a.
phase
b.
pulse-width
c.
frequency
d.
amplitude
____17.A square wave voltage varies between 0 V and 12 V. The
positive pulses are 50 s wide and there is a 50 s gap between them.
What is the DC average of the waveform?
a.
3 V
b.
4 V
c.
6 V
d.
9 V
____18.A square wave voltage varies between 0 V and 12 V. The
positive pulses are 25 s wide and there is a 75 s gap between them.
What is the DC average of the waveform?
a.
3 V
b.
4 V
c.
6 V
d.
9 V
____19.A square wave voltage varies between 0 V and 12 V. The
positive pulses are 75 s wide and there is a 25 s gap between them.
What is the DC average of the waveform?
a.
3 V
b.
4 V
c.
6 V
d.
9 V
Completion
Complete each sentence or statement.
20.A force that tends to cause rotation is known as
_______________.
21.Torque is a force that tends to cause _______________.
Name:
Date:
Learning Guide Due Date:
Pre Test Due Date:
Post Test Due Date:
RESIDENTIAL & INDUSTRIAL ELECTRICITY
�
Level 3
Task 1800
Schuylkill Technology Center-
South Campus
15 Maple Avenue
Marlin, Pennsylvania 17951
(570) 544-4748
POS # 1800
Total Hours-68
Level(s)-3
*CORE CURRICULUM STANDARDS*
*ACADEMIC STANDARDS*
� INCLUDEPICTURE
"http://upload.wikimedia.org/wikipedia/en/5/56/Tesla3.jpg" \*
MERGEFORMATINET ���
Points
Earned
Points
Available
Correct/Out of 250
Grade Percentage
Check One Percentage Task Grade
Below Basic 0%-69% 0-6
Basic 70%-85% 7
Competent 86%-92% 8-9
Advanced 93%-100% 10
NAME:
DATE:
3
10
110
140
7
THE PARTS OF A DC MOTOR / GENERATOR
164
150
130
120
5
4
6
8
9
2
1