Chapter 11 DC Motors © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Explain the operating principles.

Chapter 11

DC Motors

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Objectives

• Explain the operating principles of dc motors.

• Explain counterelectromotive force.• Identify various dc motors.• Discuss the purpose for, and operation of,

motor starting circuits.• Identify and explain the operation of various

dc motors.


Motor Operation Principles

• Electric motors are used for hundreds of useful machines, including refrigerators and vacuum cleaners

• Like poles repel each other• Unlike poles attract each other


Motor Operation Principles (Cont.)

• Practical motors

• Counterelectromotive force (counter emf, or cemf)

• Overload protection

• Commutation and interpoles

• Speed regulation


Motor Operation Principles (Cont.)

• The dc generator and motor are often interchangeable in use– dc machines

• Field magnets can be replaced by field windings


Practical Motors

• Industry– Rotational force from different source– Interaction between two fields– Armature coils connected to commutator

sections– Torque of motor increased


Counter emf

• Counter emf = Speed × Field strength × K

• K equals a constant, which varies in different motors


Counter emf (Cont.)

• Actual effective voltage when applied to the windings in the armature

• Esource – Ecounter = Earmature


Counter emf (Cont.)

• Current flowing in the armature windings at any given instant, when the resistance of the windings is known

armature

armaturearmature R

EI


Counter emf (Cont.)

• As rotation of the motor armature slows down, less counter emf is generated

• There will be an increase in current through the armature

• Current increases until the motor stops rotating


Overload Protection

• Fuses and circuit breakers– No more than 150% of the full-load current– Protect against short circuits or locked rotors

• Thermo-overload devices– Actual overload protection– 115%–125% of full-load current


Overload Protection (Cont.)

• Bimetallic overload devices– Contacts separate

when current reaches a set level


Commutation and Interpoles

• Reducing sparks at commutator brushes– Interpole– Commutating poles


Speed Regulation

• Percent of speed regulation

%100 load-full Speed

load-full Speed– load-no Speed regulation speed ofPercent


Review

What changes electrical power into mechanical power?

Electric motor


Review

What can replace permanent field magnets?

Field windings


Review

What is torque?

Turning power of the motor


Review

What is the formula for solving for counter emf?

Counter emf = Speed × Field strength × K


Review

When rotation of the motor armature slows down, is there more or less counter emf generated?

Less


Review

What percentage of full-load current should a fuse or breaker use?

No more than 150%


Review

What is a thermo-overload device?

A simple ratch wheel device that causes the circuit to open


Review

What is an interpole?

A smaller field pole placed midway between main field poles


DC Motors

• Types– Shunt– Series– Compound

• Motor starting circuits• Thyristor motor

controls

• Universal motors• Permanent magnet

motors• Servo motors


Shunt DC Motors

• Constant speed motors• Input power

– Power = Applied voltage × Total current


Series DC Motors

• Develop high torque under load• Armature speed low• Low cemf• High armature current• Increased torque

K strength Field

emfCounter Speed


Compound DC Motors

• Series winding and shunt field winding

• Cumulative compound motors– Develop high starting

torque


Compound DC Motors (Cont.)

• Differential compound motors– Starting torque is low– Used where heavy loads are applied with

varying speeds– Good speed regulation if loads do not vary

greatly


Motor Starting Circuits

• Variable starting resistance

• Maximum resistance at start, then decreased

• Manual starters• Automatic starters


Manual Starters

• Starting resistance adjusted by hand

• Resistance subject to human error


Automatic Starters

• Switches• Armature contactors• Push-button starters

– The start button is pushed closed to start

– When the stop button is pushed, the circuit is opened


Thyristor Motor Controls

• Semiconductors that control motor speed– Silicon controlled

rectifier (SCR)• Controls half-wave

– Triac• Controls full ac wave


Universal Motors

• Series motor should be used

• AC used to run a dc motor

• Not used for heavy-duty purposes


Permanent Magnet Motors

• 1/50th–5 horsepower• Magnets used instead of electrical coils

(Baldor Electric Co.)


DC Servo Motors

• Feedback determines position of mechanism the motor is driving

• Digital encoder as another feedback system


Submarines

• DC motors are propulsion units• Battery bank used to power ships when

primary source is not available• Conventional diesel engine submarines

and surface ships use electric motors for ship’s propellers

• High rpm from dc electric motor cannot be matched by addition of gear box


Review

What are the three types of dc motors?

Shunt, series, and compound


Review

What is another name for a shunt motor?

Constant speed motor


Review

What type of compound dc motor develops a high starting torque?

Cumulative compound motor


Review

How is a differential compound motor different from a cumulative compound motor?

It has two windings connected to oppose each other magnetically


Review

What are the two types of thyristor motor controls?

Silicon controlled rectifier (SCR) and triac


Review

What motors operate on either ac or dc power?

Universal motors


Review

What is a motor that is modified to give feedback information concerning the motor’s speed, direction of rotation, and number of revolutions?

DC servo motor


For Discussion

• Discuss the reasons a motor gets hot when overloaded.

• Why are starters necessary on heavy-duty motors?

• Why does a dc motor increase in speed when its field strength is decreased?

Chapter 11 DC Motors © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Explain the operating principles.

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