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AC Motors
AC motors are used worldwide in many applications to
transform electrical energy into mechanical energy. There are manytypes of AC motors, but this course focuses on threephase AC
induction motors, the most common type of motor used in
industrial applications.
An AC motor of this type may be part of a pump or fan or
connected to some other form of mechanical equipment such as a
winder, conveyor, or mixer.
Siemens manufactures a wide variety of AC motors. In addition to
providing basic information about AC motors in general, this course
also includes an overview of Siemens AC motors.
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AC Motor Construction
Three-phase AC induction motors are commonly used in
industrial applications. This type of motor has three main parts,rotor, stator, and enclosure. The stator and rotor do the work,
and the enclosure protects the stator and rotor.
Stator Core
The stator is the stationary part of the
motors electromagnetic circuit. The
stator core is made up of many thin
metal sheets,called laminations.Laminations are used to reduce energy
loses that would result if a solid core
were used.
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Stator Windings
Stator laminations are stacked together forming a hollow
cylinder. Coils of insulated wire are inserted into slots of thestator core.
When the assembled motor is in operation, the stator windings
are connected directly to the power source. Each grouping of
coils, together with the steel core it surrounds, becomes an
electromagnet when current is applied. Electromagnetism is
the basic principle behind motor operation.
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Rotor Construction
The rotor is the rotating part of the motors electromagnetic
circuit. The most common type of rotor used in a three-phaseinduction motor is a squirrel cage rotor. Other types of rotor
construction is discussed later in the course. The squirrel cage
rotor is so called because its construction is reminiscent of the
rotating exercise wheels found in some pet cages.
A squirrel cage rotor core is made by stacking thin steel
laminations to form a cylinder.
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Rather than using coils of wire as conductors, conductor bars
are die cast into the slots evenly spaced around the cylinder.
Most squirrel cage rotors are made by die casting aluminum toform the conductor bars. Siemens also makes motors with die
cast copper rotor conductors.
Advantages of AC Motor
Low cost
Speed variation
High power factor
Reliable operation
Disadvantages of AC Motor
Inability to operate at low speeds
Poor positioning control
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Servo Motors
What is a servo motor?
Servo motors (or servos) are self-contained electric devices that rotate
or push parts of a machine with great precision. Servos are found in
many places: from toys to home electronics to cars and airplanes. If
you have a radio-controlled model car, airplane, or helicopter, you are
using at least a few servos. In a model car or aircraft, servos move
levers back and forth to control steering or adjust wing surfaces. By
rotating a shaft connected to the engine throttle, a servo regulates the
speed of a fuel-powered car or aircraft. Servos also appear behind the
scenes in devices we use every day. Electronic devices such as DVD
and Blu-ray DiscTM players use servos to extend or retract the disc
trays. In 21st-century automobiles, servos manage the car's speed:
The gas pedal, similar to the volume control on a radio, sends an
electrical signal that tells the car's computer how far down it is
pressed. The car's computer calculates that information and other data
from other sensors and sends a signal to the servo attached to the
throttle to adjust the engine speed. Commercial aircraft use servos anda related hydraulic technology to push and pull just about everything
in the plane.
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Design of Motor
The magnetic force is be made by permanent magnet and the current
is used to generate torque. Then high torqueand efficiency are available at low current and small size.
It has no brush so there is little noise/vibration and no dirt. And high
precision control is available with high resolution encoder.
How does a servo motor work?
The simplicity of a servo is among the features that make them so
reliable. The heart of a servo is a small direct current (DC) motor,
similar to what you might find in an inexpensive toy. These motors
run on electricity from a battery and spin at high RPM (rotations per
minute) but put out very low torque (a twisting force used to do
workyou apply torque when you open a jar). An arrangement of
gears takes the high speed of the motor and slows it down while at the
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same time increasing the torque. (Basic law of physics: work = force
x distance.) A tiny electric motor does not have much torque, but it
can spin really fast (small force, big distance). The gear design inside
the servo case converts the output to a much slower rotation speed but
with more torque (big force, little distance). The amount of actual
work is the same, just more useful. Gears in an inexpensive servo
motor are generally made of plastic to keep it lighter and less costly
.On a servo designed to provide more torque for heavier work, the
gears are made of metal and are harder to damage.
Advantages
high performance, higher speeds, accurate positioning (becauseof feedback)
Disadvantages
higher cost (less of an issue today), requires setup/tuning
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Stepper motor
A stepper motor is an electromechanical device which converts
electrical pulses into discrete mechanical movements.
The shaft or spindle of a stepper motor rotates in discrete step
increments when electrical command pulses are applied to it in the
proper sequence. The motors rotation has several direct relationships
to these applied input pulses. The sequence of the applied pulses is
directly related to the direction of motor shafts rotation.
The speed of the motor shafts rotation is directly related to the
frequency of the input pulses and the length of rotation is directly
related to the number of input pulses applied.
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How Stepper Motors Work
Stepper motors consist of a permanent magnetic rotating shaft, called
the rotor, and electromagnets on the stationary portion that surroundsthe motor, called the stator.Figure 1 illustrates one complete rotation
of a stepper motor. At position 1, we can see that the rotor is
beginning at the upper electromagnet, which is currently active (has
voltage applied to it). To move the rotor clockwise (CW), the upper
electromagnet is deactivated and the right electromagnet is activated,
causing the rotor to move 90 degrees CW, aligning itself with the
active magnet. This process is repeated in the same manner at the
south and west electromagnets until we once again reach the starting
position.
Types of Stepper Motor
By construction the step motors come into three broad classes:
1. Permanent Magnet Stepper
2. Variable Reluctance Stepper
3. Hybrid Step Motor
These three types have been explained in detail in the following
sections.
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