Electric Motors

Electric Motors
APSC 380 : I NTRODUCTION TO M ICROCOMPUTERS
1997/98 W INTER S ESSION T ERM 2
Electric Motors
This lecture describes the principles of operation of various types of solenoids and various electric motors: synchronous, squirrel-cage, universal, shunt DC, series DC, permanent-magnet DC and stepper.
After this lecture you should be able to compute the important specifications and select the appropriate type(s) of
motor for a given application.
Linear Actuators
teraction between the magnetic field set up by field
coil windings and current flowing in an armature coil
winding. Either the field or armature windings can
be replaced by permanent magnets. Electrical connection to the rotor, when necessary, is made using
sliding electrical contacts (slip rings or a commutator).
Among the specifications that need to be considered when selecting a motor are:
For many applications the actuator need only physically move some mechanism into one of two positions. This can be done with a solenoid, which is
simply an electromagnet used to attract a magnetic
or steel core. A typical example of a solenoid is an
electrically-controlled door latch. The solenoid can
be driven from AC, DC or rectified AC.
speed range (rpm), fixed or variable
torque (maximum, starting, pull-out),
torque versus speed characteristics
output power (kW or hp) (P
W)
T , 1hp = 746
type of power supply (AC or DC, voltage, current) (e.g. 12VDC 5A, or 240VAC, 3phase,
25A)
Exercise: What type of core can be used with an AC-powered
solenoid?
Important specifications for a solenoid include
stroke length, force versus position, voltage, pull-in
current and hold current.
Solenoids can also operate valves to control pneumatic or hydraulic power sources (for example, from
a central compressor) which then control a hydraulic/pneumatic actuator or device. For example,
a hydraulic press or a paint sprayer.
Solenoids can be controlled using switching transistors.
efficiency (%)
armature inertia (for control motors)
armature inductance and resistance (for control
motors)
physical characteristics (motor and shaft dimensions, weight, ventilation requirements, etc.)
Electric Motor Selection
Exercise: An electric motor for a garage door opener lifts a
door weighing 50 kg through a distance of 1 m in 10s while turn-
There are hundreds of types of electric motors. The
most common type of motor is a rotating machine
that consists of a rotor (the part that rotates) and a
stator (fixed). The motor turns because of the inlec14.tex
ing at 600 rpm. Ignore all friction loses in the system. What are
the approximate power and torque requirements for this motor? If
the motor is 80% efficient and is supplied by 120VAC, how much
current will it draw?
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AC Motors
ratings from hundreds of kW to a few hundred watts.
AC (alternating current) motors are widely used
since most electric power is supplied in the form of
AC.
Synchronous AC motors run at an integer fraction
of the power line frequency (e.g. for a 60 Hz power
line frequency the possible speeds would be 3600
rpm, 1800 rpm, etc.). Synchronous motors are often used when a constant speed speed is required or
when various motors must operate in synchronism
(e.g. electric clocks, different parts of an assembly
line). The field windings (on the rotor) are normally
supplied with DC and the armature windings are supplied with AC.
In applications where it is desired to use an induction motor and still be able to vary its speed, a
variable-frequency AC power supply can be used.
This power supply converts the AC power to DC and
then back to AC at the desired frequency. This conversion can be done quite efficiently.
The direction of rotation is set when the motor
starts up, either by using additional start-up circuitry
or by having some asymmetry in the construction of
the motor. AC generators (typically constructed as
synchronous motors and sometimes called alternators) work in the same way as AC motors but a power
source is used to drive the rotor with the opposite
torque than when the device operates as a motor.
Some AC motors (e.g. the small “universal” motors used in many household appliances) use a commutator and operate on a principle similar to series
DC motors described below. The speed of these motors can be controlled over a wide range. The speed
control is achieved by varying the motor current either by using an external variable resistor or by reducing the supply voltage with one of the SCR or
triac circuits described earlier.
Induction motors run at a few percent less than
some integer fraction of the power line frequency
(e.g. 1800 - 5% rpm) and this fraction increases
with the load. The stator windings are supplied
with AC. The current in the rotor windings is produced (induced) by the difference in frequency between the AC frequency and the motor speed. This
means an electrical connection to the rotor (and the
resulting brushes) are not required. Induction motors are therefore simple, inexpensive, reliable and
quite widely used. Typical applications are those
where precise speed control is not essential such as
refrigerator compressors, furnace blowers, washing
machines, lawn mowers, etc. They are available in
DC Motors
The main advantage of DC motors is that their speed
and torque can be easily varied over a wide range.
For DC machines the field winding is on the stator
and the armature winding on the rotor.
A commutator is a device that is used to reverse
the direction of rotor current flow (an thus the direction of the magnetic field) as the rotor turns. The
commutator consists of two or more fixed contacts
(“brushes”) that slide over various rotating contacts
on the rotor. These contacts are connected to the ar2
mature windings.
Some DC motors use sensors, logic circuits and
transistors to switch the current flow in the stator
windings. These electronics replace the commutator and eliminate the need for sliding contacts and
brushes.
Most control applications involving DC motors
require some type of additional sensor to provide
torque, position, or speed information (“feedback”)
to the controller. These sensors can be in the form
of tachometers, pulse counters, rotary shaft encoders
or various other devices. These types of motors are
called servo motors.
Current must be supplied to both the field windings and the armature (through the commutator) for
the motor to operate. Usually the field and armature windings are connected together and connected
to the same power supply. A DC motor can be designed for the two windings to be connected in series
or parallel (shunt) or a combination.
When the two windings are in parallel the field
current is independent of the armature current. The
series motor thus has limited starting torque. Typical
applications therefore include blower motors, lathes,
etc.
When the two windings are in series the field current is the same as the armature current. This current
will be higher at low speeds (high torque) and the
motor will have a high starting torque. Series motors are therefore used for applications such as trains,
cars, hoists, etc.
The DC motor speed can be reduced by reducing
the armature current or increasing the field current.
For small DC motors the armature current can be
varied by using switching transistors to convert a DC
supply into a pulse-width-modulated (PWM) signal.
Large DC motors use AC power that is rectified to a
varying DC voltage and whose average value is controlled using SCRs as described earlier.
The direction of rotation of a DC motor can be
easily reversed by reversing the direction of the current in either the armature or field windings (usually
in the field since it has lower current).
Permanent magnets (PM) are often used in small
motors to replace either the field or armature windings. This improves the efficiency of the motor since
no field current is required. The speed-torque characteristics of a PM motor are the same as for a DC
motor using a parallel field winding.
Exercise: What are some motor applications where position
and/or speed need to be closely controller? Which of these would
use servo motors?
In some control applications the DC motor must
respond quickly to changes in the control voltage. In
these cases the moment of inertia of the rotor and the
inductance and resistance of the windings become
important specifications because they can limit the
speed with which the motor current can be changed
and the rate at which the rotor can be accelerated.
Exercise:
Identify some DC motors found in a car. What
types of motors are they? What is the smallest DC motor you
can think of? The largest?
Stepper Motors
A type of motor that is widely used with electronic
controls is the stepper motor. This type of DC motor, as its name indicates, rotates in discrete “steps.”
Stepper motors can be designed for different step
sizes. Typical steps per revolution vary from 200 (1.8
degrees) to 12 (15 degrees).
The main advantage of stepper motors is that they
allow precise position control without requiring a
feedback sensor and using only on/off outputs.
The most common type of stepper motor consists
of a permanent magnet rotor with a number of poles
and a number of stator windings. The stator windings are arranged so that they can pull the rotor into
number of stable orientations that differ by the step3
ping angle.
By varying the order in which the stator windings
are turned on and off the rotor can be rotated in either direction. By controlling the number of steps the
shaft can be moved to a known angle.
A microprocessor or a special-purpose peripheral
IC generates the logic control signals that turn each
of the windings on and off in the correct sequence to
move the rotor the desired number of steps. Switching transistors are used to switch the current to each
of the windings.
The control program or the stepper-motor control
IC must move the rotor sufficiently slowly that the
“pull-out” torque is not exceeded. If this happens the
stepper rotor will miss a pulse and get out of sync
with the controller.
The applied torque depends not only on the load
but also on the acceleration of the motor. To achieve
the best performance it is thus necessary to vary the
stepping speed to maintain the acceleration within
specified tolerances.
Exercise: Identify a motor application that can be done most
easily by a stepper motor.
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