How Does an Induction Motor Work?
Some of you have seen the Streamliner article in the April issue of Popular Science. The article editor asked me to write a brief description of how the motor in the car works. Here is my response, a short description without figures or equations that may help someone who is curious about AC motors.
The electric motor in the BYU streamliner is an induction
motor. Induction motors operate on
alternating current (AC). Alternating
current does just as the name implies, it alternates between a peak positive
polarity and a peak negative polarity within each cycle. A single phase
of AC, typical of residential power supplies, flows back and forth in a single
loop of wire. When AC power is used to
drive electrical machinery, such as a motor, the most efficient number of
phases to use is three. Using more than three
phases will not improve efficiency much but will cost a lot to produce.
Three-phase AC power requires three circuits or
wires. The three peak currents occur at three
different, but equally-spaced moments, separated so that all three peaks occur on every
cycle, giving a smoother average current than is possible with only one
phase, or one peak per cycle. The three peaks are said to be 120
electrical degrees apart, one cycle being 360 electrical degrees.
The wound stator
In an AC induction motor the electricity is applied to
the stator, or stationary part of the motor which surrounds the rotating part,
or rotor. The stator consists of
windings of wire arranged according to the number of poles in the motor. Poles come in pairs, so typical AC motors will
have 2, 4, or 6 poles. Each pair of
poles are set opposite each other in the stator and for a three-phase motor
each pair of poles requires three windings arranged symmetrically about the
poles. So a two-pole, three-phase motor (one
pair of poles) has three windings arranged so the centers of their magnetic
fields are 120 degrees apart around the circumference of the stator. A four-pole motor (two pairs) has six
windings arranged 60 degrees apart.
Here is the key to how the induction motor works. When the AC current is applied to the stator
windings each winding creates a magnetic field that alternates in strength and
polarity with the alternating current.
Because the three phases of AC power peak in 120 electrical-degree
intervals the peak magnetic field rotates from one winding to the next around
the circumference of the stator. The
speed of rotation of the magnetic field depends on the frequency of the applied
AC power and the number of poles in the winding. With only one pair of poles the magnetic
field makes one rotation around the stator with each cycle of AC. Hence an AC frequency of 60 cycles per second
(Hz) would cause the magnetic field to rotate at 60 rotations per second or
3600 revolutions per minute.
The squirrel cage rotor
Now the rotor, which rotates within this revolving
magnetic field, consists of conductive bars running axially very close to the
inner surface of the stator windings.
The appearance of these bars gives the rotor the name of “Squirrel
Cage”. Electromagnetic induction now
does the work. When the magnetic field
begins to rotate in the stator its magnetic flux cuts across the stationary
bars of the rotor inducing an electric current in the bars. This induced electric current in turn creates
a magnetic field around the conductive bars.
This induced magnetic field opposes the magnetic field in the stator
with the result that the rotating stator field pushes the induced field in the
bars causing the rotor to turn.
This explains the interesting behavior of the AC induction
motor. The torque developed by the rotor
is proportional to the strength of the induced magnetic field in the bars. This field strength is in turn proportional
to the speed with which the stator magnetic flux cuts through the rotor
bars. Thus the peak torque is developed
when the rotor is locked and the difference between the stator and rotor speeds
is a maximum. And when the rotor is
turning at the same speed as the stator magnetic field the torque is zero because
there is no electrical current or magnetic field being induced in the rotor
bars. Hence the AC induction motor
cannot run at the synchronous speed of the applied AC frequency but must always
be a little slower. The difference
between the rotor speed and the stator magnetic field speed is called “slip”. AC induction motors must have some slip in
order to run.
Speed control
Most AC induction motors run at close to the synchronous
speed of the applied AC power. For
example a two-pole motor operating on 60 Hz AC will run at about 3550 RPM, just
below its synchronous speed of 3600 RPM.
Likewise a four-pole motor on 60 Hz power will operate at about 1750
RPM, close to its stator speed of 1800 RPM.
When used as a traction motor in an automobile, where
variable speed is required, it is necessary to vary the frequency of AC power
applied to the motor. This is
accomplished in the BYU Streamliner by a variable-frequency inverter
controller. This controller uses
solid-state electronic devices called insulated gate bipolar transistors
(IGBTs) to convert direct-current battery power to three-phase AC power where
the AC frequency is varied through a simple potentiometer connected to the
throttle pedal.
Cooling
There is some heat generated by the IGBTs so the
controller is water cooled using a small, wedge-shaped reservoir located above
the battery pack and a small, 12-volt pump to circulate the water through the
controller. No radiator is needed
because the run time is less than two minutes and the interval between runs is
always at least an hour, usually more like four hours, during which time the
heated water will cool back to a safe temperature.
