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Three-phase motors will run on single phase as readily as single
phase motors. The only problem for either motor is starting. Sometimes
3-phase motors are purchased for use on single phase if three-phase
power is anticipated. The power rating needs to be 50% larger than for a
comparable single phase motor to make up for one unused winding. Single
phase is applied to a pair of windings simultaneous with a start
capacitor in series with the third winding. The start switch is opened
in Figure below upon motor start. Sometimes a smaller capacitor than the
start capacitor is retained while running.
Starting a three-phase motor on single phase.
The circuit for running a three-phase motor on single phase is known
as “add a phase” or various other brand names. “Add a phase” supplies a
phase approximately midway 90o between the 180o
single phase power source terminals.
Induction motors may contain multiple field windings, for example a
4-pole and an 8-pole winding corresponding to 1800 and 900 rpm
synchronous speeds. Energizing one field or the other is less complex
than rewiring the stator coils in Figure below.
Multiple fields allow speed change.
If the field is segmented with leads brought out, it may be rewired
(or switched) from 4-pole to 2-pole as shown above for a 2-phase motor.
The 22.5o segments are switchable to 45o segments.
Only the wiring for one phase is shown above for clarity. Thus, our
induction motor may run at multiple speeds. When switching the above 60
Hz motor from 4 poles to 2 poles the synchronous speed increases from
1800 rpm to 3600 rpm. If the motor is driven by 50 Hz, what would be the
corresponding 4-pole and 2-pole synchronous speeds?
Ns = 120f/P = 120*50/4 = 1500 rpm (4-pole)
Ns = 3000 rpm (2-pole)
The speed of small squirrel cage induction motors for applications
such as driving fans, may be changed by reducing the line voltage. This
reduces the torque available to the load which reduces the speed.
(Figure below)
Variable voltage controls induction motor speed.
Modern solid state electronics increase the options for speed
control. By changing the 50 or 60 Hz line frequency to higher or lower
values, the synchronous speed of the motor may be changed. However,
decreasing the frequency of the current fed to the motor also decreases
reactance XL which increases the stator current. This may
cause the stator magnetic circuit to saturate with disastrous results.
In practice, the voltage to the motor needs to be decreased when
frequency is decreased.
Electronic variable speed drive.
Conversely, the drive frequency may be increased to increase the
synchronous speed of the motor. However, the voltage needs to be
increased to overcome increasing reactance to keep current up to a
normal value and maintain torque. The inverter (Figure ) approximates
sinewaves to the motor with pulse width modulation outputs. This is a
chopped waveform which is either on or off, high or low, the percentage
of “on” time corresponds to the instantaneous sine wave voltage.
Once electronics is applied to induction motor control, many control
methods are available, varying from the simple to complex:
Summary: Speed control
- Scalar Control Low cost method described above to control
only voltage and frequency, without feedback.
- Vector Control Also known as vector phase control. The flux
and torque producing components of stator current are measured or
estimated on a real-time basis to enhance the motor torque-speed
curve. This is computation intensive.
- Direct Torque Control An elaborate adaptive motor model
allows more direct control of flux and torque without feedback. This
method quickly responds to load changes.
Summary: Tesla polyphase induction motors
- A polyphase induction motor consists of a polyphase winding
embedded in a laminated stator and a conductive squirrel cage embedded
in a laminated rotor.
- Three phase currents flowing within the stator create a rotating
magnetic field which induces a current, and consequent magnetic field
in the rotor. Rotor torque is developed as the rotor slips a little
behind the rotating stator field.
- Unlike single phase motors, polyphase induction motors are
self-starting.
- Motor starters minimize loading of the power line while
providing a larger starting torque than required during running.
Starters are only required for large motors.
- Multiple field windings can be rewired for multiple
discrete motor speeds by changing the number of poles.
The wound stator and the squirrel cage rotor of an induction motor
may be cut at the circumference and unrolled into a linear induction
motor. The direction of linear travel is controlled by the sequence of
the drive to the stator phases.
The linear induction motor has been proposed as a drive for high
speed passenger trains. Up to this time, the linear induction motor with
the accompanying magnetic repulsion levitation system required for a
smooth ride has been too costly for all but experimental installations.
However, the linear induction motor is scheduled to replace steam driven
catapult aircraft launch systems on the next generation of naval
aircraft carrier, CVNX-1, in 2013. This will increase efficiency and
reduce maintenance.
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