polyphase induction motor
DONE BY MY COLLEAGUES IN COLLEGE BSEE-5
Published on: Mar 4, 2016
Transcripts - polyphase induction motor
Jann Michael Nuyles
• Nikola Tesla conceived the basic principals of the polyphase
induction motor in 1883, and had a half horsepower (400
watt) model by 1888. Tesla sold the manufacturing rights to
George Westinghouse for $65,000
• Most large ( > 1 hp or 1 kW) industrial motors are poly-
phase induction motors. By poly-phase, we mean that the
stator contains multiple distinct windings per motor pole,
driven by corresponding time shifted sine waves. In practice,
this is two or three phases. Large industrial motors are 3-
phase. By induction motor, we mean that the stator
windings induce a current flow in the rotor conductors, like a
transformer, unlike a brushed DC commutator motor.
• An induction motor is composed of a rotor, known as an
armature, and a stator containing windings connected to a poly-
phase energy source . The simple 2-phase induction motor
below is similar to the 1/2 horsepower motor which Nikola
Tesla introduced in 1888.
• The stator is wound with pairs of coils corresponding to the
phases of electrical energy available. The 2-phase induction
motor stator has 2-pairs of coils, one pair for each of the two
phases of AC. The individual coils of a pair are connected in
series and correspond to the opposite poles of an
electromagnet. That is, one coil corresponds to a N-pole, the
other to a S-pole until the phase of AC changes polarity. The
other pair of coils is oriented 90o in space to the first pair. This
pair of coils is connected to AC shifted in time by 90o in the case
of a 2-phase motor. In Tesla's time, the source of the two
phases of AC was a 2-phase alternator.
• The stator has salient, obvious protruding poles, as used on
Tesla's early induction motor. This design is used to this day for
sub-fractional horsepower motors (<50 watts).
• For larger motors less torque pulsation and higher efficiency results if
the coils are embedded into slots cut into the stator laminations
• Stator frame showing slots for windings.
• The stator laminations are thin insulated rings with slots
punched from sheets of electrical grade steel. A stack of
these is secured by end screws, which may also hold the
Stator with (a) 2-φ and (b) 3-φ windings.
• the windings for both a two-phase motor and a three-
phase motor have been installed in the stator slots. The
coils are wound on an external fixture, then worked into
the slots. Insulation wedged between the coil periphery
and the slot protects against abrasion.
• Actual stator windings are more complex than the
single windings per pole in Figure above.
Comparing the 2-φ motor to Tesla's 2-φ motor with
salient poles, the number of coils is the same.
• In actual large motors, a pole winding, is divided into identical
coils inserted into many smaller slots than above. This group is
called a phase belt. . The distributed coils of the phase belt
cancel some of the odd harmonics, producing a more sinusoidal
magnetic field distribution across the pole. This is shown in the
synchronous motor section. The slots at the edge of the pole
may have fewer turns than the other slots. Edge slots may
contain windings from two phases. That is, the phase belts
• The key to the popularity of the AC induction motor is simplicity as
evidenced by the simple rotor. The rotor consists of a shaft, a steel
laminated rotor, and an embedded copper or aluminum squirrel cage,
shown at (b) removed from the rotor. As compared to a DC motor
armature, there is no commutator. This eliminates the brushes, arcing,
sparking, graphite dust, brush adjustment and replacement, and re-
machining of the commutator
• Laminated rotor with (a) embedded squirrel cage, (b)
conductive cage removed from rotor
Theory of operation
• A short explanation of operation is that the stator creates a
rotating magnetic field which drags the rotor around.
• The theory of operation of induction motors is based on a
rotating magnetic field. One means of creating a rotating
magnetic field is to rotate a permanent magnet as. If the moving
magnetic lines of flux cut a conductive disk, it will follow the
motion of the magnet. The lines of flux cutting the conductor will
induce a voltage, and consequent current flow, in the conductive
disk. This current flow creates an electromagnet whose polarity
opposes the motion of the permanent magnet– Lenz's Law. The
polarity of the electromagnet is such that it pulls against the
permanent magnet. The disk follows with a little less speed than
the permanent magnet.
Rotating magnetic field produces torque in conductive disk.
The torque developed by the disk is proportional to the number of flux
lines cutting the disk and the rate at which it cuts the disk. If the disk
were to spin at the same rate as the permanent magnet, there would
be no flux cutting the disk, no induced current flow, no electromagnet
field, no torque. Thus, the disk speed will always fall behind that of
the rotating permanent magnet, so that lines of flux cut the disk
induce a current, create an electromagnetic field in the disk, which
follows the permanent magnet. If a load is applied to the disk, slowing
it, more torque will be developed as more lines of flux cut the disk.
Torque is proportional to slip, the degree to which the disk falls behind
the rotating magnet. More slip corresponds to more flux cutting the
conductive disk, developing more torque.
• An analog automotive eddy current speedometer is based on the
principle illustrated above. With the disk restrained by a spring,
disk and needle deflection is proportional to magnet rotation rate.
• A rotating magnetic field is created by two coils placed at right
angles to each other, driven by currents which are 90o out of
phase. This should not be surprising if you are familiar with
oscilloscope Lissajous patterns.
• (Out of phase (90o) sine waves produce circular Lissajous
(X-axis sine and Y-axis cosine trace circle.)
the two 90o phase shifted sine waves applied to oscilloscope
deflection plates which are at right angles in space. If this were not
the case, a one dimensional line would display. The combination of
90o phased sine waves and right angle deflection, results in a two
dimensional pattern– a circle. This circle is traced out by a
counterclockwise rotating electron beam.
No circular motion from in-phase waveforms.
why in-phase sine waves will not produce a circular
pattern. Equal “X” and “Y” deflection moves the
illuminated spot from the origin at (a) up to right (1,1)
at (b), back down left to origin at (c),down left to (-1.-
1) at (d), and back up right to origin. The line is
produced by equal deflections along both axes; y=x
is a straight line.
Rotating magnetic field from 90o phased sinewaves.
If a pair of 90o out of phase sine waves produces a circular
Lissajous, a similar pair of currents should be able to produce
a circular rotating magnetic field. Such is the case for a 2-
phase motor. By analogy three windings placed 120o apart in
space, and fed with corresponding 120o phased currents will
also produce a rotating magnetic field.
• As the 90o phased sinewaves, progress from points (a)
through (d), the magnetic field rotates counterclockwise
(figures a-d) as follows:
• (a) φ-1 maximum, φ-2 zero
• (a') φ-1 70%, φ-2 70%
• (b) φ-1 zero, φ-2 maximum
• (c) φ-1 maximum negative, φ-2 zero
• (d) φ-1 zero, φ-2 maximum negative
The rotation rate of a stator rotating magnetic field is related to the
number of pole pairs per stator phase. The “full speed” has a total of
six poles or three pole-pairs and three phases. However,there is but
one pole pair per phase– the number we need. The magnetic field
will rotate once per sine wave cycle. In the case of 60 Hz power, the
field rotates at 60 times per second or 3600 revolutions per minute
(rpm). For 50 Hz power, it rotates at 50 rotations per second, or
3000 rpm. The 3600 and 3000 rpm, are the synchronous speed of
the motor. Though the rotor of an induction motor never achieves
this speed, it certainly is an upper limit. If we double the number of
motor poles, the synchronous speed is cut in half because the
magnetic field rotates 180o in space for 360o of electrical sine wave.
Doubling the stator poles halves the synchronous speed.
The synchronous speed is given by:
Ns = 120·f/P
Ns = synchronous speed in rpm
f = frequency of applied power, Hz
P = total number of poles per phase, a multiple of 2
The short explanation of the induction motor is that the rotating magnetic
field produced by the stator drags the rotor around with it.
The longer more correct explanation is that the stator's magnetic
field induces an alternating current into the rotor squirrel cage
conductors which constitutes a transformer secondary. This
induced rotor current in turn creates a magnetic field. The rotating
stator magnetic field interacts with this rotor field. The rotor field
attempts to align with the rotating stator field. The result is rotation
of the squirrel cage rotor. If there were no mechanical motor
torque load, no bearing, windage, or other losses, the rotor would
rotate at the synchronous speed. However, the slip between the
rotor and the synchronous speed stator field develops torque. It is
the magnetic flux cutting the rotor conductors as it slips which
develops torque. Thus, a loaded motor will slip in proportion to the
mechanical load. If the rotor were to run at synchronous speed,
there would be no stator flux cutting the rotor, no current induced
in the rotor, no torque.
• When power is first applied to the motor, the rotor is at rest, while
the stator magnetic field rotates at the synchronous speed Ns. The
stator field is cutting the rotor at the synchronous speed Ns. The
current induced in the rotor shorted turns is maximum, as is the
frequency of the current, the line frequency. As the rotor speeds
up, the rate at which stator flux cuts the rotor is the difference
between synchronous speed Ns and actual rotor speed N, or (Ns -
N). The ratio of actual flux cutting the rotor to synchronous speed is
defined as slip:
• s = (Ns - N)/Ns
• where: Ns = synchronous speed, N = rotor speed
• The frequency of the current induced into the rotor conductors is
only as high as the line frequency at motor start, decreasing as the
rotor approaches synchronous speed. Rotor frequency is given by:
• fr = s·f
• where: s = slip, f = stator power line frequency
Torque and speed vs %Slip. %Ns=%Synchronous Speed.
graph shows that starting torque known as locked rotor torque (LRT) is
higher than 100% of the full load torque (FLT), the safe continuous
torque rating. The locked rotor torque is about 175% of FLT for the
example motor graphed above. Starting current known as locked rotor
current (LRC) is 500% of full load current (FLC), the safe running
current. The current is high because this is analogous to a shorted
secondary on a transformer. As the rotor starts to rotate the torque may
decrease a bit for certain classes of motors to a value known as the pull
• This is the lowest value of torque ever encountered by the starting
motor. As the rotor gains 80% of synchronous speed, torque
increases from 175% up to 300% of the full load torque. This
breakdown torque is due to the larger than normal 20% slip. The
current has decreased only slightly at this point, but will decrease
rapidly beyond this point. As the rotor accelerates to within a few
percent of synchronous speed, both torque and current will
decrease substantially. Slip will be only a few percent during
• For a running motor, any portion of the torque curve below 100%
rated torque is normal. The motor load determines the operating
point on the torque curve. While the motor torque and current may
exceed 100% for a few seconds during starting, continuous
operation above 100% can damage the motor. Any motor torque
load above the breakdown torque will stall the motor. The torque,
slip, and current will approach zero for a “no mechanical torque”
load condition. This condition is analogous to an open secondary
Induction motor power factor and efficiency.
• Induction motors present a lagging (inductive) power factor to the
power line.The power factor in large fully loaded high speed motors
can be as favorable as 90% for large high speed motors. At 3/4 full
load the largest high speed motor power factor can be 92%. The
power factor for small low speed motors can be as low as 50%. At
starting, the power factor can be in the range of 10% to 25%, rising
as the rotor achieves speed.
• Power factor (PF) varies considerably with the motor mechanical
load. An unloaded motor is analogous to a transformer with no
resistive load on the secondary. Little resistance is reflected from
the secondary (rotor) to the primary (stator). Thus the power line
sees a reactive load, as low as 10% PF. As the rotor is loaded an
increasing resistive component is reflected from rotor to stator,
increasing the power factor.
• Large three phase motors are more efficient than smaller 3-phase
motors, and most all single phase motors. Large induction motor
efficiency can be as high as 95% at full load, though 90% is more
• Efficiency for a lightly load or no-loaded induction motor is poor
because most of the current is involved with maintaining
magnetizing flux. As the torque load is increased, more current is
consumed in generating torque, while current associated with
magnetizing remains fixed.
• Efficiency at 75% FLT can be slightly higher than that at 100% FLT.
Efficiency is decreased a few percent at 50% FLT, and decreased a
few more percent at 25% FLT. Efficiency only becomes poor below
25% FLT. The variation of efficiency with loading
• Induction motors are typically oversized to guarantee that their
mechanical load can be started and driven under all operating
conditions. If a polyphase motor is loaded at less than 75% of rated
torque where efficiency peaks, efficiency suffers only slightly down
to 25% FLT.
Induction motor alternator
Negative torque makes induction motor into generator.
• An induction motor may function as an alternator if it is driven by a torque
at greater than 100% of the synchronous speed. This corresponds to a
few % of “negative” slip, say -1% slip. This means that as we are rotating
the motor faster than the synchronous speed, the rotor is advancing 1%
faster than the stator rotating magnetic field. It normally lags by 1% in a
motor. Since the rotor is cutting the stator magnetic field in the opposite
direction (leading), the rotor induces a voltage into the stator feeding
electrical energy back into the power line.
• Such an induction generator must be excited by a “live” source of 50 or 60
Hz power. No power can be generated in the event of a power company
power failure. This type of alternator appears to be unsuited as a standby
power source. As an auxiliary power wind turbine generator, it has the
advantage of not requiring an automatic power failure disconnect switch to
protect repair crews. It is fail-safe.
• Small remote (from the power grid) installations may be make self-exciting
by placing capacitors in parallel with the stator phases. If the load is
removed residual magnetism may generate a small amount of current
flow. This current is allowed to flow by the capacitors without dissipating
power. As the generator is brought up to full speed, the current flow
increases to supply a magnetizing current to the stator. The load may be
applied at this point. Voltage regulation is poor. An induction motor may be
converted to a self-excited generator by the addition of capacitors.
Motor starting and speed control
• Some induction motors can draw over 1000% of full load current during
starting; though, a few hundred percent is more common. Small motors of
a few kilowatts or smaller can be started by direct connection to the
power line. Starting larger motors can cause line voltage sag, affecting
other loads. Motor-start rated circuit breakers (analogous to slow blow
fuses) should replace standard circuit breakers for starting motors of a
few kilowatts. This breaker accepts high over-current for the duration of
Autotransformer induction motor
• Motors over 50 kW use motor starters to reduce line current from
several hundred to a few hundred percent of full load current. An
intermittent duty autotarnsformer may reduce the stator voltage for a
fraction of a minute during the start interval, followed by application of
full line voltage as in Figure above. Closure of the S contacts applies
reduced voltage during the start interval. The S contacts open and the R
contacts close after starting. This reduces starting current to, say, 200%
of full load current. Since the autotransformer is only used for the short
start interval, it may be sized considerably smaller than a continuous
Running 3-phase motors on 1-phase
• 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
provisioning 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 simultanous with a
start capacitor in series with the third winding. The start switch is
opened 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 a static phase converter if the motor shaft is not
loaded. Moreover, the motor acts as a 3-phase generator. Three
phase power may be tapped off from the three stator windings
for powering other 3-phase equipment. The capacitor supplies a
synthetic phase approximately midway ∠90o between the ∠180o
single phase power source terminals for starting. While running,
the motor generates approximately standard 3-φ,
• 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 .
• 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.
• Variable voltage controls induction motor speed.
Electronic speed control
• 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 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
• Once electronics is applied to induction motor control, many control
methods are available, varying from the simple to complex:
Summary: Speed control
Scaler 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: 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
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
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. Line current reducing
starters are only required for large motors.
Three phase motors will run on single phase, if started.
A static phase converter is three phase motor running on single phase
having no shaft load, generating a 3-phase output.
Multiple field windings can be rewired for multiple discrete motor speeds
by changing the number of poles.
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