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Most AC motors are induction motors. Induction motors are favored due
to their ruggedness and simplicity. In fact, 90% of industrial motors
are induction motors.
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. While we include numerous illustrations
of two-phase motors for simplicity, we must emphasize that nearly all
poly-phase motors are three-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 as
shown in Figure below. The simple 2-phase induction motor below is
similar to the 1/2 horsepower motor which Nikola Tesla introduced in
1888.
Tesla polyphase induction motor.
The stator in Figure above is wound with pairs of coils corresponding
to the phases of electrical energy available. The 2-phase induction
motor stator above 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 in Figure above 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).
However, for larger motors less torque
pulsation and higher efficiency results if the coils are embedded into
slots cut into the stator laminations. (Figure below)
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 end housings.
Stator with (a) 2-φ and (b) 3-φ windings.
In Figure above, 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.
See Figure below. 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 overlap.
The key to the popularity of the AC induction motor is simplicity as
evidenced by the simple rotor (Figure below). 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 (a)embedded squirrel cage, (b)conductive
cage removed from rotor.
The squirrel cage conductors may be skewed, twisted, with respect to
the shaft. The misalignment with the stator slots reduces torque
pulsations.
Both rotor and stator cores are composed of a stack of insulated
laminations. The laminations are coated with insulating oxide or varnish
to minimize eddy current losses. The alloy used in the laminations is
selected for low hysteresis losses.
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