08-16-2017, 08:47 PM
Electric Motors
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Induction motors
These motors are probably the simplest and most rugged of all electric motors. They consist of two basic
electrical assemblies: the wound stator and the rotor assembly.
The rotor consists of laminated, cylindrical iron cores with slots for receiving the conductors. On early motors,
the conductors were copper bars with ends welded to copper rings known as end rings. Viewed from the end,
the rotor assembly resembles a squirrel cage, hence the name squirrel- cage motor is used to refer to induction
motors. In modern induction motors, the most common type of rotor has cast-aluminum conductors and shortcircuiting
end rings. The rotor turns when the moving magnetic field induces a current in the shorted
conductors. The speed at which the magnetic field rotates is the synchronous speed of the motor and is
determined by the number of poles in the stator and the frequency of the power supply.
Wound-rotor motors Although the squirrel-cage induction motor is relatively inflexible with regard to
speed and torque characteristics, a special wound-rotor version has controllable speed and torque. Application
of wound-rotor motors is markedly different from squirrel-cage motors because of the accessibility of the rotor
circuit. Various performance characteristics can be obtained by inserting different values of resistance in the
rotor circuit.
Single-phase motors These motors are commonly fractional-horsepower types, though integral sizes are
generally available to 10 hp. The most common single phase motor types are shaded pole, split phase,
capacitorstart, and permanent split capacitor.
Figure 4 - Typical speed-torque
characteristics for Design A, B, C, and
D motors.
Shaded pole motors have a continuous copper loop wound
around a small portion of each pole, Figure 5. The loop causes
the magnetic field through the ringed portion to lag behind the
field in the unringed portion. This produces a slightly rotating
field in each pole face sufficient to turn the rotor. As the rotor
accelerates, its torque increases and rated speed is reached.
Shaded pole motors have low starting torque and are available
only in fractional and subfractional horsepower sizes. Slip is
about 10%, or more at rated load.
Split phase motors, Figure 6, use both a starting and running
winding. The starting winding is displaced 90 electrical degrees
from the running winding. The running winding has many turns
of large diameter wire wound in the bottom of the stator slots to
get high reactance. Therefore, the current in the starting winding
leads the current in the running winding, causing a rotating field.
During startup, both windings are connected to the line, Figure
7. As the motor comes up to speed (at about 25% of full-load
speed), a centrifugal switch actuated by the rotor, or an
electronic switch, disconnects the starting winding. Split phase
motors are considered low or moderate starting torque motors
and are limited to about 1/3 hp.
Capacitor-start motors are similar to split phase motors. The
main difference is that a capacitor is placed in series with the
auxiliary winding, Figure 8. This type of motor produces greater
locked rotor and accelerating torque per ampere than does the
split phase motor. Sizes range from fractional to 10 hp at 900 to
3600 rpm.
Split-capacitor motors also have an auxiliary winding with a
capacitor, but they remain continuously energized and aid in
producing a higher power factor than other capacitor designs.
This makes them well suited to variable speed applications.