DC Motors
• Advantage of DC Motor is that the torque-speed
characteristic can be varied over a wide range and still have high efficiency
• 3 Basic Types - Shunt, Series and Compound
• Shunt - In this motor the field current is independent of
the armature having been diverted (shunted) through its own separate winding.
Increasing the field current actually causes the motor to slow down. Torque and
power however are higher.
• Series - The series motor is identical in construction to
the shunt motor except the field is connected in series with the armature. At
startup, armature current is high, so flux is high and torque is high. If load
decreases, speed goes up. Series motors are for high torque, low speed
applications such as the starter motor of a car or the motors used for electric
locomotives.
• Compound - A compound motor carries both a series field
and a shunt field. The shunt field is always stronger. As load increases, the
shunt field remains the same but the series field increases. At no load it
looks like a shunt motor.
Induction Motors
• Most frequently used in industry (simple, rugged and easy
to maintain)
• Essentially constant speed from 0 to full load
• Not easily adapted to speed control
• Parts:
Stationary
stator
Revolving
rotor (slip ring at end)
Conventional
3 phase winding
Squirrel-cage
windings (copper bars shorted at end)
The characteristics of the induction motor are illustrated
below:
Synchronous Motors
• The most obvious characteristic of a synchronous motor is
its strict synchronism with the power line frequency.
• Its advantage to the industrial user is its higher
efficiency and low cost in large sizes
• Biggest disadvantage is added complications of motor
starting.
• A synchronous motor is identical to a generator of the
same rating.
• Synchronous motors are only selected for applications with
relatively infrequent starts since starting is more difficult and usually
requires the use of induction (squirrel cage) motor.
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