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.

The diagram shown below illustrates the basic characteristics of these motors:

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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