Answer / tariq mahmood

The slip ring or wound rotor motor is an induction machine
where the rotor comprises a set of coils that are terminated
in slip rings to which external impedances can be connected.
The stator is the same as is used with a standard squirrel
cage motor.
The slip ring motor is used primarily to start a high
inertia load or a load that requires a very high starting
torque across the full speed range. By correctly selecting
the resistors used in the secondary resistance or slip ring
starter, the motor is able to produce maximum torque at a
relatively low current from zero speed to full speed. A
secondary use of the slip ring motor, is to provide a means
of speed control. Because the torque curve of the motor is
effectivley modified by the resistance connected to the
rotor circuit, the speed of the motor can be altered.
Increasing the value of resistance on the rotor circuit will
move the speed of maximum torque down. If the resistance
connected to the rotor is increased beyond the point where
the maximum torque occurs at zero speed, the torque will be
further reduced.
NOTE:
The all above said characteristic of slip ring motor is due
to slip ring as well as due to external resistance. All
characteristic are related to is starting period or for
speed control. when we disconnected the resistance from the
circuit then slip ring motor and induction motor will be
same. So, hope the reason, "why slipring motor , at full
speed run as squirrel cage motor" will be cleared.

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