Monday, July 2, 2012

Three-Phase Wound Rotor Induction Motor

Wound Rotor (Slip Ring) Induction Motors
 
This type of motor is identical to a squirrel-cage induction motor except for the rotor, which consists of a symmetrical, three-phase winding. The three terminals are connected to three slip rings. Three external variable resistors are connected to the three slip rings via carbon brushes. Motor and resistor connections are shown 

 
Carbon brush wear is considerably higher in a humid environment or when the current density is lower than the recommended value. The carbon dust deposits on the slip ring and adjacent metal parts can cause a ground fault that may initiate a flash over and fire.

 A slip-ring flash over protection that will open the motor controller
upon detecting a ground fault is required to prevent a fire in the housing. For large motors, a liquid rheostat is used to dissipate the generated heat more efficiently.

Wound-rotor or slip-ring induction motors are used where high starting torque or small variation in speed is required. The most common applications are:
  • Conveyor drives: Higher starting torque is required for loaded systems.
  • Cranes: For lifting heavy loads and changing speeds.
  • Crushers, ball and sag mills: High starting torque is required.








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