How Wound Rotor Motors Work
Wound rotor motors are an extremely versatile breed of induction motors. Featuring a rugged design, these machines provide the unique ability to gradually bring up to speed high-inertia equipment and large loads smoothly and easily. Wound rotor motors also can develop high starting torque at standstill - while maintaining low inrush. Long motor life is ensured with the use of external resistor banks or liquid rheostats that dissipate heat build-up generated during motor start-up.
What makes the wound rotor motor a unique induction machine is its rotor. Instead of a series of rotor bars, a set of insulated rotor coils is used to accept external impedances. The rotor windings are similar to those found on a DC armature, with the coils connected together to a set of rings that make contact with carbon-composite brushes. The circuit is completed by connecting the brushes to a set of impedances such as a resistor bank or liquid rheostat. This rotor construction design allows for a varying resistance from almost short-circuit condition to an open-circuit condition with infinite external resistance. By modifying the resistance, the speed-torque characteristics can be altered. This allows for the torque to remain high, the inrush low, and the speed varied.
