A wound rotor motor is a three phase induction variant that features speed control capacity and greatly reduced current and increased torque values during start up. Conventional induction or squirrel-cage motors have a rotor made up of laminated steel bars joined at one end. Wound rotor motor rotors are profiled to accept three separate wire windings terminated at three slip rings on the motor shaft. During start up, a variable resistance is applied in series with the rotor windings via the slip rings which results in a reduction in overall starting current drain and an increase in available torque. This allows smaller motors to be used with high slip and high inertia loads.
Machines such as heavy, large diameter ventilation fans, long conveyor belts, and slurry pumps are known as high inertia loads or high slip loads. In other words, due to their high inertia potential, they take a considerable time to reach operational speeds when started. If conventional motors drive such loads, they would have to be rated to handle these extended periods of start up current and torque demands rather than the far lower running values. To achieve a workable solution, the motor, its starters, and the power supply system would have to be far bigger than necessary to actually run the machine. An alternative to this conundrum is a wound rotor motor.
The rotors of conventional induction motors are made up of closely spaced steel bars electrically shorted or joined at one end. The rotor of a wound rotor motor looks outwardly similar but features an inner profile designed to accommodate three separate windings. These windings terminate at three slip rings mounted at one end of the rotor shaft. During operation, a set of static carbon brushes run on these slip rings and connect the motor to a variable resistance device. This allows the motor operator or an automated system to vary the rotor resistance when the motor starts.
Raising the resistance of a induction motor rotor during start up greatly reduces the overall current draw of the motor and increases the amount of available torque. Once the motor is running at its full operational speed, the resistances are shorted out, thus effectively replicating a conventional rotor. Gradual resistance increments also allow the motor speed to be varied to a degree. These beneficial characteristics of a wound rotor motor allow smaller motors and starters to be used when installing high inertial load machinery, thereby making the installations far more efficient and cost effective.