Wound rotor motors have reduced speed control and current ratings, but increased torque during startup. They have rotors with three wire windings terminated with slip rings, allowing for variable resistance during startup, reducing current consumption and increasing available torque. This makes them ideal for high inertia loads, such as large ventilation fans, conveyor belts, and slurry pumps, as they can use smaller motors and starters, making installations more efficient and cost-effective.
A wound rotor motor is a three-phase induction variant that features greatly reduced speed control capabilities and current ratings, and increased torque during startup. Conventional induction or squirrel cage motors have a rotor made of rolled steel rods joined at one end. The rotors of the wound rotor motor are profiled to accept three separate wire windings terminated with three slip rings on the motor shaft. During starting a variable resistance is applied in series to the rotor windings via the slip rings which results in a reduction of the overall starting current consumption and an increase in the available torque. This allows smaller motors to be used with high slip and high inertia loads.
Machines such as heavy and large diameter ventilation fans, long conveyor belts and slurry pumps are known as high inertia loads or high creep loads. In other words, due to their high potential for inertia, they take a long time to reach operating speeds at startup. If conventional motors are driving such loads, they should be rated to handle these long periods of starting current and torque demand rather than the much lower operating ratings. To achieve a viable solution, the motor, its starters, and fuel system would need to be much larger than needed to actually run the machine. An alternative to this conundrum is a wound rotor motor.
The rotors of conventional induction motors consist of closely spaced steel rods, electrically shorted or joined at one end. The rotor of a wound rotor motor looks similar on the outside but has an internal profile designed to accommodate three separate windings. These windings terminate in three slip rings mounted at one end of the rotor shaft. During operation, a series of static carbon brushes slide across these slip rings and connect the motor to a variable resistance device. This allows the engine operator or an automated system to vary the rotor resistance as the engine starts.
Increasing the rotor resistance of an induction motor during startup greatly reduces the overall current draw of the motor and increases the amount of torque available. Once the motor is running at its maximum operating speed, the resistors are shorted, thus effectively replicating a conventional rotor. Gradual increases in resistance also allow the engine speed to vary by one degree. These advantageous characteristics of a wound rotor motor allow smaller motors and starters to be used when installing high inertial load machinery, thus making installations much more efficient and cost effective.
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