Wound rotor motor
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A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.[1]
Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-up a typical rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable power resistor. When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the resistors reduce the field strength at the stator. As a result, the inrush current is reduced. Another important advantage over squirrel-cage motors is higher starting torque.
The speed and torque characteristics of a wound-rotor motor can be adjusted by changing the external resistance, unlike a squirrel cage motor which has a fixed characteristic. This is useful for speed control of the motor.[1]
A wound-rotor motor can be used in several forms of adjustable-speed drive. Common applications include hoists, elevators, and conveyor systems. Also, the travel mechanism of gantry cranes or overhead cranes also used this type of motor, because it has both adujustable speed and high torque. Certain types of variable-speed drives recover slip-frequency power from the rotor circuit and feed it back to the supply, allowing wide speed range with high energy efficiency. Doubly-fed electric machines use the slip rings to supply external power to the rotor circuit, allowing wide-range speed control. Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.
Advantages of this motor are mainly high starting torque, high rotational torque, variable speed control with no need of complicated electronics, good efficiency. Disadvantages are mainly the higher maintenance due to wear out of slip rings, they are quite noisy (buzzing ) at start-up especially with high loads and have less efficiency compared to squirrel cage rotor motors. Also, wound-rotor motors are not fully sealed. While this allows a better cooling, it needs to be protected from excessive moisture (rain ) or excessive dust, and limits somehow it's usage in explosive and damp enviroments. They are mostly replaced with induction motors with variable frequency drives, or with permanent magnet synchronous motors, depending on applications.