Wound-rotor motors deliver speed control of heavy and high-torque loads.

What's special about the wound-rotor motor? This type of 3-phase induction motor has high starting torque, which makes it ideal for applications where standard NEMA design motors fall short. The wound-rotor motor is particularly effective in applications where using a squirrel-cage motor may result in a starting current that's too high for the capacity of the power system.

In addition, the wound-rotor motor is appropriate for high-inertia loads having a long acceleration time. That's because you can control the speed, torque, and resulting heating of the wound-rotor motor. This control can be automatic or manual. It's also effective with high-slip loads as well as adjustable-speed installations not requiring precise speed control or regulation.

The configuration and construction of wound-rotor motors are different from squirrel-cage motors, basically in the design of the rotor. Also, they incorporate brushes and slip rings.

The windings (bars on a laminated core) on a squirrel-cage motor are short-circuited by an end ring. The windings on a wound-rotor motor are not short-circuited, but are connected in a 3-phase arrangement. Here, one end of each phase is brought out via leads to three separate slip rings, which are mounted on the end of the shaft.

Stationary brushes ride on each slip ring, forming an external "secondary" circuit in which you can insert any desired value of resistance. It's this special feature (insertion of variable resistance in the secondary circuit), with manual or automatic control that permits the wound-rotor motor to perform in its unique fashion.

For example, with the slip rings shorted (no resistance in the rotor circuit); the rotating machine behaves very much like a NEMA Design B motor. The rotor winding alone has a low resistance, resulting in low slip and high efficiency.

If you add resistance to the secondary circuit, the speed-versus-torque curve goes through a series of changes, depending on the value of the added resistance. When you insert resistance in the rotor (secondary) circuit, you reduce the rotor current and proportionally the stator (primary) current.

In addition, there's a significant effect on motor starting torque when different values of resistance are in the secondary circuit. You can understand and appreciate the performance characteristics of a wound-rotor motor with changing values of resistance by reviewing the accompany curves. Usually, you start wound-rotor motors with some value of resistance in the circuit.

This method results in a lower value of starting current, but a higher value of torque, depending on the amount of resistance used. This characteristic makes wound-rotor motors suitable for starting heavy loads and accelerating them gradually and smoothly. Typical applications include conveyer belts, hoists and elevators.

The technique of starting wound-rotor motors with resistance in the rotor circuit is also used for speed-control applications. You can reduce the resistance in steps to permit the motor to come up to normal speed. And, you can select a particular operating speed in accordance with the load. At full load, you can reduce the speed effectively to about 50% of the motor synchronous speed, particularly when driving variable torque/variable speed loads such as printing presses or compressors. Reducing the speed below 50% results in very low efficiency.

A widely used type of wound-rotor motor controller is one furnished with metallic, wire-wound resistors that are progressively shorted out manually or by magnetic contactors controlled electronically. The resistance can be in the form of a drum controller for manual operation, or in the shape of wire, cast iron grids, or as electrolyte liquid with metallic electrodes (although the latter types are falling into disuse). Varying the resistance of the metal-type resistors usually requires "tapping" of the resistors in steps. These resistance components can be used on a continuous basis, if the unit has been designed to dissipate heat produced by such operational reduced speeds.

Modern wound-rotor controllers use solid-state devices to obtain stepless control. These may incorporate thyristors that serve in the place of magnetic contactors. The latest solid-state controllers provide smooth acceleration with practically constant torque up to running speed. The motor then returns to running torque while maintaining a nearly constant speed. You can use solid-state controllers to reverse motor rotation.