The slipring induction motor.

The slipring induction motor

The wound rotor or slipring ac machine addresses some of the disadvantages of the cage induction motor, but with the handicap of extra cost and the complexity of brushes and insulated rotor windings.

With the correct value of (usually) resistance inserted in the rotor circuit, a near-unity relationship between torque and supply current at starting can be achieved, such as 100 per cent full load torque (FLT), with 100 per cent full load current (FLC) and 200 per cent FLT with 200 per cent FLC. This is comparable with the starting capability of the dc machine. Not only high starting efficiency but also smoothly controlled acceleration historically gave the slipring motor a great popularity for lift, hoist and crane applications. It has had a similar popularity with fan engineers, providing a limited range of air volume control (either 2:1 or 3:1 reduction) at constant load by the use of speed-regulating variable resistances in the rotor circuit. Although a fan presents a square-law torque–speed characteristic, so that motor currents fall considerably with speed, losses in the rotor regulator at lower motor speeds are still relatively high, severely limiting the useful speed range.

Efficient variable-speed control of slipring motors can be achieved by slip energy recovery in which the the slip frequency on the rotor is converted to supply frequency. It is possible to retrofit variable frequency inverters to existing slipring motors simply by short-circuiting the slipring terminations (ideally on the rotor thereby eliminating the brushes) and treating the motor as a cage machine.

Variable voltage control of slipring motors has been used extensively, notably in crane and lift applications, though these applications are now largely being met by flux vector drives.

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