What is a wound-rotor (slip-ring) induction motor?
Working principle
A wound-rotor induction motor has the same stator and rotating field as a squirrel-cage motor, but its rotor carries a three-phase winding whose ends are brought out to three slip rings on the shaft. Carbon brushes ride on the rings and connect the rotor circuit to an external resistor bank. The rotating field induces rotor currents as in any induction motor; the difference is that extra resistance can now be put into the rotor circuit from outside, and taken out again, while the motor runs.
Rotor resistance and the torque curve
Adding resistance to the rotor circuit does not change the peak, or breakdown, torque. It only moves the slip at which that peak occurs: the larger the resistance, the higher the slip. Starting torque therefore rises until the peak sits at standstill, while the locked-rotor current falls because the rotor circuit impedance grows. A squirrel-cage motor must accept its one fixed curve, but a slip-ring motor can combine a high starting torque with a much lower inrush current.
Stepped starting and speed control
A rotor-resistance starter inserts the full resistance at switch-on and cuts it out in steps as the motor speeds up, so each step moves the operating point onto a higher curve and torque stays high until the rotor is shorted. The same resistance can control speed: at constant load torque, more resistance means more slip and a lower speed. The price is slip power. The share s of the air-gap power lost in the rotor circuit becomes heat in the resistor, so efficiency falls roughly in step with speed.
Where it is used
Wound-rotor motors drive loads that are hard to start: crushers, mills, hoists, cranes, conveyors and large pumps and fans on weak supplies. High starting torque at low current suits long, heavy acceleration, and resistance speed control was common on cranes and hoists before power electronics. Today slip-ring machines also appear as doubly fed generators in wind turbines, while cage motors with variable frequency drives have replaced many of the rest, partly because brushes and rings need maintenance.
Frequently asked questions
- Why does rotor resistance increase the starting torque of a wound-rotor motor?
- At standstill the rotor current lags its induced voltage because of the rotor leakage reactance, so much of the current produces little torque. Extra resistance brings the current more in phase and raises the torque per ampere while limiting the current. Torque at standstill peaks when the rotor resistance roughly equals the leakage reactance; beyond that, more resistance lowers it again.
- Does rotor resistance change the breakdown torque?
- No. The maximum torque depends on the supply voltage and the leakage reactances, not on the rotor resistance. Rotor resistance only changes the slip at which the maximum occurs, moving it towards standstill as resistance grows. In the simulator the curves keep the same height while their peaks shift towards lower speed.
- Why is rotor-resistance speed control inefficient?
- The slip power is not recovered; it is dissipated as heat in the external resistor. With the rotor at 1200 rpm on a 1500 rpm field, about 20 % of the air-gap power is lost there, so efficiency falls roughly in step with speed and the resistor bank must be rated for continuous heat. Drives or slip-power recovery schemes are used when long-term speed control is needed.