<P> An alternate design, called the wound rotor, is used when variable speed is required . In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft . Carbon brushes connect the slip rings to a controller such as a variable resistor that allows changing the motor's slip rate . In certain high - power variable - speed wound rotor drives, the slip - frequency energy is captured, rectified, and returned to the power supply through an inverter . With bidirectionally controlled power, the wound rotor becomes an active participant in the energy conversion process, with the wound rotor doubly fed configuration showing twice the power density . </P> <P> Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices . Transistorized inverters with variable - frequency drive can now be used for speed control, and wound rotor motors are becoming less common . </P> <P> Several methods of starting a polyphase motor are used . Where a large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals (direct - on - line, DOL). Where it is necessary to limit the starting inrush current (where the motor is large compared with the short - circuit capacity of the supply), the motor is started at reduced voltage using either series inductors, an autotransformer, thyristors, or other devices . A technique sometimes used is star - delta (YΔ) starting, where the motor coils are initially connected in star configuration for acceleration of the load, then switched to delta configuration when the load is up to speed . This technique is more common in Europe than in North America . Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load . </P> <P> This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor . </P>

Describe the main features of an ac electric motor