26 Electric Motors The speed is changed by switching the stator windings to change the number of pole pairs in the stator. By switching from a small pole-pair number (high speed) to a high pole-pair number (low speed), the actual motor speed can be dramatically reduced, for example, from 1500 to 750 RPM. With rapid switching from higher to lower speed, the motor runs through the regenerative range. This places a considerable load on the motor and the mechanism of the driven machine which can cause damage to motor and machinery. Slip control Controlling the motor speed using slip can take place in two diff erent ways: either by changing the stator supply voltage or by modifying the rotor.
It should be mentioned that these methods involve considerable thermal losses. Please refer to other sources of information if more is needed. Rotor control Controlling the motor speed using the rotor can be made in two diff erent ways: • Resistors are inserted in the rotor circuit. These types of motors are called “slip-ring” motors. The trade-off using this method is higher power losses in the rotor. • Rotor circuits are cascaded with other electrical machines or rectifi er circuits. The rotor circuit is then connected via slip rings to DC machines or to controlled rectifi er circuits instead of resistors.
The DC machine supplies the rotor circuit with additional variable voltage making it possible to change the rotor speed and magnetisation. Frequency regulation With a variable frequency supply, it is possible to control the motor speed with minor additional losses. The rotational speed of the magnetic fi eld and hence the rotor speed changes with the frequency. To maintain the motor torque, the motor voltage must change together with the frequency as shown in Fig. 1.18 Torque characteristics with voltage/frequency control. With a constant ratio of motor supply voltage to frequency, the magnetisation in the rated motor operating range is also constant. Fig.
1.18 Torque characteristics with voltage/frequency control