25 Electric Motors The motor speed can therefore be changed by changing: • The pole pair number p of the motor (for example, pole-changing motors) • The motor slip s (for example, slip-ring motors) • The motor supply frequency f (for the motor) (1 - s) x f p n = No. of pole pairs Frequency Slip Rotor Starter voltage Resistance Cascade coupling Fig. 1.16 Diff erent options for changing the motor speed Pole number control The rotational speed of the magnetic fi eld is determined by the number of pole pairs in the stator. In the case of a two-pole motor, the rotational speed of the magnetic fi eld is 3000 RPM at a motor supply frequency of 50 Hz. For a four-pole motor the speed is 1500 RPM.
Fig. 1.17 Torque characteristics when changing pole number Motors can be designed to have two or more diff erent pole-pair numbers. This is done by using a special arrangement of the stator windings (Dahlander winding) in the slots and/or by using more separate and isolated windings in the slot.