15 Electric Motors a) b) Fig. 1.4 Two coils in one phase connected in series to a) two poles b) four poles Table 1.2 Pole pairs (p) or pole number and synchronous motor speed – lists the number of poles corresponding to synchronous speed (n 0 ) at 50 and 60 Hz supply. Higher pole numbers are possible but rarely used nowadays. Table 1.2 Pole pairs (p) or pole number and synchronous motor speed Synchronous means “simultaneous” or “the same”. This means in synchronous motors the speed of the rotor is the same as the speed of the rotating fi eld.
If the rotor speed is aff ected by slip (see also section 1.3.3 Slip, Torque and Speed) and therefore lower than the speed of the rotating fi eld, the motor is classifi ed as asynchronous, meaning “not simultaneous” or “not the same”. 1.2.6 Effi ciency and Losses The motor draws electrical power from the mains. At a constant load, this power is greater than the mechanical power the motor can output to the shaft, due to various losses in the motor. The ratio between the output power P 2 and input power P 1 is the motor effi ciency:
η = P 2 P 1 output power input power = The effi ciency depends on the motor principle, components (for example lamination quality), amount of active material (for example, due to lamination or use of magnets), size of the motor (rated power) and number of poles. Pole pairs (p) 1 2 3 4 6 Pole number (2p) 2 4 6 8 12 n0 [min-1] (50 Hz supply) 3000 1500 1000 750 500 n0 [min-1] (60 Hz supply) 3600 1800 1200 900 600