28 Electric Motors Pay special attention to the rated motor voltages in star and delta. If the supply voltage is higher than the rated voltage of the applied confi guration, the motor will be damaged. The connection itself can be often changed by rearranging the jumpers at the motor terminal. U1 V1 W1 U1 V1 W1 a) b) a) b) W2 U2 V2 W2 U2 V2 Fig. 1.20 Star (a) and delta (b) confi guration of motors via jumpers on the terminal block In delta connection the full supply voltage is applied to each motor phase but the current is reduced by the factor 3. In star connection the current is maintained, and the voltage is reduced.
Therefore the power is the same regardless of the connection due to the fact that the feeding voltages are diff erent. L1 lpp = lp Up = Upp L1 lpp = 3 3 = lp Upp 3 3 Upp lp L2 L2 a) b) L3 L3 Fig. 1.21 Current and voltage distribution in star (a) and delta (b) confi guration So-called star/delta starters utilise this behaviour for reducing the starting current of a motor. In delta connection, the motor must suit the supplying mains. This means on 400 V mains the motor must have a 690 V star and a 400 V delta rating. At start the motor will be connected in star, reducing current, power and torque to one-third. After the motor has been accelerated the connection will be changed to delta.
Motor voltages in catalogues are often expressed by mentioning the star and delta voltages together (example: 400/230 V Y/Δ or 690/400 V Y/Δ). The lower voltage is always related to delta and the higher to the star connection. The relation of the current is vice versa: the lower current relates to star confi guration, and the higher current relates to delta confi guration.