Three-phase motors can also be rewound for a change in voltage. The only changes necessary are in the number of turns and the size of the wire. EXAMPLE: If a 220-volt motor is to be rewound to operate an 440 volts, use twice as many turns on each coil and one-half the circular-mil area of wire. In other words, if forty turns of No. 17 wire were used on the original motor, eighty turns of No. 20 should be used on the new motor.
RECONNECTING THE POLYPHASE MOTOR FOR CHANGE IN SPEED
It has been mentioned that the speed of a three-phase motor decreases if the number of poles is increased and vice versa. (A change in speed is also caused by a change in line frequency.) Most methods of changing speed involve rewinding the coils and altering the coil pitch. However, one method of obtaining a different speed is to change the number of poles by reconnecting. If the applied voltage remains the same when changing from a low speed to a higher speed, the number of turns per phase must be decreased. From a high speed to a lower speed, the numbe is increased. EXAMPLE: It is desired to reconnect a six-pole, 220-volt, two-circuit delta to a four-pole, 220-volt motor. What connection should be used? Use the following procedure:
1. Regroup the coils for 3 phases X 4 poles= 12 groups.
2. If reconnected as the original, that is, two-circuit delta, the motor should operate on 1,800 = 150 per cent of original voltage, which is 330 volts. 1,200
3. To operate this motor on 220 volts, change from two-circuit delta to four-circuit star since the four-circuit star requires 330 X 86.6= 286 volts. This will prove satisfactory since the pitch of the coils has not been changed. Rewinding for Change in speed. To solve previous problem by rewinding, proceed as follows: 1. Change the coil pitch to 1 and No. of coils -1. Thus for a 48-slot motor the pitch is 1 No. of poles...