Of all AC motor available, a poly phase induction motor is one which is most widely used for several kinds of industrial drives. This is because it is simple, extremely rugged, low cost, reliable, high efficient and requires little maintenance.
Among the induction motors available, a squirrel cage machine is the best. However, such induction motor can never be started by direct switching, as the motor can never be started by direct switching, as the motor would take 7 to 10 times their full-load current and would develop only 1 to 2 times their full-load torque. The initial excessive current is highly objectionable because it produces large line voltage drop, which affects the operation of other electrical equipment connected to the same lines. So to prevent this, starter is used.
This circuit “3 Phase Induction Motor Starter” is one such starter. It is very-low cost but highly reliable gadget, besides being fool-proof.
Circuit Description of Automatic Induction Motor Starter
The circuit “3 Phase Induction Motor Starter” basically consists of a 555 timer chip (IC1). This timer switches on the 3-pole/3-change relay (connected in star mode for starting and delta mode for running) after a delay of about 60 seconds approximately, hence switching on the motor safely and also preventing line voltage drop. Here capacitor C1 starts charging as soon as the power is switched on.
When C1 is charged to 2/3 Vcc the output goes low and energizes the relay which changes the contacts and hence the motor connections. The LED in 3 Phase induction motor starter is on during the charging period of C1 and acts as a delay indicator. Diodes D2 and D3 protect the circuit against reverse polarities.
Another advantage of the circuit 3 phase induction motor starter is that even in the case of power interruption, the motor would re-start in its usual manner. Automatic induction motor circuit is very simple and can be assembled in less than an hour. Care has to be taken about the high voltage power lines.
PARTS LIST OF 3 PHASE INDUCTION MOTOR STARTER
Resistor (all ¼-watt, ± 5% Carbon)
R1 = 500 KΩ
R2 = 10 KΩ
R3 = 1 KΩ
C1 = 67 µF/25V
C2 = 1 µF/25V
C3 = 0.01 µF
IC1 = NE555
D1 = 1N4007
D2, D3 = 1N40007
LED1 = RED LED
RL1 = 12V-3poles/3change relay
SW1 = TP ST Switch