For a 3 phase motor to start it is deemed necessary to have a starter because 3 phase motors specifically induction motors are started by about 6-8 times the rated full load current.This is a lot of current but it does not cause harm to the motor due to the shot duration of time.
METHODS OF STARTING 3 PHASE INDUCTION MOTORS
- Direct on line
When the on button is pressed or activated by remote, the conductor coil A becomes energized and its normally open contacts are closed. Thus the motor gets connected across the 3 phase terminals. The hold on contact keep the motor in supply even after push button is released. When the off button is pressed or activated by remote, the coil gets de-energized and hence main contacts of the conductor are open and motor stops. fuses in this case are provided for circuit protection. In case of an overload on the motor the normally closed contact of the overload activates a relay which opens and subsequently stops the motor.
- Star delta starter– they are of three types; manual, semi-automatic and automatic star delta starter. they wont all be discussed here but their difference is that one is manually activated while the other is partially activated manually and part automatic while the last is automatically self activating. WORKING PRINCIPLE
When the push button is pressed or activated by remote control, the star part of the circuit is activated hence connecting stator terminals in star. The windings get 3 phase supply from the mains contactor which is also activated. The motor starts rotating with stator terminals star connected and when motor picks up to normal speed the time delay contact opens and closes star connection hence activating delta connection i.e stator terminals in delta across supply
- Auto-transformer starter – it basically consists of an auto transformer and a switch.
When the start button is pushed, a small voltage is applied to the motor and it goes to start, when the motor reaches 80% of its speed the switch goes to run position and at this stage the auto transformer is cut out of the circuit.it should be noted that the auto transformer can be tapped to select proper starting voltage which may vary form motor to motor.
- Slip ring motor starting – This method of starting involves rotor rheostat. The slip ring induction motors are always started with full line voltage on stator terminals and is adjusted by variable resistance in rotor circuit. When the motor gains speed the rheostat is cut out
Note – slip ring is done to phase wound ( slip ring induction motors) while the rest i.e direct on line, star delta and auto transformer starting is done to squirrel cage induction motors.