When the AC Voltage is directly applied on to the stator windings of a 3 phase induction motor, the motor draws a high starting current which may disturb the system voltage. The starting current may be of the order of 6 to 7 times the full load current of the motor. In order to reduce the starting current, there are three types of starters, normally used in industrial applications.
TYPES OF STARTERS:
DIRECT ONLINE STARTER (DOL):
In this the AC Voltage is directly applied to the stator. No effort is taken to reduce the starting current. The control circuit of DOL starter is as under
DOL starters are used up to 10 HP normally.
STAR-DELTA STARTER:
In this starter, the stator windings of the motor are connected initially in star and once the motor reaches its 2/3rd of its normal speed, the stator windings are changed to delta connection. In this starter, the starting current is /3 times compared to DOL starter.
The λ-∆ starter can be of
- manual
- semi – automatic type
- fully – automatic type
The λ-∆ starters are used for motors from 12.5 HP to 30 HP.
AUTO TRANSFORMER STARTER:
In this starter, an auto transformer which has 70% of its winding tapping is initially connected to the stator windings and once the motor reaches its 2/3rd of its normal speed, the 100% of the winding tapping is connected to the stator windings. In this starter, the starting current is reduced by √3 times compared to λ-∆ starter.
Auto-transformer starter are used normally for motors above 30 HP.
THERMAL OVER-LOAD RELAY:
These starters have thermal over load relays to trip the motor on over load situations. These Bi-metal relay elements carry the full or proportionate current to the motor and when the current through these elements crosses the pre-set value, these elements bend and open a normally closed contact, incorporated with the relay.
This disconnects the electricity supply to the contactor coils of the starter
0 Comments