Traditionally, in many regions there was a requirement that all motor connections be fitted with a reduced voltage starter for motors greater than 4KW 5HP.This was to curb the high inrush of starting currents associated with starting induction motors.
Advantages of this method is low or reduced cost as compared to other methods...
For me to be able to explain this method properly we shall need to digrams for the Star Delta:
(i)Power Circuit Diagram
(ii)Control Circuit Diagram
Power Circuit Diagram
Initial States:KM3 (Main Contactor)-NC
KM2 (Delta Contactor)-NO
When the power is fed into the circuit, KM3 allows current to flow to the motor since it is NC (Normally Closed).Current flows into the motor and out to the KM1 which is the star-connected starter.After a specified period defined by the clock delay (Usually 5sec) the KM2 (Delta) Closes and KM1 opens to allow the motor to receive the full load current and run at delta.
Control Circuit Diagram
By using the control circuit diagram one can easily connect the terminals of the contactor easily.
An ExampleI will take a motor with some characteristics then we shall examine its values of current & voltage in star and Delta.
Power Rating: 120kW
No.of Poles: 4
Connection(Terminal Box): Delta
Running Conditions (Delta):Power Intake: 120kW
Starting Conditions (Star):Power Intake: 120kW