For Motors which using Star/Delta starting method, how we can choose, contactors and CB rating, Over current relay and timer relay Setting.?

For wye-delta motor controller, main CB sized-up to motor full load current, while the delta, main contactor and overload relay sized-up to only 57% while wye contactor is much smaller because it takes smaller current. All rating will have its safety factor or sized a step higher than computed or given values.

At First Star/Delta motor starting method is the most effective cost and common method.

So, let us know how to size the circuit component.

The motor load is split between 2 contactors but the overload only sees the current carried by one.It is not a 50/50 split but has to be calculated by a special formula which, simply put, means the overload and each of the delta and line contactors only 0.58 x the motor current (line).

Delta and main contactor will be (Motor Current (line) * .58) sure will us the above nearest available value.

Star contactor will be (Motor Current (line)*.5) sure will us the above nearest available value.

Main CB will be > I Staring Motor Current (2.5 Motor Current (line)).

Overload depend on its positon: -

·         After Main or Delta Contactor will be ( 1.05 * .58* Motor Current (line)).

·         After the Main CB or Before Main or Delta Contactor will be (1.05 * Motor Current (line)).

Cables according to the cabling tables with reference to the current flow in the circuit. And must applied the de rating factors to make our design more accuracy and safe.

Timer setting based of motor data sheet.

It is more safe and protection for the motor at the starting method to chose the Start/ Delta connection with selection the rating current of the circuit breaker as per the Load current and rating current of the Motor. The setting of the Over current relay and time setting has to be a minimum setting during start-up of the motor.

CHOOSE THE C.B RATING DEPENDS ON CALCULATION OF CURRENT FROM THE LOAD