Setting Time Delay on Automatic Star-Delta Starter 2

[ing001]

I know that when the motor has reached 80%-85% of the synchronous speed, it can be switched from star to delta.
I also know that this moment can be determined using a clamp ammeter on the supply lines from the main contactor. Will someone describe how this is done if, say, the motor is rated 7.5KW ?

 

[rbulsara]

ing001:

Not sure why you need a star delta starter for a 7.5 HP motor.

Regardless, if you try the clamp on meter method, you will see the current quickly dropping off when motor reaches almost full speed that should be your transition time. Or otherwise you can do it by trial and error. Start out by say 6 seconds and can go up to 10 sec. But for this size motor I doubt even 5 second is needed.

 

[Marke]

Hello ing001

Depending on the type of load and the inertia of the load, you can often estimate the expected time to reach full speed. (provided that the motor develops enough torque in star to reach full speed)

Set the transition time to longer than the expected time, monitor the current and listen to the motor while timeing its operation. You should set the timer to the time taken for the motor to either run out of torque, - no more acceleration, or the current to halve, whichever occurs first.
There is no point in leaving the motor in star if the rate of acceleration is close to zero. Additionally, there is no point in using star delta if the motor will not accelerate to full speed in star.

Best regards,

Mark Empson

http://www.lmphotonics.com