Star/delta motor trips on Delta

SparkyUK · Jul 1, 2015

Hi,

I have a 90KW 3phase electrical motor. It is set up on the star/delta configuration for high current start ups. The problem comes when I start the motor. It is fine in star but when it switches into delta it will trip. It won't trip every time and when it does stay on it will run fine. But stop the motor and try to run it again and sometimes it will trip on delta. I have checked the phase windings,nice low values. Meggered it out- everything ok there. The circuit will switch just fine with no motor connected, so everything brings me back to an overload issue. The thermal OL are set correctly and the supply voltage is coming from a 500KVA Genset. Plenty of power and no problems there. I am in a cold climate so ambient temp is no a problem for over heating. Any ideas??

Darkwood · Jul 1, 2015

Is the motor getting to speed before it changes to Delta?
Are all bearings, gears etc been lubricated where necessary, this could be a sign of stiffening of workload over time and its taking longer to get to speed in star thus not achieving the changeover speed and taking out the overload device.

snowhead · Jul 1, 2015

I'd go with checking the S/D Time first, is it consistent and as above is the motor up to speed before the timer changes to delta?

What load is it driving, has that changed, conveyor, fan?

SparkyUK · Jul 1, 2015

snowhead said:
I'd go with checking the S/D Time first, is it consistent and as above is the motor up to speed before the timer changes to delta?

What load is it driving, has that changed, conveyor, fan?

Thanks for your feedback guys. Yes the motor is getting up to speed before the changeover. Timer is set up correctly (20S) the motor is driving a fluid pump mixing brine. I always ensure the operators start the motor offload but the bearings sounds interesting. The humidity is high here and the brine + chemicals are highly corrosive..

davesparks · Jul 1, 2015

What's the generator doing during all this? Is it noticeably labouring more than usual during starting? Has it been serviced recently?

Pete999 · Jul 1, 2015

SparkyUK said:
Hi,

I have a 90KW 3phase electrical motor. It is set up on the star/delta configuration for high current start ups. The problem comes when I start the motor. It is fine in star but when it switches into delta it will trip. It won't trip every time and when it does stay on it will run fine. But stop the motor and try to run it again and sometimes it will trip on delta. I have checked the phase windings,nice low values. Meggered it out- everything ok there. The circuit will switch just fine with no motor connected, so everything brings me back to an overload issue. The thermal OL are set correctly and the supply voltage is coming from a 500KVA Genset. Plenty of power and no problems there. I am in a cold climate so ambient temp is no a problem for over heating. Any ideas??

How old is this Motor? what type of over current protection does the starter have? Have I missed some thing what exactly is tripping?

Lucien Nunes · Jul 1, 2015

What is motor rated FLC and voltage? What current is the overload relay set at? What current and voltage readings do you get in star and delta just before and after the transition? Is it the overload relay that is tripping the starter off or the circuit OCP?