Motor's circuit breaker trips

[DrWhoF]

We have a lathe with a 3-phase motor.
There are two big contactors to turn the lathe on and off and to change direction.
Also, there are another two big contactors to start the motor in the following sequence: star-to-delta.
When the mpotor is turned on the star contactor comes on and after a while the delta contactor is activated but at that moment from time to time the main circuit breaker trips.
I have no idea why, do you have any clues?

 

[Awarapunshee]

Due to high spikes.

 

[IanP]

If 'spikes' caused circuit breakers to trip there wouldn't be power anywhere ..

What is the power rating of the motor (name plate) and what is the rating of the circuit breaker?

Regards,
IanP

 

[DrWhoF]

Motor: 14kW
Circuit Breaker: 500Vac 20A
DrWho

 

[IanP]

My granfather would say this: JUNGE, JUNGE ..

Dear friend, have a look at the following rough estimations:

14kW motor (divided by a typical pf≈0.9) gives you ≈15.5kVA .. divided by 3-phase voltage, say 400Vac, that is ≈39A ..
As a rule of a thumb you have to select a circuit breaker that is ≈ 2 x rated current but if 20A circuit breaker trips only from 'time to time' you may be just right with 63A-rated unit ..

So, replace 20A circuit breaker with 63A one and it should stop tripping ..

Regards,
IanP

 

[DrWhoF]

Hi IanP
Thak you for your prompt answer.
Well, we can't touch circuit breakers!
We replaced motors by their physical sizes and I wasn't aware of current problems.
What can we do now?
Thanks
DrWho

 

[IanP]

Now, my friend, you can try to run this motor in STAR configuration, without switching to DELTA ..

Regards,
IanP

 

[DrWhoF]

OK.
will try it at the earliest opportunity.
Thanks
DrWho

 

[DrWhoF]

I tried something but it didn't work (big bang).
Contactors are actuated in a sequence and they also use auxilliary contacts.
How to modify this circuit with minimum changes?
DrWho

 

[IanP]

Try to re-configure the second contactor (DELTA) to STAR ..
When the motor is powered up it will start in STAR mode (the first contactor) and will continue in STAR mode after the second contactor is engaged ..

Regards,
IanP

 

[DrWhoF]

OK. How to do that???

 

[IanP]

Take a look at the attached pictures: the first shows a typical STAR-DELTA configuration, the second modified STAR-STAR connections ..
Just cut/remove 3 wires and make the bridge and don't touch anything else ..

Regards,
IanP

 

[DrWhoF]

Thanks Ian.
It is working as we speak!!!
Thank you once again.
DrWho

 

[IanP]

You're welcome ..
I hope you have made the bridge of a decent wire ..

Regards,
IanP

 

[DrWhoF]

Waht do you mean by a decent wire?

 

[IanP]

Motors like yours should be wired with 8-10mm² conductor (8 AWG or larger) ..

Regards,
IanP

 

[DrWhoF]

Thanks.