Star Delta Starter, Blowing Contactor

[SparkyBud]

Hi there,
I got a call to a job, table saw (wood) tripping C/B straight away.
4kW motor on Star Delta Starter.
So I get to the job, after some investigation I find that the main contactor is welded shut and there is a phase to phase short circuit, I find that the motor tails to the windings are shorted together which I found to be caused by loose terminations on the winding side causing the cables to heat up and the lug had melted through the insulation on another phase.

Those tails were cleaned up and tested the resistance, found to all be equal, so the motor was put back in and I put in the replacement contactor, tested the motor side and all seemed okay. On start-up, had a good start on Star but when delta kicks in it would seem I had the phase rotation wrong and an awful noise from the motor, all tripped out, another contactor down.
I get the next contactor and this time confirm rotation by disconnecting the motor tails and using a phase rotation meter and test both sides found my mistake, all sweet, working great without motor connected. Double check the motor at this point with 3.6ohms on 2 windings and 3.7 on the other.

It starts on Star no worries, then I stop because rotation is wrong for saw blade, change rotation at main isolator for machine (my mistake from fault finding).
Go for another start, no go, main contactor A phase dropped. Check motor windings, all the same. I have more contactors on order but I can't keep buying contactors...

Question 1: where am I going wrong?

Question 2: Does it really need to be a star delta starter? It is not a big motor can I just wire it up in Delta?

Question 3: Why isn't the O/L stopping the contactor from burning out? Is the contactor not suitably rated?

Any help or suggestions is much appreciated.
Cheers

 

[pc1966]

Question 3: Why isn't the O/L stopping the contactor from burning out? Is the contactor not suitably rated?

Not sure I can add much to what you have done so far, other than a few photos might help folks.

However, the overload side of the contactor is really intended to protect the motor windings against an excessive mechanical load drawing too much current.

What protects (or should protect...) the contactor itself is the MCB or fuses that supply it. Here you would need to check the contactor specification for what sort of protection it needs, and see what sort of protection is actually in use. If there is something drawing a LOT of current, say a short somewhere or the motor being switched direction on star-delta change over, then damage often comes down the to I2t let-through energy of the upstream protection.

There are some motor contactors that have both thermal overload for the motor windings and the equivalent of MCB fault protection built in (forget number but Schneider make them) but even then they are limited to breaking a 3kA fault so you would need to check the PSCC and how any upstream OCPD (fuses, MCB, or MCCB) coordinates with it.

 

[SparkyBud]

Not sure I can add much to what you have done so far, other than a few photos might help folks.

However, the overload side of the contactor is really intended to protect the motor windings against an excessive mechanical load drawing too much current.

What protects (or should protect...) the contactor itself is the MCB or fuses that supply it. Here you would need to check the contactor specification for what sort of protection it needs, and see what sort of protection is actually in use. If there is something drawing a LOT of current, say a short somewhere or the motor being switched direction on star-delta change over, then damage often comes down the to I2t let-through energy of the upstream protection.

There are some motor contactors that have both thermal overload for the motor windings and the equivalent of MCB fault protection built in (forget number but Schneider make them) but even then they are limited to breaking a 3kA fault so you would need to check the PSCC and how any upstream OCPD (fuses, MCB, or MCCB) coordinates with it.

Thank you, in the motor controls I have experience with this has been the usual set up I am used to, apologies for my ignorance. I will have a look at what protection is needed as I believe it was changed recently but this protects the entire machine and an auxiliary circuit, not just that motor. Maybe I should look at a motor circuit breaker to replace the TOL. Thanks for your input!