Motor on SD-Starter Trips Breaker on Startup

I assume that someone from your organization will have to sign some sort of acceptance instrument before the contractor will be paid.

If you are that person you can either modify the agreement or just not buy it off until you are happy.

Otherwise, just stand back and watch.

Good luck.

The current draw in Star can be no more than 33% of the maximum locked rotor current, even if the unloader valve is stuck. So with a nameplate of 57A FLC and assuming 600% LRC, that makes it 342A LRC in Delta. Therefore in Star, the highest current that motor can draw is going to be 113A!

I would say that your assessment is dead on, except to my mind, he has BOTH scenarios going; connected in Delta AND the unloader is stuck. It might not, by the way, be connected improperly, but the starter transition time may be set too low and it is transitioning into Delta almost immediately. I have seen this trick used time and again when technicians don't want to bother finding the REAL problem, otherwise they might have to fix it. Someone had a problem, so they cranked down the timer to essentially force it into Delta and power through it. Just another reason why I hate Star-Delta starting....

By the way, the OL relay setting is a little too low, should be set at 58% of the motor FLA, so 33A. Not likely the cause, but it also might give some nuisance tripping later on.

If the FLA is 57, the conductors should be sized for 57 x 1.25 ≈ 71A. I'm rusty on it, but I thought breakers could be even higher.

If the motor draws 114A (= 57 x 6 ÷ 3) for a significant time, no wonder it is tripping the breaker.

I bet the next breaker up from 50-63A is a larger frame size....

^{If the motor draws 114A (= 57 x 6 ÷ 3) for a significant time, no wonder it is tripping the breaker.}

The motor nameplate FLA is 57A, not 57A x 6 / 3. Don't understand where that came from... The current draw in Delta will be split between the 2 sets of conductors; (hence the 58% setting of the OL relay). You size both sets of conductors based upon the 58% value because even though 58 + 58 would technically be over 100% current, they are out of phase with each other, so each set will carry an amount of current equivalent to 58% of the motor current (not 58% of the load). So each set of conductors is sized based on 33A in this case.

The OP is not in North America (he's in New Zealand). They have different protective device rules than we do. The 125% sizing of the conductors is a US standard (NEC 430.22(C) specifically), no idea if the Kiwis have a similar requirement, but probably.

In the RN we calculated start current to be usually around 6-8 X the FLA, so the current you are reading would appear to be about right.

So the main breaker is actuating too soon maybe or the thermal switch is activating. You could try removing the temp sensor in some way and starting once and once only...

The unloader may be at fault too of course, but I doubt it......the measured load sort of fits in my experience.....

Of course there might be a wiring error in delta......

If you look at this link:-

http://en.wikipedia.org/wiki/Induction_motor#Starting_of_induction_motors

You will see that it mentions a start current of between 5 - 9 X the FLA......

Sadly I have been out of that business too long to be of much further help!!

Best of luck.