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Discussion Starter · #1 ·
Here's the situation. We have a machine which uses two 100HP motors to drive hydraulic pumps. The first motor, M1, starts without an issue. The second motor, M1A, intermittently trips the breaker. When I observe the start-up, it starts fine in star (wye) but when it changes over to delta run, 1 in 10 times the breaker trips. We have changed contacts, mechanical interlocks and even coils. The motor has also been rewound once, as we believe this starting issue eventually caused the windings to short to ground. This problem has persisted since the machine was installed (brand new). The manufacturer even sent us all new starters and overload. The problem has still persisted.

Has anyone experienced a similar problem or have any thoughts that I may have overlooked?
 

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Here's the situation. We have a machine which uses two 100HP motors to drive hydraulic pumps. The first motor, M1, starts without an issue. The second motor, M1A, intermittently trips the breaker. When I observe the start-up, it starts fine in star (wye) but when it changes over to delta run, 1 in 10 times the breaker trips. We have changed contacts, mechanical interlocks and even coils. The motor has also been rewound once, as we believe this starting issue eventually caused the windings to short to ground. This problem has persisted since the machine was installed (brand new). The manufacturer even sent us all new starters and overload. The problem has still persisted.

Has anyone experienced a similar problem or have any thoughts that I may have overlooked?
your timer how long does star run before it changes to Delta?

Sent from my HUAWEI Y210-0100 using Tapatalk 2
 
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Discussion Starter · #4 ·
Ultrafault: the initial inrush current is about 280A. Once it gets up to speed on star, it draws about 56A. Delta run current is about 86A.

Nickson: the timer was one of my thoughts as well. I've adjusted the time from approximately 2.5 seconds (factory setting) to about 4.5 seconds. We've started and stopped the motor without any issues this morning, but as I stated earlier the problem is intermittent (1 in 10 times) so its now a waiting game to see if that was the issue. Both motors were set to the same time, and M1 motor has never once had this issue, only M1A.
 

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The fact that it runs most of the time but not all the time says to me it's not likely a fault. I think your protection is likely going out on instantaneous because something is set wrong.

What makes you believe there is a ground fault and why? Has anyone looked at the transition time? Has anyone looked at the hydraulic load?
 

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Discussion Starter · #7 ·
Big John: The motor went to ground previously (about 10 months ago). It has since been rewound and is no longer faulting to ground. My suspicion was that the ground fault was caused by this intermittent problem reoccurring over 4 years of run time. This issue has never really been addressed over the years, just reset and forgotten about. It wasn't until I took over the department about a year ago that we started to look deeper into the causes. That's when we started replacing components. Until recently I wasn't too familiar with star/delta starters. I'm big on soft starters. Lower cost, fewer problems, higher starting torque. I've increased the transmission time from 2.5 to 4.5 seconds.

We have looked into the hydraulic load as well. The solenoids bypass to the tank on start-up, so the load should be minimal. I'll be looking into the possibility that a solenoid is sticking open later today.
 

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This is a known issue with Star-Delta open transition starters and the reason I never recommend them. There is a phase shift that occurs in transition from star to delta. Depending on the magnetic permeability of the motor cores, the impedance of the windings and the phase angle at which the exact transition point occurs, it can cause the motor to connect in Delta while the fields are not yet collapsed, so it's like connecting two out-of-phase generators. The result can be a voltage transient that can then create a current transient that in theory, can be as high as 2200% of the motor FLA, far exceeding the mag trip settings of the breakers. If you google "star delta motor starter transient" you should find some articles on it.

Your only cheap solution is to "roll" the conductors, in other words change which winding they are connected to, but keep the same rotation pattern. What this does is to take advantage of any possible slight differences in impedance between the windings and the phases, which might keep the transient below a tolerable level. Do it twice only, no point in doing it a third time because you go back to the original. So it that doesn't work, you must either go to a closed transition star-delta scheme, which adds a 4th contactor and a resistor bank to absorb the transient, or start over using a soft starter.
 

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Discussion Starter · #9 ·
Try a new breaker. Seems you have tried everything else.
This will be my last option once everything else is ruled out. It has been tripped quite a few times. But again, it's intermittent, so I suspect its not the breaker. There is a breaker protecting the starters for each motor. The trip settings are identical, and the run currents are very close as well.
 

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This will be my last option once everything else is ruled out. It has been tripped quite a few times. But again, it's intermittent, so I suspect its not the breaker. There is a breaker protecting the starters for each motor. The trip settings are identical, and the run currents are very close as well.
Swap out the breaker with one next to it if they are exact and see.
This seems like the obvious next step to me.
Swap breakers and at least then you know for sure its not the breaker.
Breakers do go bad and just because its intermittent does not mean the breaker is good.
I have seen this before and that is why I suggest trying another breaker.

I would also check all the breaker connections upstream and downstream. Do you have bus duct above? If yes, pull it and check the stabs.
 

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By the at, that voltage transient can also damage the motor winding insulation and eventually lead to failure. See the connection?
 

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Discussion Starter · #12 ·
Swap out the breaker with one next to it if they are exact and see.
This seems like the obvious next step to me.
Swap breakers and at least then you know for sure its not the breaker.
Breakers do go bad and just because its intermittent does not mean the breaker is good.
I have seen this before and that is why I suggest trying another breaker.

I would also check all the breaker connections upstream and downstream. Do you have bus duct above? If yes, pull it and check the stabs.

No buss duct, but I've also seen breakers do some odd things, including pass voltage but not current. We'll give this a try.
 

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Discussion Starter · #13 ·
By the at, that voltage transient can also damage the motor winding insulation and eventually lead to failure. See the connection?

Fabulous advice, thanks so much. I'll spend some time over the next day or two doing some research into this. We'll try rolling the conductors as you suggested. If it was up to me, I'd scrap the whole damned thing and replace it with a soft starter. It just makes sense, not to mention saves money in the long run on peak demands.
 

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You switch from wye to delta after 2.5 to 4 seconds, but the switch is instantaneous.
Once the motor runs at full speed in wye, disconnect the power to the wye contactor coil, let the speed fall to half, and then connect power to the coil of the delta contactor . The counter emf should have been reduced.
This would require another timer. Induction motors under power generate a counter emf, which opposes the applied voltage. By switching from wye to delta, that counter emf can add to the supply voltage.
 

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Your only cheap solution is to "roll" the conductors, in other words change which winding they are connected to, but keep the same rotation pattern. What this does is to take advantage of any possible slight differences in impedance between the windings and the phases, which might keep the transient below a tolerable level Do it twice only, no point in doing it a third time because you go back to the original."
just asking jraef how is this possible.becoz practically when I start changing the phase pattern am also changing the rotation
maybe I am not getting the point in your explanation please assist
Sent from my HUAWEI Y210-0100 using Tapatalk 2
 

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you must either go to a closed transition star-delta scheme, which adds a 4th contactor and a resistor bank to absorb the transient, or start over using a soft starter.

I did read the link you posted
however I was expecting to get a circuit diagram which has the four contactors with it in the star Delta. if you have it please post..

was also asking is it possible not to provide a resistor bank and
still be able to solve the open problem. thank-you
Sent from my HUAWEI Y210-0100 using Tapatalk 2
 

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just asking jraef how is this possible.becoz practically when I start changing the phase pattern am also changing the rotation
maybe I am not getting the point in your explanation please assist
Sent from my HUAWEI Y210-0100 using Tapatalk 2
You are just moving the wires, not changing the phase relationship. So you move A to B, B to C, C to A. The rotation remains the same, the conductors shift. All it does is possibly change the circuit impedance a little, but sometimes it's enough.

Most of the motor starter mfrs will publish diagrams on the 4 contactor arrangement, do an image search for "Closed Transition Star Delta Diagram".

And no, there is no option for fixing it without the resistors, that's what the 4th contactor is for. The "no resistor" option is to scrap the star-delta starter and put in a soft starter.

Or click here:
 
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