MCP Required?

[trallio]

I have a reversing motor contactor for a large (300HP, 480VAC). It came with an MCP in the cabinet. It is fed directly from an adjustable branch circuit breaker. This motor has to be estopped within 2 seconds. Plugging is used for this.
When the motor is E-stopped 10 minutes after starting, all is well.
When it is E-stopped (plugged) within 5 minutes of starting, the MCP trips. It's max setting is 5000A. The FLA is 345A. I believe the max amps which should be seen for plugging should be something a little in excess of locked rotor. The windings are type G. 1800A+ maybe as high as 2300V. It should not be tripping. In fact, when we jumpered out the MCP, the upstream breaker held even when the motor was started and estopped multiple times within a half hour period.
We suspect that the MCP is not able to clear heat fast enough after a start to handle the plugging inrush. OR, it is a bad MCP.
My question is this. Is the MCP actually required. It is at the contactor on a mezanine above the production line where the motor is. The upstream breaker is about 200 ft away in a mechanical/electrical room.
Can we not just replace the MCP with a non-fused disconnect?

 

[Buck Parrish]

Can we not just replace the MCP with a non-fused disconnect?

Sure, as long as the breaker is sized correctly.

 

[quogueelectric]

Is there gfci protection on the mcp?

 

[Jraef]

An MCP breaker's operation is strictly magnetic, so it is not very sensitive to the thermal condition of the breaker itself. Most likely, the reason it trips if e-stopped too soon after starting is that when the motor was started only 5 minutes ago, the heat from the starting current is still primarily in the rotor, so the stator windings are cooler. If cooler, they have less resistance (motors have a Positive Temperature Coefficient of resistance). So when plugged earlier on, the inrush (magnetization current, not starting current) can be a lot higher. After the motor has been running a while, the heat gets into the windings, the resistance goes up and plugging has less of an effect. I should also point out that plugging is hard on a motor circuit anyway because it is usually done instantaneously and there are issues of flux penetration that cause extra high inrush currents than just starting from a dead stop. Adding the PTC resistance issue probably nudges it over the top of what the breaker can hold in on.

I believe the max amps which should be seen for plugging should be something a little in excess of locked rotor.

If it is already tripping, you are allowed to adjust the magnetic settings higher than normal, i.e. 1700% of motor FLA if it is a high efficiency motor, 1300% if not. Locked rotor amps is usually around 600% so if you have it set there, it is likely too low. Remember, the MCP is providing only the instantaneous short circuit protection, not thermal overload protection. That is being done by an overload relay as part of the motor starter. If the 5000A you mentioned earlier is the setting of the Mag Trips right now, that may already a code violation unless it is a high efficiency motor. If so, you may be stuck with either putting up with nuisance tripping or putting in a soft starter so you can soft-plug the motor. Either that or just set up the process so that the motor must be run for 10 minutes when first started before allowing a process operation that may end up requiring a plug stop.

 

[augie47]

most impressive answer, Jraef

 

[trallio]

Jraif wrote: I should also point out that plugging is hard on a motor circuit anyway because it is usually done instantaneously and there are issues of flux penetration that cause extra high inrush currents than just starting from a dead stop. Adding the PTC resistance issue probably nudges it over the top of what the breaker can hold in on.


Thanks for the information.
We have run tests with the upstream breaker set properly, and it holds ok. This seems to match up with your answer.


The plugging is only used for Estop, not normal stopping. The Estops are terribly hard on the motor and, I believe, on the reversing contactor as well. And, unfortunately, the Plant's past practices on similar equipment are to use Estops instead of normal stops most of the time. This is hard enough on the smaller motors they already have. It is much worse on this motor. We have been trying to impress on them that they will tear this thing up if they use Estops when not required.

Again Thanks!

 

[Jraef]

... We have been trying to impress on them that they will tear this thing up if they use Estops when not required.

Again Thanks!

Been there, done that. It is a very frustrating thing to watch. I have seen workers plug stop giant wood chippers just because they don't want to wait to change out the blades. They would tear up the contactors, the breakers would occasionally explode, the transformers would overheat... they couldn't care less. Then the poor electrician has to "fix it" which never meant stop it from happening, it just meant "spend as little as possible to get back up and running".

If the normal start-stop operations have long wait times like that which are making the operators plug stop out of frustration, you could propose DC injection braking. Not as fast for emergencies, but not as hard on the equipment either.

 

[trallio]

...If the normal start-stop operations have long wait times like that which are making the operators plug stop out of frustration, you could propose DC injection braking. Not as fast for emergencies, but not as hard on the equipment either.


I have hesitated to mention this because I wanted a legit answer to this problem, but the OTHER motor they refused to let be at 2300V is a 600HP motor. It also has to estop. It is on a soft start and we are using Allen-Bradley's SMC smart braking which is a form of DC injection.

I've not witnessed it, but I'm told watching that thing stop in less than 2 seconds is a little terrifying.

I have told our project management that if they do not change plant policy, at least one of these two motors or their starters will be destroyed by Spring.

 

[augie47]

I would think if you were at the controller during an e-stop, "a little terrifying" might be a mild statement :grin:

 

[jim dungar]

The Estops are terribly hard on the motor and, I believe, on the reversing contactor as well.

As well as the plant electrical system. Plugging is effectively paralleling a generator that is not even close to being in synch.

 

[Jraef]

I would think if you were at the controller during an e-stop, "a little terrifying" might be a mild statement :grin:

No kidding. I have been in front of one of those on just a little 250HP 460V spindle motor, the sound is scary. It also wreaked havoc on the building's electrical system, causing the UPS systems in the accounting dept. to initiate an automatic shutdown. The bean counters were not happy (but who cares, right?).

Those braking systems are not DC injection by the way. They are essentially a reverse sequence cyclo-converter; a softer form of plugging. That's why you can hear the pulsations in the starter and the motor when they are stopping. But I don't think anyone has DC injection brakes for MV motors as of yet, so that may be your best option.

 

[trallio]

Those braking systems are not DC injection by the way. They are essentially a reverse sequence cyclo-converter; a softer form of plugging. That's why you can hear the pulsations in the starter and the motor when they are stopping. But I don't think anyone has DC injection brakes for MV motors as of yet, so that may be your best option.


Honestly, if I had it to do over again, I am not sure I would not have suggested a mechanical air brake.

This thing has been nothing but trouble from day 1. It did not help that our Elec Contractor was poorly vetted and turned out to be better at home security than 480V motors of any size. They were "overmatched."

Thanks for the help!