Repeated Starting Causing Magnetic Breaker to Trip?

[adamscb]

Forum,

Long story short, we had a 60 hp motor that was started 21 times in about 13 minutes. The system was set up so that the motor started/stopped based on a level indication, and that level hovered around the start/stop point. What I'm confused about is that the HMCP circuit breaker eventually tripped. I thought HMCP's only operated on magnetic principles?

(it was eventually restarted later, we learned of these excessive starts after the fact, so there wasn't a fault, thankfully). Could you guys provide some insight into this? Much appreciated. ​

 

[ptonsparky]

What value is the HMCP?
What type of load?

I’ve had small motors that would trip on occasion. They would not always hit the magic #. We caught it with a PQ meter

 

[Ingenieur]

you are lucky it tripped
the motor is rated for max 6 starts per HOUR
with a min of 85 sec cool down

you had >80 with maybe 15 sec

 

[ptonsparky]

I’d be more interested in why his overloads didn’t trip. Somethings screwed up.
Time to check the contacts on the starter as well.

 

[Ingenieur]

I’d be more interested in why his overloads didn’t trip. Somethings screwed up.
Time to check the contacts on the starter as well.

:happyyes:

probably smokin' or fused lol

 

[JFletcher]

You're going to fry that motor starting it so many times and such a short amount of time. You need to adjust your operational processes so that it does not start so many times, install a smaller motor that can run longer, or put in a vfd

 

[Jraef]

Most likely as the motor windings heated up, the resistance dropped and the inrush increased. Either that, or the re-starts were so fast that a re-start took place when the magnetic fields of the previous one had not yet fully collapsed, so the motor was still a generator being connected out-of-phase with the line. That can cause a huge current spike. It can also cause a huge torque spike, one that I have seen twist the shaft off of a 500HP motor (a similar thing can happen in Wye-Delta starters). So in a way as others have said, you were lucky that's all that happened.

 

[adamscb]

This is all good insight, thanks everyone. Unfortunately I don't have the power to change this system, being in the maintenance department, but I told everyone what needs to happen.

At first operations thought this was a PLC wiring issue in the bucket, because the PLC said the breaker was tripped but they thought it should just say that the motor was off. Another day in the world of maintenance watching operations trash their own equipment (I'm in an alcohol industrial plant, the old Hiram Walker plant in Peoria, IL)

 

[adamscb]

Most likely as the motor windings heated up, the resistance dropped and the inrush increased.

Jraef, I was under the impression as the temperature of a conductive material increases, the resistance also increases? Or am I ass-backwards?

 

[ptonsparky]

This is all good insight, thanks everyone. Unfortunately I don't have the power to change this system, being in the maintenance department, but I told everyone what needs to happen.

At first operations thought this was a PLC wiring issue in the bucket, because the PLC said the breaker was tripped but they thought it should just say that the motor was off. Another day in the world of maintenance watching operations trash their own equipment.

Wow...PLC.... even I could have thrown a warning, flag or shutdown on that operation sequence. Sounds easy from here.

 

[Jraef]

Jraef, I was under the impression as the temperature of a conductive material increases, the resistance also increases? Or am I ass-backwards?

No, that is true of the CONDUCTORS, they have a Positive Temperature Coefficient (PTC) of Resistance, so the resistance increases as temperature increases. But at the same time, extreme excesses in core temperature can affect the magnetic permeability of the steel laminations (sometimes permanently) and since the overall inrush current is a combination of wire resistance AND impedance, which is affected by the magnetic flux in the core, the overall effect can show up as a slightly Negative Temperature Coefficient of Resistance, especially as motor sizes go up. When I was involved in making DC Injection Brakes, we discovered this (the hard way). A motor is PTC up to and including normal operating temperatures, but if it OVER heats, it starts to become NTC. We blew up quite a few DCIB power devices over that little nugget of knowledge... Turned out we should have read more about it first.

I realize now after re-reading my earlier post that I did say the "resistance" would decrease, so that was incorrectly worded. I was just thinking of the inrush increasing, which would be because impedance can decrease if it is over heated.