Breaker trips when motor runs for six seconds on 45 min intravels

[dvhamil]

We have a scenario in which a motor comes on every 45 minutes and runs only 6 seconds. After about 6 to 8 cycles of this the motor circuit breaker trips. When in the manual mode of continous run, there is no problem. The motor is 3 HP, 480V with 15 Amp MCP set on "B" (a 60 amp trip setting). What could be causing the trip.

 

[buzzp]

I am not sure what kind of breaker you have but motor protection uses thermal capacity of the motor to determine when it should trip.
I think of it as a bit bucket. As the motor runs above SF, a bit is added to the bucket. The number of bits deposited in the bucket with each increment of time, is determined by how much over the limit you are, the more over you are the more bits you need to add. As the condition persists, more bits are added with each increment of time. If the motor runs at nameplate, bits are taken out of the bucket. This repeats itself over and over until the bucket fills up (trips the motor). Each start adds a lot of bits to the bucket in a short amount of time, whereas once it starts and runs at nameplate, the bits are slowly taken out of the bit bucket.
Does this make sense? I suspect what you are experiencing is simply protecting the motor. I think if you looked at the trip curve for the breaker (or whatever is tripping) you will see exponential curve that reduces the trip time based on how much over the limit you are- you will not just see an instantaneous trip at 60A)

 

[Barry1961]

A few ideas.....

The motor could be having a hard time starting from time to time due to a mechanical problem and the circuit breaker is just doing it's job. For example a compressor failing to unload.

There is a high ambient temp. in enclosure causing a quick trip.

The motor may have a hard time starting all the time resulting in a 1 in 8 chance of the breaker tripping during start. You just have not hit the 1 in 8 chance yet in manual mode.

The breaker is bad.

Barry1961

 

[electricpete]

Can you describe what the motor is driving? Is it possible that after 5-6 starting cycles, the mechanical load during start begins to increase?

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[dvhamil]

Electricpete,

The motor is driving a belt on a magnet, and the trip occurs even when there is no additional load.

 

[peebee]

Are you tripping the MCP, or the thermal overload?

It would make sense that you're tripping the overload giving the severe duty that you're imposing on the motor. But it sounds like the OL is holding in and you're tripping the MCP -- that's strange.

The MCP shouldn't be tripping -- not unless, perhaps, you're trying to start the motor while it's still spinning. . . . You're MCP is already set beyond the 800% of FLA permitted by 430.52 (granted, there's a semi-applicable Exception here relating to motor starting).

It's critical that you identify if the MCP or OL is tripping. Periodic starting like you're doing is tough on a motor due to the high magnitude starting currents. Wiring and OL sizing may need to be increased.

430.22(E) and Table 430.22(E) may be applicable -- these would required sizing the wiring at 200% of motor FLA rating. 430.32(C) might also be applicable, this would permit you to increase the OL size to 130 or 140% of FLA. If your OL is in a high-temperature ambient, that could also be causing the problem, you might need to derate the OL

430.33 would permit you to run without an OL device -- short circuit protection only. BUT -- then you'd be required to stay within the requirements of Table 430.52, which you've already exceeded. . . . If this is the way you're set up, with no overload, and you can't hold in at 800% of FLA, then it would seem that either 1) you've got a problem with your motor, or 2) your motor is undersized for the application, or 3) you've got a bad MCP.

 

[peebee]

Oops -- I originally misread your post -- thought you said it ran for 6 seconds every 45 seconds.

BUT, you didn't say that, you said 6 seconds every 45 minutes. That is NOT a very severe duty cycle.

Ignore most of what I said above. Sorry.

My guess is a bad MCP or bad/undersized OL.

 

[busbar]

Might do a per-pole trip test of the MCP with a small breaker/motor-overload test set.

In a pinch, something might be kludged with welding cable, C200-C400 CT and a 2-5 kVA Powerstat.

 

[jraef]

I will add that there is also a potential problem with your process. The curious factor here to me is that it runs fine in "manual" continuously, but trips only when in that 45 minute "auto" mode. That is far from a severe duty cycle, your thermal trip elements are not even close to coming into play here. I would suspect that something else in your control scheme is occasionally hanging up, allowing the load to be coupled on startup one out of every 6-8 cycles. When coupled to the full load at startup, the motor sees locked rotor conditions and exceeds the magnetic trip settings of your breaker. This happens quite often on hydraulic pumps where a bypass valve sticks.

"Venditori de oleum-vipera non vigere excordis populi"

 

[sreid]

There can bee a fairly high pulse of current into the motor depending on where in the AC cycle the motor is turned on. If the start contactor closes at or near zero voltage crossing, the current will be "well behaived." If the contactor closes at or near the AC voltage peak there can be a brief, larger current surge. If the circuit breaker is a magnetic type, it may unconditionally trip above some current level.

 

[JesperMP]

Does it trip immediately (MCP trip) or after a little while (OL trip). Granted, it can be difficult to see, so you may have to provoke some starts to be able to see it.

Also, measure the starting current to see how long time before it drops down. It should be less than a second for a small belt drive. Compare the current with the breaker setting and the motor nameplate.

Is this a small belt conveyor with a magnet inside for removing metallic items from another conveyor ?
If so, is the magnet a constantly magnetised type or an electrical type ? If it is an electrical magnet, does it have its own supply, or is it supplied from the same circuit as the motordrive ? There will probably be a transformer/rectifier for the magnet, and such things have very high inrush currents. Could be that this one is tripping the breaker.

 

[light1]

A lot can be deduced from monitored motor current waveforms. The current amplitude could be compared to the motor LRA, and protective device settings. A defect in MCP might be deduced, if the amplitude waveform is normal.