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Discussion Starter · #1 ·
Hi everyone, this is my 1st post so if I'm doing something out of the norm please forgive me haha.

I currently have an issue I've only seen one other time and can't seem to get to the bottom of it.

Both cases involve 600v 3 phase motors tripping a breaker in a bucket of an MCC. One motor is turning a piston-like pump, and the other is a roof top exhaust fan. Both motors are controlled by a contactor with overloads. The overloads aren't tripping, it's the breaker tripping so that typically points to a short. In both cases I've used a megger and no shorts are present. I've tightened lugs in the circuit. I even went as far as to replace the $1000+ breaker.. Everything is seemingly normal including running amps equal across the 3 phases.


Canadian code says the instantaneous trip setting can be 1300% of the FLA. I have dialed that setting up way beyond code allows, and that seems to only hold for a short while, maybe a day or two. Of course I don't want to leave it that high.. I must be missing something else. Does anyone have any insight to a problem like this? If you need more info please ask. Any help would be awesome!
 

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Master Electrician - Ontario
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I can't say for sure about the pump, but I have seen this when a fan (cooling tower) is spinning backwards because of wind then the conditions require the fan to start and it has to stop the reverse rotation. Essentially because the fan is stopped, it is like shorting out the motor vice overloading it thermally.

Perhaps there is a similar back pressure on the pump which is stalling the pump.

Cheers
John
 

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instantaneous trip setting can be 1300% of the FLA

That is what the breaker uses when the motor starts. As the motor is starting adjusting this might allow a short inside the motor to be ignored but that is a band aid that wont last long.

With a standard meter you will struggle to see whats really going on. A lot of stuff is happening with in 1/60th of a second and a normal meter will not detect the problem.

we measure the amps and see if there is a flicker (unstable reading, overload). We ohm the motor winding"s, We meg the wires to ground, we disconnect the wires and insulate the ends, then meg between them, We meg the disconnect open and closed and we do voltage tests.
(as you have 2 motors be extra careful that you no not have a short between the wires of both motors. Turn both off while testing in case they share a conduit and meg between them, this normally shows up if both trip at the same time)

The best option unless you are on a emergency, is to call a rewind place and have them bring the equipment on site and do a motor test. (i can get this done for free but if your a small company then they will want to charge for the service)

If you are on a emergency and you do not have the tools then its a guess. Either the motor or breaker.
 

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Old Grumpy Bastard
GOV/MIL contracting
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Welcome [email protected]!

I have seen what Navyguy said about fans many times and agree with what he said he said about pressures on the pump. Does the pump have some type of unloader that could be malfunctioning.
 

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ProcessAutomate07

Welcome to the fourm in here and now the next question I going to ask you .,,

What size breaker it is ?

What size motors it is for fan and the pump?

Is the motor set up is DOL ( direct on line ) or it have variable speed control on it.

the more info the easier for us to help you on this details I know you mention 600 volts system so we know that part.

Are both motor started the same time from that breaker ?

Let us know more details one of us will help you on that.
 

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I'm with DVR... the problem figures to be mechanical.

BTW, there's NO WAY that you managed to get the rotation reversed...?

Nothing is seized, galled, ...

You are not looking at the end game of VFD bearing destruction...?

Etc.
 

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I would check to see if you a mechanically bound up at start.

Then i would check for electrical connection problems....I would check the contactor and all connections points. Excessive arcing from loose connection or jiggly contactor could cause intermittent spikes in amps over the normal lock rotor.
 

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I pretwist and then use wire nuts. Solder pots rule.
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A pump and an exhaust fan on the same bucket?

I wonder what the overloads are set to protect?

I'm sure it's designed so both motors will only fail together, at the same time.. :plain:
 

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Discussion Starter · #9 ·
Thanks for the replies. Let me clarify some details.

So the current issue I am having with the exhaust fan is similar to another issue I have had once before with a piston pump motor(raw sludge pump at waste water treatment plant). So 2 separate problems.

Here's some specs:


*New* Exhaust fan - 3 phase, 600 volt, 1.45 FLA, SF 1. Fed from an existing MCC bucket for the old fan so older breaker, contactor and overloads. It's on a 3 amp breaker with instantaneous trip setting ranging from 9-30 amps.


Running amps are L1-1.2a, L2-1.1a, and L3-1.0a

Inrush peak amps were somewhere between 18.6-21.4

I turned the pullies by hand an there's no mechanical blockage although the weight of the fan was a lot for an across the line start. Either way the fan came as a unit so they make them that way all the time.

We did a high-pot test of 2200volts at various points throughout the circuit and 2 of the wires failed the test which wasn't seen by the 1105volt test from the megger.

I don't think that was the issue though as we temporarily strung out a teck cable and bypassed everything by connecting straight on to the breaker and the new disconnect switch supplied with the fan. The breaker still tripped until the instantaneous setting was at 24amps. Then it held.

In appendix B for rule 28-210(b) it states that newer energy efficient motors can have a higher locked rotor current, therefor the rule allows for either 1300% of the FLA OR 215% of the locked rotor current which is roughly where the breaker holds. (if you read the whole rule+appendix the setting may be upwards of 1700-1800%)

This is where we have set it and if the breaker trips then we will probably have to put a new $1500+ 15amp breaker in which is what a motor app is suggesting. The 3 amp breaker worked for years with the old fan and I don't think the specs were much different between the old and new.

If anything else comes up I will post an update. Cheers
 
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