Gremlin in the Motor? 1

[brinkmann27]

Monday I'll be visiting a customer with a motor problem. Motor is 10 HP 230/3/60. Wye- delta 9 wire motor. At one time this motor was running just fine in the system.Then the motor was blowing the 60 amp fuse along with tripping the motor overload in the controller after running five minutes. They removed the motor and sent it to a motor shop for testing. The motor was tested and returned. Not sure of repairs (if any) that were made. No evidence of burnt wires and motor connection box is fine.

Once re-installed the motor ran for four hours and blew the fuse/tripped O/L again. Replace fuse and ran for 2 hours. Replace fuse and ran for 15 minutes. (gets progressivily worse).

They claim 1)Voltage is correct 2)Motor amperage is correct. 3)Rotation is correct.

I had them dis-connect the motor (at the motor) and contactor pulled in and fuse did not blow. They left it all day.

My ideas: It appears something in the motor is changing once the motor comes up to temperature. And gets worse the more times they change fuse. I not sure what could change enough to short circuit the motor and then "heal" itself to run again after replacing the fuse. Usually when they "break" they are "broken"

If they are losing a phase (momentarly) would this cause the fuse to blow or just trip the O/L?

If the motor was connected incorrectly (Wye or delta) would it even run?

But what would cause a short circuit (blow fuse) and not completely damage the motor or wiring?

Thanks in advance for any suggestions

 

[itsmoked]

For starters from all you have stated... I would go right to the load! The motor shop sez okay and the motor spins. This all screams "Load Problem".

That said, make sure you have some way to examine the load when you get there.

Yes certainly single phasing the motor could easily result in blown fuses and tripped overloads. Again not a problem with the motor!

The varying times to trip also scream load. It is running fully loaded or slightly overloaded the fuse warms up to where it is seriously considering tripping then the load peaks for whatever reason, fuse smokes instantly.

Make sure you take a clamp-on amprobe so you can check this possibility.

Also if the motor is supposed to be running delta and is wired wye, depending on the load you would also be blowing fuses.

 

[Marke]

Hello brinkmann27

There are two possibilities that I would be following.
1. An intermittent shorted turn which is temperature related. I have seen this sort of problem before and it only showed up under test when we put the motor into an oven and megger tested frequently. In this case, it was phase to phase in the centre of the windings, just not quite enough energy to make a big splash. A shorted turn within a winding is not quite so easy to prove while it is intermittent, but I would once again heat in an oven if possible and monitor the impedance of each winding. (AC impedance rather than DC.
2. I have seen this sort of problem due to the autoreclose effect. - momentary interruption of the supply. I have found faulty contactor coils and control circuit problems that have caused such intermittents. A common problem is related to auto/manual switching where the switch is able to momentarily open while switching from auto to manual or similar.

If the fault energy is sufficiently limited, an intermittent short can cause the fuse to fail without creating a permanent fault. This is a real problem when there are semiconductor fuses in the circuit.
If the problem is an intermittent shorted turn or similar, one could argue that the motor is already faulty, so increasing the fault current (larger fuse) may change the fault from an intermittent to a permanent fault.

Check that there is not a poor termination near the fuse that is causing the fuse or holder to get hot. This will cause the fuse to fail at a lower current.

Best regards and good luck,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

I'm with itsmoked on the problem being with the load. I would take a strobe lite with you as well as a clamp-on ammeter. If the motor is wired correctly, it will slip more than the nameplate value if it is overloaded. You should also see higher than nameplate current under overload conditions.

 

[jraef]

Excellent responses from Marke and itsmoked, I agree with both of them as things to investigate. Another one that I would add is that did you check continuity on the main Delta (run) contactor? It possibly could be that one contact on that contactor is not making properly, and essentially single phasing the motor AFTER it is in full run condition (or partially single phasing by having a high resistance connection). If it were an Across-the-Line starter, the motor would not even start with single phase. Because it is a Wye-Delta however, it starts but it can't run continuously. Then if the load is variable as itsmoked says, it would take different amounts of time for it to blow fuses or trip the OL under 1 phase conditions. It may take a recording ammeter to see the problem, may places rent those f you don't have one.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Barry1961]

The voltage could be fluctuating either across all three legs or just one getting pulled down by another load turning on. Just a little imbalance can have a big and sudden effect which they might not have caught.

Barry1961

 

[brinkmann27]

Thanks to all for your ideas...I know it's hard to formulate an answer with limited information.

Round 1.. Went to site this morning and found the following:

Motor is 20 HP 460/3/60 Fed from Square D MCC with across the line Square D NEMA rated contactor. (found T3 lead loose on bottom of contactor)

Then it goes to an explosion proof enclosure with dis-connect outside, with GE fused dis-connect inside. (Disconnect is in poor condition with some corrision). Customer had actually by-passed the dis-connect and jury rigged connections the the tops of the fuses. (for testing only, they say)

Then it goes to an exposion proof enclosure (junction box) with terminal strips. In good condition.

Then it goes to the motor.

Motor and centrifugal pump had been removed and were tested satisfactory. Megger readings on the motor were fine.

Cables- All meggered fine except 2 wires from exp. fused dis-connect to exp j-box. T2 was 400K ohms to ground and T3 was 500K ohms to ground. This was verifed using 2 different meggers. As these cables are in conduit with seals no visible inspection was made. total run of the cable was 10 feet.

With these faults I didn't see any reason to power up the system and blow up a few more fuses.

My recommendation was to replace all 3 wires from the dis-connect to the j-box, replace the GE dis-connect switch and check/tighten all connections. I believe a combination of the the loose connection (T3), low readings of the cables
(not a dead short) and the dis-connect switch condition allowed the motor to run, heat up and overload the circuit. As stated above this would happen in a matter of minutes.

Customer appears as they will respond quickly (as system has been down for a couple of weeks) so we should know if we were correct.........

Thanks again

 

[Marke]

Hello brinkmann27

Thanks for the update. It is great to get some feedback. All too often, the "input" disappears into a black hole and we never learn whether we have actually helped solve the problem!!

Your findings certainly could be the answer, but often, I have found that the first findings are a smokescreen. Be prepared for round 2, hopefully, it will not happen!!

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

They claim 1)Voltage is correct 2)Motor amperage is correct. 3) Rotation is correct..

Looseness can locally generate heat which melts the fuse, single phasing the motor supply, and then unbalance currents trip the overload protection.

 

[itsmoked]

Nice out-ta left field call there, jraef. jraef gets the Kewpie doll. (for round one)