Motor Issue.

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Fordean

Senior Member
Location
New Jersey
Occupation
Electrical Contractor


1st test was to Move motor giving problem to another MCC Bucket. But still used same UG branch circuit to it. This ruled out bucket problems.

Voltage were good to all three phases. Cant remember exact number but 274, 278, And maybe one had a 269. Had 479, 478, and maybe 469 on all. The bad phase I believe had the lower voltage.
Only minor difference their. Did alot of moving around. Wires definitly damaged underground. Megger should bad Real Bad actually But still they worked. Only one came up out of bad range to good , but only in lower ohms scale. Still wasnt a good phase

Next question How to test motor. I megger it. I did not get bad reading on the way I did it. Want to double check procedure.. I need correct megger test for motor.

Test #1 Dont think this was a correct test.
I left all wires connected. 4 thru 9 wired for standard High voltage diagram. ( except the T1 T2 T3, I megged from their. Someone said I should megger T1 (Red lead of meg) and Ground T2,t3, to Case and to ( Black Lead meg).

Test #2 This test I know was a correct test.
I megger T1 (Red Lead Meg, Black to Casing) Fine. T2 (Red Lead Meg, Black to casing) T3 (red Lead Meg, Black lead to Casing) All these meg well. ( Remember All 4 thru 9 leads Tied for 480volts together,
Did not isolate these).

New Motor Got damaged I believe in very short period. Should have meg first. But wires looked brand new so I ignored. But they were damaged. After New motor had same results with original underground wiring.

Actually thought I found A complete sever of Yellow phase. Trying to find out if someone cut for a jumper, as we were testing, but I believe it was severed and fell apart after wiggling. And no one touched it. It was in bundle. Almost positive it was severed. THis was the Above Ground Portion. Could not pull nothing out of conduit underground.
 

Besoeker

Senior Member
Location
UK
I still think OP has lost a phase, probably in his questionable underground conductors. That lost phase maybe is grounded on the motor side of the break therefore motor does run, but there is less voltage on that phase and results in unbalancing of current.
I'm inclined to think that a ground fault that sinks 3.some Amps and reduces the voltage would make it presence known thermally to the extent that it would not likely be a steady state condition. But the motor swap seems to indicate that the conditions were replicated after this.
The currents have bean measured. Measuring the voltages would seem, to my simple way of thinking, to be the next logical step.


Clamping meter around all three leads will give a reading of any current returning via other paths.
I agree.
 

kwired

Electron manager
Location
NE Nebraska


1st test was to Move motor giving problem to another MCC Bucket. But still used same UG branch circuit to it. This ruled out bucket problems.

Voltage were good to all three phases. Cant remember exact number but 274, 278, And maybe one had a 269. Had 479, 478, and maybe 469 on all. The bad phase I believe had the lower voltage.
Only minor difference their. Did alot of moving around. Wires definitly damaged underground. Megger should bad Real Bad actually But still they worked. Only one came up out of bad range to good , but only in lower ohms scale. Still wasnt a good phase

Next question How to test motor. I megger it. I did not get bad reading on the way I did it. Want to double check procedure.. I need correct megger test for motor.

Test #1 Dont think this was a correct test.
I left all wires connected. 4 thru 9 wired for standard High voltage diagram. ( except the T1 T2 T3, I megged from their. Someone said I should megger T1 (Red lead of meg) and Ground T2,t3, to Case and to ( Black Lead meg).

Test #2 This test I know was a correct test.
I megger T1 (Red Lead Meg, Black to Casing) Fine. T2 (Red Lead Meg, Black to casing) T3 (red Lead Meg, Black lead to Casing) All these meg well. ( Remember All 4 thru 9 leads Tied for 480volts together,
Did not isolate these).

New Motor Got damaged I believe in very short period. Should have meg first. But wires looked brand new so I ignored. But they were damaged. After New motor had same results with original underground wiring.

Actually thought I found A complete sever of Yellow phase. Trying to find out if someone cut for a jumper, as we were testing, but I believe it was severed and fell apart after wiggling. And no one touched it. It was in bundle. Almost positive it was severed. THis was the Above Ground Portion. Could not pull nothing out of conduit underground.

Simple testing would mean verifying continuity through windings - for typical 9 wire dual voltage motors you can start with leaving all connections in place and just disconnecting from supply leads. You should have continuity from L1 to L2 and to L3 all being similar in resistance. You should not have any continuity to ground and a meg from any of those leads to ground should result in very high resistance readings. Any failure of testing so far is likely going to mean bad windings. What is harder to find is if you would have a short from turn to turn in same winding and I'm not even sure if I can tell you how to verify something like that. You probably need equipment that is accurate at measuring very low resistance, and make comparisons from one winding to the next. Faults to ground or open circuits are much more obvious to find.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
What is harder to find is if you would have a short from turn to turn in same winding and I'm not even sure if I can tell you how to verify something like that.
Since the overall impedance of the winding at DC will only change by a small fraction of the winding resistance when a turn is shorted, one standard way to check for shorted turns is to measure the impedance using an AC driving voltage (very low voltage). That can be either the normal 50/60 Hz or a higher frequency to make the measurement easier. A shorted turn will significantly decrease the inductance of the winding and will also greatly decrease the Q factor. The general purpose device to make these measurements is called an Impedance Bridge.
On a running motor, a shorted or partially shorted turn will increase the current through that winding.
 

kwired

Electron manager
Location
NE Nebraska
Since the overall impedance of the winding at DC will only change by a small fraction of the winding resistance when a turn is shorted, one standard way to check for shorted turns is to measure the impedance using an AC driving voltage (very low voltage). That can be either the normal 50/60 Hz or a higher frequency to make the measurement easier. A shorted turn will significantly decrease the inductance of the winding and will also greatly decrease the Q factor. The general purpose device to make these measurements is called an Impedance Bridge.
On a running motor, a shorted or partially shorted turn will increase the current through that winding.

I figured someone would offer advice on doing this. I usually can confirm the motor has winding issues, but usually can not confirm exactly what is wrong other than ground faults or open circuits. There really isn't too much else that can go wrong other than turn to turn shorts. Small motors we usually just replace, larger motors go to motor shop and they deal with it.

Of course if you open it up and see discolored insulation on windings, you pretty much consider it toasted, even if it does still run.
 

Haji

Banned
Location
India

Next question How to test motor. I megger it. I did not get bad reading on the way I did it. Want to double check procedure.. I need correct megger test for motor.

Test #1 Dont think this was a correct test.
I left all wires connected. 4 thru 9 wired for standard High voltage diagram. ( except the T1 T2 T3, I megged from their. Someone said I should megger T1 (Red lead of meg) and Ground T2,t3, to Case and to ( Black Lead meg).

Test #2 This test I know was a correct test.
I megger T1 (Red Lead Meg, Black to Casing) Fine. T2 (Red Lead Meg, Black to casing) T3 (red Lead Meg, Black lead to Casing) All these meg well. ( Remember All 4 thru 9 leads Tied for 480volts together,
Did not isolate these).

New Motor Got damaged I believe in very short period. Should have meg first. But wires looked brand new so I ignored. But they were damaged. After New motor had same results with original underground wiring.

Actually thought I found A complete sever of Yellow phase. Trying to find out if someone cut for a jumper, as we were testing, but I believe it was severed and fell apart after wiggling. And no one touched it. It was in bundle. Almost positive it was severed. THis was the Above Ground Portion. Could not pull nothing out of conduit underground.

I believe meggar readings may not reveal the full winding condition of the motor. The best course of action was suggested by Phil Corso

I suggest you inter-change phases L1-L2, and then do it again for phases L2 and L3. The double interchange will not alter rotation. If the line curent-imbalance does not shift, then the supply is at fault.

So if the line curent-imbalance shifts, the motor is at fault.
 
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