AC motor overload after rewinded 4

[mtchufi]

Please help my problem, Thanks
I've several AC motor on my production line used for cutting the edge of ceramic corner (grinding), one of them has broken and I've already repaired in whork shop of motor rewinder. The problem is, after rewinded and installed to the machine, the thermal protector is always trips after several minutes started. Here is the data of the motor:
V=380 V 3 phase, Amph = 43 A
Kw = 22KW, HP = 30, Rpm=2880 , Hz = 50 Hz
When the motor started, its seem normaly, the current normal after started is about 12 Amp. but after the load applied the current rise up to 50 Amps and more. I compared with other motor, the other motor is, after started 12 A, and when loaded, its about 32 Amps. why??? thanks your help.
for addional info: the current was normal after I change the motor. The mechanical properties dosn look any problem. Try to rewind again..the problem is still same..some one can help me ??

 

[jOmega]

It sounds as though the motor does not have the same torque capability after the repair.

To determine, motor needs to go back to repair shop and operated on dynamometer to determine how much torque it can produce.

The may not have enough turns in the slots to produce the required air gap flux.

 

[electricpete]

I agree... a torque test would show whether or not the problem is in the motor.

Another possibility - low or unbalanced voltage to this motor (and not to others).. perhaps high resistance connection.

Another possibility - have the characteristics of the mechanical load changed? Maybe the blade got duller?

What happened to the original motor that it needed to be repaired?

 

[rbulsara]

Did you ever investigated as to why the original motor failed?

You may try this motor to a different load with similar HP and see what happens. If it misbehaves there too then its the problem with motor re-wind.

May be its rotor is not the same as before? Was it damaged in the original motor?

 

[UKpete]

If the winding specification was changed maybe by mistake during the first rewind, there isn't much you can do about it now. The second rewind won't have helped, the repair shop will just copy what was done in the first rewind.

What is strange is that the no-load current appears correct. A winding mistake or a faulty rotor may be expected to affect the no-load current as well as the load current.

Are you completely sure of your checks?

 

[jOmega]

UKPETE:

For what its worth.... an unloaded motor doesn't need a lot of current to keep it rotating. Most of the current that is measured is for magnetizing the iron.. and is reactive. The torque producing current is the in-phase current component, which you don't see on an ammeter.

Consider this: with a VFD that allows the V/Hz to be adjusted, and with no mechanical load on the motor, measure the current.... then reduce the V/Hz in 0.5 V/Hz increments, recording the amps at each step.

Did you see much of a change in the amps, even though you are effectively reducing the air gap flux.... ?

Not really. (unless the motor has a lousy power factor)..

Just something to think about.

Kind regards,
jO

 

[mtchufi]

Thanks to all replied my question..
I want to explain what the action i ve taken.

1. I try to change with the other motor ( its contain 11 motors in the machine with the same spesification). When I use the motor to drive the load in the problem side, the motor is OK, It means that the thre is no mechanical problem ( Overload or friction ), and when the repared put in the normal side, the problem is coming, so the problem is still in the motor.

2.I try to change over the Rotor with the normal motor,..when its started together, what happen is :
a. The motor that using the originalwinding with the repaired Rotor have problem, the current is abnormal, too high.
b. The motor with rewind the coil with original Rotor seems normal.
so... the problem is in the rotor of the rewinded motor..
The Question is: Why the Rotor is problem???, Its seem no difeerent between the NG Roter and the original Rotor...
As Additional Info: The motor was Broken by the jammed bearings, and the winding is burned out at last.
Any way.. I dont useed that motor anymore, I used the new spare, but I just want to know why It could be happened?
Thanks and warm regards to all..

Mtchufi

 

[UKpete]

jO - thanks for putting me straight. I don't to look at faulty motors very often, I confess it didn't come from first-hand experience hence the tentative language.

Mtchufi - thanks also. There are many reports of faulty induction motor rotors that I am sure others can supply, it doesn't suprise me.

 

[edison123]

mtchufi,

Broken rotor bars due to locked rotor operation ?

 

[electricpete]

edison's scenario (cause of rotor damage was locked rotor) makes sense to me, since:
1 - the problem started when the bearings jammed.
2 - locked rotor can cause high heating in both the rotor and the stator.
3 - rotor bars of small cast-aluminum rotors like this are not visible for inspection.

 

[mtchufi]

Thanks to all,
I Thinks "electricpete" give reasonable answer and for moment this the answer what I need and I will use it as my refference for another similar problem.
Thanks so much for all your helps.

warm regards,
Mtchufi

 

[jbartos]

Suggestion: The power supply should be monitored for its quality. The motor shaft load should be monitored for HP or torque. The motor should be properly grounded. The repair shops usually do a good job since they are testing the motor before shipping out. Else, they would not stay in business.

 

[jOmega]

JB:
You would be surprised at the number of motor shops around today that DO NOT have dynamometers. Often (more often than not)... the only test they see before being returned to the customer is a power up test... and no load amperage check.

Consider: if the shop that repaired mtchufi's motor had dyno tested the motor before returning it, they would have discovered the problem; all other things being equal and proper.

 

[mtchufi]

To jbartos and jOmega,
Actually it is right as you said that here we (I) cannot find a good shop repair completed by the standard test of repaired motor. just for information : my location is in a small city in a south east asia country. so, do not compares with yours, never thinking about dynamometer, or dyno test.
What they do is just merger test and no load test using the variable voltage output transformer. The power supply given by the electrical suppiler cannot cover the big motor. Normally they only had under 20 KVA (32 Amps). Thats why almost the repair motor do not be tested at load condition in the repair shop.
All of your reply above are very heplfull for me, thanks

mtchufi

 

[GordS]

I have also experienced motor problems following rewinds. My application involves conveyors where I sometimes need a high breakaway torque to start a belt that has tripped with a load. I have had 2 cases recently. In one case we found an undersized rotor. In another case we found excessive wire in the end turns that looked like were hammered in to fit into the motor. In both cases, we noticed that the starting current drawn was much less than was supposed to be ( in the range of 60%).

As a result, we now do an inrush current check before we install motors on conveyors. Your shop should be able to do this either with a chart recorder or a clip-on with a peak hold function. This could be another check to for rewind quality.

 

[jbartos]

Suggestion: 43A drawn by the motor is reasonable. 50A or more drawn current is excessive. There may be some winding inter-turn shorts. They might be observed by suitable resistance/impedance measurements/testing.