Dear Expert,

Please, all your input will be highly appreciated. We have 160 kW, 6.3 kV motor. This is vertical motor and direct couple to Sundyne pump. When we tried to run, motor could not start!!. And there is some message appeared in our Sepam M20 motor protection. Earth Fault and Phase fault.

We did some test for motor.
We megger motor with 5000V and result as follow R-G=68 Mohm, S-G= 94 Mohm and T-G= 87 Mohm.
We measure the winding resistance R-S = 3.5 ohm, S-T= 3.5 ohm and T-R= 3.5 ohm,
also we megger nmotor power cable also (we disconnected from VCU to motor terminal), and result R-G=75 Gohm, S-G=59.5 Gohm and T-G = 59.5 GOhm also R-S = 118 GOhm, S-T= 142 Gohm and T-R = 141 GOhm (with 5000V)

We did also Pressure Test with 9 kV DC / 10 minute / phase and result is ok. all value less than 1 microvolt when 9kV DC applied to each phase (the leakage).

We took data also with All Test Pro and test result is good winding

For Polarization Index is 2.4.

We tried once more to run but motor still fail. and also Earth Fault and Phase Fault messages appear in our MPR display.

What is happen with this motor?

For a 6kv motor, your megger readings are a little on the low side. IEEE43 specifies 100 megaohms temperature corrected minimum and typical readings are 500 megaohms or more. What was the winding temperature and ambient temperature? Were the motor space heaters kept energized?


Is it a new installation (motor and relay?). Any maintenance lately?

This message has been edited. Last edited by: electricpete,

Dear Pete,

ambient temperature was around 30 deg. C. The space heaters (200Watt) kept energized during motor out of service. This is old installation ( 1996) But for relay 2003 installed (replacement with this type) and always calibrated every 2 years. Lat calibrated on June 2007. No maintenance before for motor.

You might as well re-check the relay (since motor seems to check ok unlesss it is an intermittent fault).

And since this is 4kv I assume relays are fed by CT's - it doesn't hurt to do some routine checks on the ct when associated secondary and primary circuit is deenergized. Disconnect the relays from ct circuit, lift the ground (probably at relay end of the circui) and megger with 250 volt. Using this type test, we once found a ground on the leads to a transformer multi-ratio ct once (ground was at a terminal box on the side of the transformer). That unintentional ground combined with the one intentional circuit ground to change the circuit in a way that changed the effective CT ratio and caused an different effective CT ratio which tripped the differential relay.

Also do a magnetization check of the CT (again disconnected from the relay with secondary and primary circuit deerngized). It involves applying varying ac voltage using a variac to secondary and measure current to recreate excitation curve. Start low and increase voltage until you see the knee of the curve (all three phases should look similar). As a very rough ballpark, the knee occurs approximately at the number of turns times one volt (one volt per turn). We actually did have a ct go bad internally (shorted turn) once and cause a 13.2kv motor trip on starting "negative sequence current" (which is like unbalance) and the one failed ct showed up much different than the other two ct's on this magnetization check. It is not a very common scenario - to my knowledge that is the only CT we ever had that failed internally during the 20+ life of our plant.

Were your insulation resistance tests temperautre corrected? If not, I would like to know the winding temperature if you have it so I can compute temperuatre correction.

If your readings are temperature corrected, it would still be helpful to know winding temperature and compare it to ambient temperate as an indicator of how effective are the space heaters... they should keep winding tmempearture somewhere aroud 5C above ambient temperature. And if your readings are temperature corrected, it is a little puzzling to see all three readings so low. I assume these were all taken by meggering one phase with other two phases grounded? These results would not be consistent with a localized fault. Maybe they do suggest a wet winding, although that typically would be accompanied by low PI.

Another question to ask. Is there anything else connected to the circuit within the relay protection zone downstream of the breaker that may have been disconnected during your megger? Power factor correction capacitors? Surge capacitors? Partial discharge monitoring capacitors?

Also is there any idea how long between the breaker closure and the trip? Did anyone see whether motor shaft rotated or not?

If motor checks good and relay/ct check good you are in a trickier position and really need to start looking at smaller details. I will brainstorm a few possibilities.

One thing I noticed you reported winding resistance to one decimal place. A good bridge measures to milliohms and gives better chance of detecting solid turn short. Of course I think the All Test pro high frequency test probably does a better job of detecting a solid turn short than a dc resistance test since solid turn shorts show up more dramatically at higher frequency than at dc. Maybe if you provide the specific All- Test results, some of the other readers will recognize something unusual in there.

You might also want to consider trying a dc step voltage test and surge test. These are slightly more probing (and potentially destructive tests... ESPECAILLY if you are conducting them on a machine with marginal insulation resistance measurement). If there is a weakness which causes an intermittent fault during starting, these might expose it. Of course if you can pull the motor for shop inspection, there are a few more options. Lights out test with ac hi-pot applied is one.

Another option that might be feasible in-place without removing the motor (if it is a horizontal motor) would be to pull the end-bells and do a visual inspection of the end windings to see if there is any obvious indication of a fault (that presumably is intermittent since didn't show up in the test.) By the way, did the fault show up one one phases or two? If it showed up on two as an intermittent fault, I would say the endwinding is a much more location for an intermittent phase to phase fault than the slot.

If it's a vertical motor and you can't pull the endbells, you might try instead peaking into accessible locations like inlet/outlet vent ducts with a boroscope. I have done that myself a few times, but it's a longshot that you'd ever find anything that way. You only look at a very small area at a time that way and it's very easy to get "lost" and forget where you are and go around in circles.

One more option we have done is to install monitoring equipment to try to measure the currents during starting. Most likely your 4kv cables are shielded so you are forced to monitor the ct's.... is not quite as independent of a test as you'd like but canstill be helpful. Compare measured current to relay setpoint (or relay indication). If it doesn't match, it points to a relay problem. You can also compare to expected starting parameters to see if it matches what you think it should be.

Maybe others have some more ideas.

This message has been edited. Last edited by: electricpete,

Hello Janoajah,
If you started 6,3 kV motor couple times with same fault mesage and after that you don't have big hole in stator core and windings and good megger readings I think that your motor is OK and you must, like Pete said, pay antention on your motor protection and his protective current transformers
In my factory, recently, we had similar problem with earth fault trip of 2 MW motor. We have old earth fault directional rele with residual current and voltage transformers. The problem was instalation of cable through the residual current transformer because the neutral wire of 6kV cable is not back through transformer and connected to the ground and it always give 80mA false earth fault current. Then we had sometime earth fault on another motor 3km away and 10V residual voltage on earth fault rele and tripping of motor.
Another idea is shecking of HV breaker. If one pole is slower then another two you have diference in current and sometime good reason for earth fault and trip of the motor.
Happy New Year and good hunting for everyone!
Panta Serbia

This message has been edited. Last edited by: Panta,

Dear Pete and Panta,

am sorry for delayed to reply. I had annual shutdown. For your information, I have a new data for this motor. 2 days ago, the motor protection relay vendor came to our plant and they checked, everything is ok (function, wiring, control and so on. And they show me about the current starting motor and result very unbelievable. For U phase we got 315 ampere, V phase 516 amp and W phase 65 Amp!!. What is this? I informed you last time, we have a good insulation resistance, a balance winding resistance and PI 2.4, why starting current like this? what is happen with this motor..? Should I apply surge test or another test for the winding (like Mr. Pete said)? it need to rewinding?
All input are welcome and I will be appreciated it. I attached also data from All-Test Pro Condition Calculator3

What is the weather like? Is this motor TEFC (totally enclosed fan cooled) or ODP (open drip proof)? If there is a lot of moisture in the air, the motor will have low meg ohms, especially if it is an older winding. Also, if the windings are coated with a lot of dirt and oil, this can result in low meg ohms as well.

quote:

R-G=68 Mohm, S-G= 94 Mohm and T-G= 87 Mohm

quote:

GOhm also R-S = 118 GOhm, S-T= 142 Gohm and T-R = 141 GOhm (with 5000V)

These values should be nearly equal.

If you can eliminate the the control system as being the problem, then I would lean towards the insulation breakdown in the motor windings. Somehow the electricity is tracking to ground and phase fault. Your windings might be saved by cleaning and retreating - Which still means pulling the motor.

This message has been edited. Last edited by: THMotorMan,

the humidity around 80%, this is TEFC.
THMotorMan, you mean, it no need to rewind? only cleaning and retreating?
Would you like to explain to me, why starting current is big different? what is cause it?

Thank You

Did you check your incoming voltages? Are they even as well? If they are even, then likely your motor has gone bad.

Sometimes a motor will get dirty and start tripping offline. Moisture is a big cause of this. Sometimes it just needs to be cleaned and the moisture removed, then retreated. Hopefully the winding has not been damaged to that extent. However, if your starting amps are that different, check your incoming voltage to make sure that is not the issue.

Whether it is a rewind or not, it should be pulled and inspected throughout.

The full load current on this motor should be roughly 18 Amps. It means the locked rotor current must be about 5* 18 = 90 Amp. Therefore, the 516 and 315 Amps must be the result of a short.
I believe, there is a short between phase U and V. This short comes and goes with the dynamic forces between the conductors of the winding. The forces of course appear only with substantial current, in other words with substantial voltage. As usual, the low voltage shows nothing.
jank

Dear All

We disassemble the motor and we found (see attach ment). A part of winding (U and V)look like worn out. Like Jank said, maybe there is short between of them. We found also some moisture trapped inside them.
The Question : why we can not detect it? because the winding resistance, insulation resistance and also all test pro showing the good value?
Please, any suggestion, advive or comment are welcome

PICture_GB-151.doc (532 KB, 41 downloads)

Hello
its quite interesting example.can you upload your ATP IV results here if you have tested with it? have u tried with any other low voltage testing tool?
i request to leaders in low voltage testing to give atleast reply why low voltage tester has failed to detect this. ( PDMA and Mr.MotorDoc ????) or you will still say there is nothing wrong with the motor???



thanks

I, along with many others that use the low vltage testers, do not believe that a turn to turn short can be picked up by low voltage testing. I have some documents from GE from testing they did, which says the same thing.

The RIC test falls into the same category.

This horse has been kicked.

D

I wanted to kick in my two cents here. I usually just watch and learn, but hopefully I have something to contribute as well. Let's look at your winding a couple of different ways. First let's look at the insulation test. I'm assuming a megger or hi-pot was used to perform this test. It only measures groundwall resistance. As an example, take a 1000 turn coil of bare copper and wrap it in mica-mat and tape it for on a DC pole piece. You can test this coil and get a good insulation reading (megger or hi-pot) and still have a totally shorted coil. The resistance reading will be off though. Now use wire with a film coating only, and make the same coil, wrapped and taped. The insulation reading will still be good, AND the coil resistance will be good with a low resistance meter. Finally, scrap off the film between two adjacent turns in the last coil we made. Everything will still pass the previous tests of coil resistance and insulation resistance. There is still a physical gap in between the turns that must be bridged. The only way to bridge it is with voltage between the two turns. If you do not exceed that voltage, you will not see that fault. I believe it is a pretty substantial voltage to jump across an open air gap. I don't know for sure what that number is, but I have been told it is around 350 volts to detect a turn short in an armature. I don't know what the ATP voltage is, but since it is being called a low voltage tester, it probably isn't sufficient to brige the gap. Surge testing may or may not have found the fault in this case, but is what I would have recommended. Hope it helps. Please feel free to correct me on anything I may have said. Sean

quote:

Originally posted by Sean McGrady:
Surge testing may or may not have found the fault in this case, but is what I would have recommended.

I would bet on the surge tester for this fault also. Seen it too many times prove out.

D

quote:

Originally posted by THERMOGRAPHER:
Hello
its quite interesting example.can you upload your ATP IV results here if you have tested with it? have u tried with any other low voltage testing tool?

thanks

I submitted result of ATP 3.1 already. I am using another tool (DC Low Ohmmeter ADEX)also we got same value

This message has been edited. Last edited by: jonoajah,

quote:

Originally posted by RRS_Dave:
I have some documents from GE from testing they did, which says the same thing.

D

Dear Dave, if you dont mind, would you like to send me that documents to electrical@styrindo.co.id , please?

Jonoajah,

I have sent the documents. However, the GE one was about the RIC test, or measuring the change in inductance as a function of rotor position. I have a problem with this test also. Depends too much on residual currents, and the phase of the moon.
I have included another document that you may find intersting on measuring negative sequencing as a result of turn to turn short.

Hope they help.

Dave

Dear All,

I would like to say thank you very much for all.
Our motor could started now after partial repair of winding.

Regards,

jonoajah