11 kw 2pole motor burnt out connected in star 4

[speeddial]

Hi
I have had 2 11 kw 2 pole pumps burn out at the same time.
Both had a converter starters and were found to be connected in star.I have wired the replacements in delta and run no problem.
The motors were 400 v delta 660-690 star.
The manufacturer is insisting that the connection in star would not cause them to burn out but as I understand running at 400 v in star is running undervoltage and not only would the the torque be reduced but this would cause increased temperatures in the windings.
Any technical info to give to the manufacturer would be usefull
regards
speeddial

 

[Skogsgurra]

You will have a lot of slip and a very hot rotor if you run them star connected for any time. The rotor will heat the stator winding and cause failure. The current will probably also be on the high side - adding fuel to the fire.

Cannot understand why your supplier/manufacturer says what he says - does he think you were running them with no load?

 

[aolalde]

If the drive is providing 400 volts and the windings are wye connected, the voltage applied is only 57% of the nominal. This is a severe under- voltage condition, which will increase the line current more than 73% of the nominal. The I^2*R losses of the stator winding are around 3 times the nominal at 690 volts, overheating the insulation to failure.

The motor performance with under voltage is complex, since the rotor slip will increase, and the speed will drop reducing some of the load but the motor runs close to the breakdown torque area, this condition reduces the impedance seen by the line and the line current demand increases.

 

[jraef]

I think the confusion is in the way it is worded. You said the motors are rated 400V DELTA, 660/690 STAR. If you supply them with 400V, you MUST connect them in the delta pattern. If you wire them to 690V, you will get similar HP if you connect them in Star, because the voltage is reduced by the sq. rt. of 3, so 690/1.732 = approx. 400V. Connecting them in the Star pattern AND only giving them 400V was giving them low voltage / low torque, essentuially like giving it only 230V and only 33% of Full Load Torque. This would cause the motor to pull more current in an attempt to maintain slip speed. Even if the motor were completely uncoupled it would have had difficulty doing that, having a load on it guaranteed failure.

I am curious as to why your OL relay did not drop out the starter. It is probably because the OLR was wired into the circuit as if it were Delta, so when you connected in Star it was only seeing 57% of the motor current. Make sure you check that out.

"Venditori de oleum-vipera non vigere excordis populi"

 

[speeddial]

Just a note to help any confusion.
The two motors were each driven by its own inverterdrive.
The motors were delivered pre wired and set up by a major manufacturer and commissioned by them. Unfortunately the commissioning engineer failed to notice the motors had been wired in star.After 1 year both motors burnt out at the same time resulting in the closurer of a sizeable factory for half a day.
As a result,the claim is also sizeable. The manufacturer agrea's that they should have been wired in star but insists that this would not have caused the motors to burn out -hmmmm

 

[jraef]

Well that depends. Again, even with a VFD, that motor was going to be unable to produce very much torqure for the applied current (or conversely pull more current for the required torque). So perhaps the VFD went into current limit on a continual basis that, although within the VFD's capability, may have been above the motor's capability as a continuous current draw. And if the VFD were slowing the motor down, any fan cooling would have been reduced. This of course always happens, but at a reduced speed you typically have a reduced load as well. So a combination of reduced cooling and increased current consumption could definitely lead to failure.

I will however deny any of this in a court of law!

"Venditori de oleum-vipera non vigere excordis populi"

 

[Skogsgurra]

I do not understand that the motors survived one full year! Did you increase/decrease production prior to the failure - so that the motor speed/load was increased? Or (in the case of deplacement pumps) decreased so that the cooling was less efficient?

If it comes to proving who's right and who's wrong - I suggest that you bring in spare motors and connect everything as it was when they were commissioned. Make sure that you can make a fast replacement once the motors burn out.

If the manufacturer does not agree to do this experiment, then he has (indirectly) admitted that such a situation is not good for the motors. Costs? Yes, but - as you say - there are substantial claims. And the one that loses the "bet" has to pay it all.

I would love to hear if he agrees to such a test.

 

[jraef]

Great suggestion.

"Venditori de oleum-vipera non vigere excordis populi"

 

[digitrex]

I had a similar experience 2years ago when I found 4nos 30kW 415V running at higher than normal temperature(2 of them at particular high load and at higher temperatures) on the bodies. These motors were direct online start type and made by a well known European manufacturer.

Electrical measurements (current, kW etc) did not indicate any problem. However the no-load currents were only about 15% of the rated FLC. Some experts around advised me that normally no-load current of a motor, 2 or 4-pole, should be roughly around 33% of FLC.

I removed one of these motor and opened it in the workshop and found the rotor surface had some blue colour indicating that some overheating had occured.

I contacted the manufacturer directly and raised the possibility that the motor nameplate could have been wrongly indicating as Star connected. The reply from manufacturer was positive, confirming that the motor windings should have been connected in Delta. In admitting their mistake, they send us new nameplates and give us 2 new motors free of charge. Eventhough we had to pay import taxes for the new motors, we accept them graciusly as spares.