Motor Winding Resistance

"there is a winding turn fault"

Please give full & complete detail about winding turn fault. What are the Amps drawn by each phase?

Pls clarify more.Aftr disconnecting the delta connection,the problem was found out.But in delta connection,the values must have been unbalanced isn't it? For small winding turn faults,will there be not any change in equivalent resistance of the all 3 phase.?

Unless you can measure at milliohms level! What led you to measure resistance in the first place. What observation of yours started this hunt?

The motor tripped on unbalance during running.So as usual we checked the winding resistance (ph-ph)without seperating the windings at motor TB.Took trial & motor tripped again during starting itself. My question is doesn't the inter turn shorts make the resistance unbalance,but sufficient for the protection relay to trip the motors.?

The motor tripped on unbalance during running.The winding resistacne was found 0.2ohm.When it tripped again on starting,we found the 2 of the winding short after seperating the motor cables.My question is,why this fault didnt reflect on the meter.Whether micro ohmmeter is being used everwhere for motor winding resistance & we should also employ that method.?

When measurement levels are in decimal, it is obvious what kind of instruments should be used. This is my commonsense postulation. I am not aware, what everybody uses, under such circumstances. Somebody, sooner or latter, will tell us about that.

"why this fault didn't reflect on the meter"

Because earlier you did not isolate the motor cables before taking measurement. Or there might have been a slip-up somewhere. It is a thought. I am not accusing anybody of anything.

Due to the low resistance values you are reading, yes, you should use a micro-ohmeter, but even this won't reveal significant unbalances, for that you should perform a surge test, it will alert on minute turns count diferences between windings, insulations integrity and turn to turn shortings, even at the scale of one single turn.

Regards

lead resistance is normally done with DC. For a single shorted coil with say 100 turns will give only a 1% change. When the coil is excited with AC the shorted turn now appears as a short to the AC flux change. That is the principal of flux linkage. That shorted turn will give a high current for the AC.

I had a DC machine, 500kw, 500vdc, short a turn in the shunt field. Now you would expect that a dc coil fed with DC would not have any problems. But the DC supply was an unfiltered phase controlled bridge. At the rising edges of the waveform a very high current flowed and opened the SCR protective fusing. The root cause of the failure was a partially blocked cooling duct - the motor over heated, but the klixon was installed at the inlet and not the outlet. The winding failed on the "hot" end.

Motor faults can occur from the winding heating up. The thermal expansion will then cause a short, but when it cools, the windings return to their normal position and show no signs of shorting.

A good way to test motor problems is to run an ammeter (three if three phase). Remember that measuring the resistance of windings can only be done when the motor is not energized, hence a static condition. Also, if a few windings are shorted, a meter may not read the difference in resistance. The resistance of the winding is based on length of wire and resistance of the wire material (micro or milli ohms per foot). You can see how a small short (10-20 windings) will not have much effect on the resistance, but measuring current or impedence will show the short.

Thanks a lot to all......by the way can anybody tell what's the nromal range of resistance & inductance of any rated induction motors.This would help veryeasily.

Dear giri,

Best way to check this type of fault by using Electirc Motor Checker instrument. In case of inter turn fault, when you measure % inductance,(R-N,Y-N, B-N,if neutral point is accessible, otherwise,R-Y,Y-B,B-R,),there will be variation of the readings. Normally,if the readings are differing more than 3to 5%, it indicates there is a interturn fault.

Manindra

Resistance of winding for 127 kW motor 460 V Delta 60Hz 4 pole

• colt motor, at 27.2 °C winding, frame and room temperature
• U1-U2 : 0.02174 Ohm
• V1-V2 : 0.02168 Ohm
• W1-W2 : 0.02169 Ohm

• warm motor, at 96.4 °C winding, 74.5 °C frame, 32.2 °C room temperature
• U1-U2 : 0.02790 Ohm
• V1-V2 : 0.02776 Ohm
• W1-W2 : 0.02787 Ohm

These values come from a type test of a 127 kW motor.

For a 160 kW motor the values will be smaller.

Values depend on pole number, voltage, motor construction

• more poles, higher resistance
• higher voltage, higher resistance

Because the values are very small, it is difficult to measure them.

The resistance measurement will be affected by difference in length of connection cables,

• difference in torque of connection bolts
• difference in material from cable shoes, washers, Cu bolts, Fe bolts; etc
• difference in material from test clips micro ohm meter

Almost impossible to make a correct measurement

Have you tried to do a insulation test using a mega-ohm meter when the the motor is cold and when it tripped? The windings should be totally isolated from the supply and from each other.