we have a 110 KW pump motor run by a 132 KW VFD. The supply voltage is 415 V,motor name plate voltage is 415v, three phase,60 HZ.Motor insulation class is "H".The motor is delta start and the phase sequence is red,yellow,blue.The full load amperes of the motor are 188 amperes. the set points for the currents in vfd are 187 amperes. first this motor pump was taking 167 amperes and 170 amperes. but now it is taking 184 amperes and 183 amperes. we changed the grease of the motor and oil of the pump also but it is taking the same amount of currents. we did the megger tests of it, the results are as under:
coil insulation test to ground = >1000 M OHMS i.e infinity
coil insulation resistance = 0.08 OHMS.
coil to coil insulation resistance = >1000 mega ohms i.e. infinity
cable insulation resistance is also ok
Also the voltages and currents which we are taking at the output of vfd & at motor terminal box cables are:
R Y B
187 A 180 A 181 A

The voltages at the output of vfd are:
R/Y Y/B B/R

460 V 464 V 466 V
But the voltages when we measured by opening the terminal box of the motor are:
R/Y Y/B B/R
343 v 370 v 338 v

we are confused why the voltage difference is coming i.e by measuring the voltages at the vfd output and at the motor terminal box. the currents are almost the same.

Also we have checked the motor by changing the phases in the terminal box. i.e. we changed the red phase with yellow, yellow with blue and blue with red, the rotation of the motor was the same as previous and we got the results as:

B R Y
186 A 180 A 181A
We compared all the currents with before phase change currents, so we got a result that the cause of unbalance is in the electrical system. but that unbalance is with in 5% so its not an issue.
but we got the voltage difference there by changing the phases.
This pump motor when runs for 8 to 10 hours then it shows the "inhibit option on vfd" and gets tripped, we have another pump like the one having problem, that has the same problem,but when it trips it does not show the inhibit option. we did all the electrical checks which shows that the motor is healthy. now i need your help that what shall we do with the motor so that it comes to healthy stage. the motor alignment is ok.
one more thing which i think is to decouple the motor and run it without load, but for that i need your help as i am confused that without load how much current the motor will take, i need the answer of this question. and the second thing is that why the motor is showing the inhibit option on the vfd, whether because of over current or over heat, since the class of motor is "H" so i think its not getting tripped because of overheat. also please tell me which other checks or tests we need to rectify the motor problem.
i need your guidelines in this issue. please when you get free then brief me about it. i will be waiting your precious reply.

thanks and regards
engr tahir khan

 

you have an unacceptable voltage drop. Something somewhere is getting warm. Current rises as slip increases, a product of low voltage. Review the integrity of your power circuit. And don't wait.

 

we have checked the whole system, there is nothing which is going warm. all is normal. the motor winding temperature is 92 centigrade and the bearing temperature is 80 centigrade. the motor is ambient temperature is 40 centigrade and its insulation class is H. We found no other thing which is getting warm. so please give me some details or checks through which we can rectify the problem.

 

If the voltage at the motor is lower than the voltage at the output of the VFD, there MUST be a resistance between the two. Check for loose terminals at each and every connection between the two, including the connections at the output of the VFD. As said by GetDeviceInfo, that much current and that much voltage drop will create a lot of heat.

 

Has there been any change in the way in which the cable(s) from the VFD to the motor terminal box are run or arranged?

 

If the voltage at the motor is lower than the voltage at the output of the VFD, there MUST be a resistance between the two. Check for loose terminals at each and every connection between the two, including the connections at the output of the VFD. As said by GetDeviceInfo, that much current and that much voltage drop will create a lot of heat.

AT 180 amps current, the extra resistance in question is small ~ 0.5-0.6 ohms. (but the effect is large)

 

nope same cable is used,there is no break in the cable.