electric motor problem

[tahir mehmood]

august sir;
once again i need your help regarding a problem with a motor. hope you will share your precious time. sir we have a 110 KW pump motor run by a 132 KW VFD. The supply voltage is 415 V, 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 are:
R Y B
187 A 180 A 181 A
we have checked the current unbalance which comes less than 5% according to NEMA standards.
The voltages at the output of vfd are:
R/Y Y/B B.R

460 V 464 V 466 V
we have checked the unbalance voltages which comes less than 1%.


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.


This pump motor when runs for 8 to 10 hours then it shows the "inhibit option on vfd" and gets tripped. 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.
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.

 

[Jraef]

This is difficult with the limited information I can glean from the pile of superfluous information provided, but here goes:

The only way you can have a higher output voltage than you input voltage is of you have an Active Front End (AFE) drive. If so, then secondly the only way to have a higher output voltage than the input voltage is if you COMMAND the VFD to do so. So then we must try to determine why someone would do so. It has to be either that the motor nameplate voltage (which you did not provide) is 460V, or someone thought they were clever and increased the speed of the motor above the base speed, which would require a corresponding increase in voltage in order to avoid a loss of torque. If nobody did that on purpose, but entered the motor nameplate voltage as 460V, or left it on a default setting like that, an AFE drive may make the assumption automatically that it was 60Hz and created the same effect.

Assuming the latter, then the problem is that if you increase speed on a centrifugal pump, you increase the power required by the pump by the cube of the speed change. If the motor was sized for providing full required torque at base speed, you may now be overloading it. Going with the above assumptions, 460V implies 110% speed, which means the pump now requires 135% power. So you may have a 110kW pump that is now requiring 148kW because you are running it at 110% speed, but yor drive is now rated to supply that.

 

[Jraef]

I'm replying to your PM in here for the benefit of others who may search for similar answers.

really sir i am very thankful to you for the reply, as you said that some body will have increased the voltage upto 460 V from the vfd setting, but i have seen the voltage coming in the vfd screen is 415 v but when we measure the voltage on the vfd output then it shows 460 v like this. and also the name plate voltage of the motor is 415 volts. and also the motor full load current is 188 amperes, but when we measure the amperes at the vfd output then it comes 184,183 and 181 amperes. since the same motor was taking 167 amperes and after some times 177 amperes but now its running at the peak. we have done all electrical checks of the motor. meggering,alignment ,greasing, from that side its ok.but well we dont know whats the actual problem with the motor. in the previous message i have cleard all the details. so if possible and if you have time then have a quick overview over it and tell me some solution. i know you will be busy but the problem is critical.
thanks and regards

tahir mehmood

If it is a centrifugal pump, any incease in flow from when you took the earlier readings will result in an increase in motor power and therefore amperage. Given the slight percentage change in the amp draw, it could even be a slight difference in viscosity or a slightly different valve position that could cause a small increase like that.

Also, if you are taking readings of voltage and current on the output side of a VFD, anything short of the most expensive of meters will not likely have sufficient filtering to be accurate. False readings on the load side of PWM outputs are a notorious quirk of the industry. It is better to use the values shown by the VFD display, as they have the necessary filtering and algorithm adjustments to make the information more accurate.

 

[tahir mehmood]

electric motor problem

thanks mr.jerif.
i got good information from your replies. but we are wondering that why our motor reached to the peak ampere values. shall a motor whose FLA is 188 A must run at 184 or 185 amperes ? and then why it get trips by showing "inhibit" on the VFD. as you said that it must be because of change in viscosity of the fluid, since our motor is used for the purpose of condensing cooling water circulation, so may be the coolant amount has been increased in the water.also the water flow is the same as previous, at that time motors were taking 170 and 176 amperes. we have four motors all are now having the same problem. since all electrical checks are ok, and also mechanical alignment, greasing,oiling of the pump all are ok. anyhow mr.jerrif thanks once again.

 

[GoldDigger]

thanks mr.jerif.
i got good information from your replies. but we are wondering that why our motor reached to the peak ampere values. shall a motor whose FLA is 188 A must run at 184 or 185 amperes ? and then why it get trips by showing "inhibit" on the VFD. as you said that it must be because of change in viscosity of the fluid, since our motor is used for the purpose of condensing cooling water circulation, so may be the coolant amount has been increased in the water.also the water flow is the same as previous, at that time motors were taking 170 and 176 amperes. we have four motors all are now having the same problem. since all electrical checks are ok, and also mechanical alignment, greasing,oiling of the pump all are ok. anyhow mr.jerrif thanks once again.

The viscosity of the coolant can definitely be affected by the concentration of additives to the water, as well as by temperature. Do you have an independent flow meter? A reduction in viscosity could cause an increase in flow which, paradoxically, could increase the pump motor load.

 

[tahir mehmood]

same problem got more complicated

same problem got more complicated

mr.golddigger i got your point but i we got another issue also, when we measure the volatges at the output of vfd then we got the following fluctuating voltages:
R/Y=460 V, Y/B = 464 V, B/R = 466 V and the currents taken are RED=187 A, YELLOW= 180.8 A, BLUE = 181.5 A.
but when opened the terminal box of the motor and checked the voltages and currents there then we were surprised and got more confused.
the voltages and currents are:
R/Y=343, Y/B=370, R/B=338
R=179A,Y=183A,178A.
we checked the cable size but that is good. so we are confused why it is like this,why the motor is not getting rated voltage which is 415 v

 

[GoldDigger]

mr.golddigger i got your point but i we got another issue also, when we measure the volatges at the output of vfd then we got the following fluctuating voltages:
R/Y=460 V, Y/B = 464 V, B/R = 466 V and the currents taken are RED=187 A, YELLOW= 180.8 A, BLUE = 181.5 A.
but when opened the terminal box of the motor and checked the voltages and currents there then we were surprised and got more confused.
the voltages and currents are:
R/Y=343, Y/B=370, R/B=338
R=179A,Y=183A,178A.
we checked the cable size but that is good. so we are confused why it is like this,why the motor is not getting rated voltage which is 415 v

If you get different current readings using the same meter at opposite ends of the same cable, then there is a load somewhere else that you are not aware of.
(Separate cooling fan for motor, for example.)
If you took the measurements at the input and output of the VFD, then differences in current would be possible, even expected. The input and output voltages and currents at the VFD may measure differently just because the waveforms are very different and you are not using a true RMS meter.

 

[tahir mehmood]

motor problem issue

motor problem issue

actually the current readings at the vfd output and at motor terminal box are the same,just only voltage difference is present there.so thats why i asked it.anyhow thanks for the nice replies. i got much more from them.