480V Motor possible operating slower

[Nardie]

Is there a damper that prevents some of the air from going backwards through the second fan?

There is an inlet damper that always remains open for both fans. The outlet damper is controlled by the control circuit for the motor. Once the fan is energized the outlet damper is opened, and once the fan shut off the outlet damper closes. The outlet damper is a pneumatic damper controlled by a solenoid. We have checked the inlet damper for both fans, and they are positioned correctly. During our issue the operators checked the position on the outlet damper and reported it was open. However, we plan to start the troubled fan tomorrow, and double check the damper positions, as well as any air leaks to the instrument air system that supplies the outlet damper.

 

[Jraef]

Just curious could a rotor bar issue cause the motor speed to slow down?

Loss of torque, increase in slip, decrease in RPM. But you would also see an increase in current.

 

[synchro]

There is an inlet damper that always remains open for both fans. The outlet damper is controlled by the control circuit for the motor. Once the fan is energized the outlet damper is opened, and once the fan shut off the outlet damper closes. The outlet damper is a pneumatic damper controlled by a solenoid. We have checked the inlet damper for both fans, and they are positioned correctly. During our issue the operators checked the position on the outlet damper and reported it was open. However, we plan to start the troubled fan tomorrow, and double check the damper positions, as well as any air leaks to the instrument air system that supplies the outlet damper.

It sounds like you'll be doing the right things. I think some additional focus should be put on making sure that the outlet damper on the standby fan is fully closing. Perhaps there's a stop on it that can be adjusted.

 

[2xcheck]

I have a 480 VAC motor that normally pulls 50 in wc for exhaust. The other day the in wc dropped to 38 in wc. There was an initial concern of the motor was single-phasing, which I didn't think. However, today we checked the fuses, heater contact, and supply voltage. The fuses were good, and the heaters weren't tripped. The supply voltage was 479 VAC, so this was adequate. We also perform an ohm check on the motor winding's. They all read the same 0.6 ohms. From the operators inspection the inlet and discharge dampers were fully opened, so the motor wasn't dead headed. We are going to check the positions of the dampers again for verification. I also plan to perform static and dynamic testing on the motor to check the internal components of the motor. So I am wondering is there anything else I should do, or what else could cause the sudden drop in exhaust pressure? Thanks

Remove and blow out sensing line to sensor

 

[Nardie]

So to give everyone an update, we performed the static test on the troubled fan today. The test consisted of performing a polarization index test on the motor. The PI reached a measurement of approximately 1100 mohms (normal is 30,000 mohms) and stayed around that figure for the duration of the 10 minute test. I am in South Carolina, and we have had a lot precipitation yesterday. The room the fan is located in draws in air from outside, so we believe the test results from the static test might just be due to moisture from the last 24 hours. When we attempted the dynamic test the outlet damper didn't open and we noticed a leak in the instrument air line. The motor operated for about 5 to 10 minutes. After this we re-performed the static test with the same results. I thinking we didn't run the motor long enough to burn off the moisture. So tomorrow we plan to repair the instrument air line, and perform a megger test. I am not able to perform a static test, so I am doing the next thing available. Once the instrument air line is repaired we plan to run the motor for at least 3 hours if available, and then perform another megger test. So my question is what is everyone's thoughts on the PI test results, just moisture? Thanks