VFD Question

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knoppdude

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Sacramento,ca
Hello, I was sent to check out a conntactor controlled VFD that was making a loud buzzing noise under load. I changed out the contactor as the customer ordered, but the noise continues. It appears to be comming from the VFD, and I don't have any theories on what could be causing the noise. The fan motor appears to be ok, but I can't tell for certain with the cover in place. I am not very knowledgeable in this area, and wonder if any of you have experienced this before. Thanks for all responses.
 
I've said this before and as simplist as it is ... a paper towel to you ear will isolate a sound... you can hear a difference one foot way! Granted I never worked with a VFD that one had to take a cover off! PPE required!
 
quinn77 said:
check the integrity of the entire circuit before assuming the contactor or vfd has an issue...that contactor is called an "isolation"contactor.
Click to expand...

Thanks for the definition, I actually did not know what purpose it served, but maybe you are onto something. What would you check in this situation? Power electronics is a tough area for me, and if you are right, I should start at the motor and work my way back. Is it possible that another branch circuit could be causing this?
Cadpoint, this is a AB power flex 70 powering a 10 hp motor. To see the integral fan which is on the underside, you have to remove the cover, which disables the drive. You are right about the PPE. Everyone on this site should use it when required. A few hundred dollars beats a stay in the burn unit anyday. Thanks
 
well knoppdude, i am no engineer, but the last time i ran across a functioning vfd making abnormal sounds i shut it down, checked connections at drive and motor, megged the motor and feeders and found faulted conductors underground. re-pulled the motor feeders and the drive returned to a normal state. when i troubleshoot vfd's and soft starters i use my knowledge of ohms law, proper measurements, and NEC to exaust any and all posibilities. when that gets me nowhere, i ALWAYS call the engineers at tech support. hope that helps.:)
 
ps...high resistance connections( loose and or improper ), length of motor feeders exceeding manufact. specs, poor installation procedures, etc...can all effect the way the vfd performs. i have very little vfd system design experience, so please consult an engineer if you find nothing wrong with the install.
 
What type of noise is this - mechanical vibrating or "hummm"? If the latest, it can be due to harmonics (as their are in hearing frequency range). Try to change the VFD switching frequency and see if the noise disappears or changes.
 
Is it coming from the drive or the contactor?,,,If it's the drive it's probably normal.

If it's the contactor, and you changed it and it still does it, I would be taking a serious look at the Power supply to the coil voltage. The last time I saw a coil "buzz" real loudly,it was poor power quality on a size 5 starter causing the problem. Do you have an oscilloscope?
 
Thanks for the replies. Strange as it may be, the noise went away. I don't have any idea why or how. No fault codes appeared during the time the VFD was making noise, and the motor was running normally, with current and voltage levels measuring normal. The next time I get the chance, and the plant is down, I will check the circuit from the motor back. I did talk with an electronics instructor at a local junior college, who said solid state power control is the future, and that I might need to learn the basics of electronics to begin to understand these devices. I think it is time for this ignorant electrician to go back to school.
 
I used to have a VFD sample case that my kids liked to play 'Star Trek' with, as they changed the speed of the 1/3HP motor the audible noise would change.
 
An audible whirring / whining noise from a VFD is typical.

Inside the VFD is a rectifier, a DC Bus, control circuitry, and then the inverter. If you heard clatter or chatter it is not normal, because there are no moving parts, save for a cooling fan.

Yes, everybody should take a class on drives, because now they are everywhere. I guess the best advice would be to keep the output of the drive, to the motor, isolated from everything else. That is not your standard 480V, it is more like radio waves.
 
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