A little history first. This varible frequency drive operates a exhaust fan. The HVAC guy told me that the VFD keeps tripping out on Overload. while doing some troubleshooting, I notied that the frequency reading on the drive was fluctuating up and down. I then went up to the motor and noticed it was kicking on and off. It did this for about 5 minutes and then ran steady after that. I tried to see if any sensors may have been telling the VFD to do some crazy stuff. Nothing yet. I then decided to monitor the voltage and current. The current on the line side of the VFD was 4 on phase a, 4 on phase c, but only 1 on phase b. Is this normal.
VFD tripping out on Overload
- Thread starter JdoubleU
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donselectric
Senior Member
- Location
- nh
i'd disconnect the motor and megg it.
Check incoming voltage underload, check winding resistance on all three windings (should be pretty close, say withing 10%) then check voltage drop across drive ( however most newer drives have self diagnostics and will show a fault number), Check Line and Load terminations, check control input
ie: 4-20ma, 1-5v, 120v, what ever it is....make sure its not fluctuating....
ie: 4-20ma, 1-5v, 120v, what ever it is....make sure its not fluctuating....
If it as unbalanced as that on the line side of the inverter it strongly suggests an inverter fault, possibly somewhere in the input rectifier stage.
Can you try a different inverter on the same motor and/or a different motor on the same inverter?
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
I agree with another poster that you probably have an issue with the input section of the VFD. They should be very close unless one of the rectifiers is fried or not working for some other reason.
I would have guessed maybe an intermittent connection on one of the input phases. In my experience, when diodes fail, they almost invariably* go short circuit. If one was short circuit you would have fault current drawn from the supply all the time it it wouldn't be possible to run at all.
*I've never known one to fail in any other way, but there's always a first I guess.
petersonra
Senior Member
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- Northern illinois
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- engineer
could be the control circuitry rather than the semiconductor itself.
but its odd that it is tripping on overload. thats a sign that the motor is drawing too much current, not that the vfd is.
he needs to measure the motor current. it may be the overload value is set wrong.
If the input ac to dc conversion uses a three phase diode rectifier, and most do, the controls would have no influence on that conversion stage.
The unbalanced input currents strongly suggest a problem in the input stage. The degree of unbalance is not normal operation.
Quite.
There could be more that one problem and one shouldn't preclude such a possibility.
I looked at the the settings and the overload was set at 15amps and the FLA rateing of the moter is 15.5. I also checked the midpoint voltage and the factory setting is 80volts. The motor is 208 three phase. is this a correct midpoint.
I generally wouldn't set the overload trip at less than the FLA rating of the motor.
I have been playing around with variable speed drives since before the Dead Sea reported sick.
In the context of variable frequency inverters I don't know what parameter midpoint voltage refers to but I accept that it could just be different terminology between different VFI manufacturers.
But none of that really explains the input current unbalance.
richwaskowitz
Senior Member
- Location
- Shelby Twp. MI, near Detroit
I suggest that you are single-phasing the input to the drive either due to an open fuse or a poor connection. I have seen this on more than one occasion. The drive will still operate with only two phases supplying it but will not have enough power to run the motor properly since the output stage will be taking power from the DC bus faster than the front end can supply it under a load. Not reading balanced current on all three phases is NOT normal.
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