AB VFD fault

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Electromatic

Electromatic

Senior Member
Location
Virginia
Occupation
Master Electrician
Yesterday I was troubleshooting an Allen Bradly Powerflex4 VFD running an exhaust fan in a commercial hood. The restaurant manager said that for some time it will turn off with an F3 fault code (power loss). Sometimes multiple times a day, sometimes not. He tried to associate it with some other piece of equipment turning on, but never saw a pattern.

It happened while I was there but the power never dropped out. (I was able to get my meter on the line terminals within seconds of the fault.) I forced a couple of AC units to turn on and never saw a drop in voltage: everything about 210V line-line at the drive.

I see in the manual that the fault is actually triggered when the DC bus drops below 85% of nominal. What else could I check or could cause the DC bus voltage to drop?

The drive is connected via ethernet cable to a Melink hood control panel. Could this fault be triggered by a bad communication signal? We did call Melink/CaptiveAire, and all they had to say was to check the incoming power or they'll quote a replacement drive.

Thanks in advance for any help!
 
while it won't fix the problem there are parameters that can be set in the VFD to have it reset the alarm automatically. might not be compatable with the control system though.
 
A phase loss while running under load is what causes an F3 fault. The PF4 does not have direct phase loss sensing so it determines loss of input phase by looking at DC bus ripple.

Is the feed to this drive 3 phase or is it single phase? A lot of people will use a small VFD like that to allow the use of a single phase source to feed a 3 phase motor, because the VFD will create the 3 phase output from the DC bus regardless of what feeds the rectifier to make the DC. BUT, in doing so, you must EITHER buy a VFD that is already designed for 1 phase input, or you must de-rate a 3 phase drive. That's because the input to the rectifier will have more DC bus ripple, so you need more capacitance inside of the drive and must increase the size of the VFD relative to the motor size. Some people think it is OK to just up-size by one size, but that's not enough. The PowerFlex 4, because of the way the capacitors are configured, requires a 65% de-rate. So for example if you have a 1HP 230V motor rated 4.2A, you must use a VFD rated for at least 12A, so that's a 3HP drive, not a 2HP. If someone under sized the drive, it will trip at full load.
 
It is fed with 3-phase and the motor is 3-phase (208V). I didn't go to the roof to look at the motor, but it was drawing about 2.5A and the drive is rated for 5.2A or 4.5A-continuous. All of the equipment has been in place for some time with no recent modifications.

I'm just trying to cover all bases before having to buy a new drive.

Thanks.
 
Electromatic said:
I see in the manual that the fault is actually triggered when the DC bus drops below 85% of nominal. What else could I check or could cause the DC bus voltage to drop?

The drive is connected via ethernet cable to a Melink hood control panel. Could this fault be triggered by a bad communication signal? We did call Melink/CaptiveAire, and all they had to say was to check the incoming power or they'll quote a replacement drive.

Thanks in advance for any help!
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Could be something as simple a a loose connection, maybe on one of the input phases if it's an intermittent problem.
I'd think it unlikely to be a component failure - their failure is usually terminal.
 
Electromatic said:
It is fed with 3-phase and the motor is 3-phase (208V). I didn't go to the roof to look at the motor, but it was drawing about 2.5A and the drive is rated for 5.2A or 4.5A-continuous. All of the equipment has been in place for some time with no recent modifications.

I'm just trying to cover all bases before having to buy a new drive.

Thanks.
Click to expand...

These intermittent issues are painful to work out.

OTOH, the drives are relatively inexpensive compared to your time. Might be the most logical choice is to get a new one. IME, the PF4 has a lot of quirks, especially when using the Ethernet adaptor.
 
I'd vote for a loose /corroded connection somewhere, could be anywhere in the circuit, i.e. all the way back to the breaker and its connection to the panel bus. Replacing the drive before finding the problem risks a call back and an angry customer...
 
Jraef said:
I'd vote for a loose /corroded connection somewhere, could be anywhere in the circuit, i.e. all the way back to the breaker and its connection to the panel bus. Replacing the drive before finding the problem risks a call back and an angry customer...
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Totally agree.
 
I've checked all the connections I'm aware of: bus to breaker, breaker to branch wires, and terminals on the drive. As far as I can tell, the circuit is a straight run of MC from panel to drive - no junctions. All connections look very good and are tight. I agree that replacing the drive is not ideal without knowing for sure what's wrong, but I'm out of ideas--that's why I'm here! :)

One thing that I thought was minor but might be a clue: I could not clear the fault from the drive keypad. I had to power cycle to drive to clear it. (Which seems should just create another power loss fault.) Also it appeared that the parameter to automatically clear faults was set to do that, but it obviously isn't. Further indication of the drive going bad? Some kind of interference from the hood control board via ethernet?

Thanks as always.
 
It's remotely possible that the restaurant has some sort of large single phase load that comes on and creates a temporary extreme voltage drop on 2 of the 3 phases, which the VFD interprets as a phase loss because of its effect on the DC bus ripple.
 
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