1hp VFD blowing 1 of the line fuses.

[philly]

We have a 480V Powerflex 40 1hp VFD that continuous to blow one of the fuses on the line side of the drive and then continue to operate on single phase supply. The fuse size that keeps blowing is a 6A fuse.

This motor is a 9-lead motor which origonally was wired incorrectly, which we thought was the problem, however now we have confirmed that the motor is wired correctly and it is still blowing this same phase fuse every time.

I would'nt suspect the fuses are too small because after the fuse blows and the drive is running on single phase supply we would expect the current on the two remaining phases to increase and thus blow the other two fuses if they were marginal.

Anybody have any idea what can be causing the same phase fuse on the line side to keep blowing?

 

[gar]

100301-1200 EST

What is the circuit diagram of the input diode rectifiers in the VFD?

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[petersonra]

It is doubtful that a fault on the load side would blow a single line side fuse.

My guess is an intermittant ground fault of some kind on that line. Is this a grounded power system? It would be unusual for a GF to open a fuse like that if it was not a grounded power system.

Once in a long while the case of an input rectifier can develop a flaw and create a current path from the line to the frame of the drive, which will open a fuse on a grounded power system PDQ. It could be heat related so it only happens when the rectifier is hot.

Try powering it up and putting a heat gun on the input rectifier with the motor not running.

 

[philly]

100301-1200 EST

What is the circuit diagram of the input diode rectifiers in the VFD?

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I'm not sure I follow what you mean here.

It is doubtful that a fault on the load side would blow a single line side fuse.

My guess is an intermittant ground fault of some kind on that line. Is this a grounded power system? It would be unusual for a GF to open a fuse like that if it was not a grounded power system.

Once in a long while the case of an input rectifier can develop a flaw and create a current path from the line to the frame of the drive, which will open a fuse on a grounded power system PDQ. It could be heat related so it only happens when the rectifier is hot.

Try powering it up and putting a heat gun on the input rectifier with the motor not running.

There is actually a set of 6A fuses at the drive and then another set of 6A fuses at the panel feeding the drive. The fuse that is blowing is at the panel. So it is hard to tell if the problem is on the feeder cable thus blowing the panel fuse, or the problem is at the drive, and the panel fuse is blowing first thus removing current and not causing fuse at drive to blow.

This drive is on a solidly grounded wye system.

 

[gar]

100301-1425 EST

philly:

Look at the circuit diagram in the manual for the drive. There should be at least 3 diodes used to rectify the AC input. Quite likely 6 diodes.

Your new information that the fuse at the panel is blowing may imply that there is short or leakage before you get to the VFD fuses.

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[Jraef]

I'm not sure I follow what you mean here.



There is actually a set of 6A fuses at the drive and then another set of 6A fuses at the panel feeding the drive. The fuse that is blowing is at the panel. So it is hard to tell if the problem is on the feeder cable thus blowing the panel fuse, or the problem is at the drive, and the panel fuse is blowing first thus removing current and not causing fuse at drive to blow.

This drive is on a solidly grounded wye system.

That scenario right there shows me the distinct probability that you have a fault on the cable going from the panel to the drive. The only possibility of the VFD making only one fuse blow consistently is if it had a bad diode or something else failed on the front end. That would cause the fuses closest to the drive to blow (assuming the fuses are the same type) because the added wire resistance would make the upstream fuse see slightly less current if the fault were at the VFD. A problem on the load side would cause the VFD to trip. The only thing you are left with is a fault on the cable run. Most likely a ground fault due to a nick in the insulation from pulling.

 

[Besoeker]

Anybody have any idea what can be causing the same phase fuse on the line side to keep blowing?

Possibly a ground fault on that phase.

 

[ptonsparky]

Megger time.

 

[Jraef]

Megger time.

Don't forget to remove the conductors from the VFD first!

 

[steve66]

That would cause the fuses closest to the drive to blow (assuming the fuses are the same type) because the added wire resistance would make the upstream fuse see slightly less current if the fault were at the VFD.

Fuses in series will see the same current, so you can't really predict which one will blow first. So I think it could be the cable, or it could be the drive.

You could try rotating all the input leads to the VFD. Then, if the same fuse blows, there is a good chance the cable is bad. If a different fuse blows, it's probably the VFD or motor.

Steve

 

[philly]

Fuses in series will see the same current, so you can't really predict which one will blow first. So I think it could be the cable, or it could be the drive.

You could try rotating all the input leads to the VFD. Then, if the same fuse blows, there is a good chance the cable is bad. If a different fuse blows, it's probably the VFD or motor.

Steve

I was thinking the same thing, that the fuses are in series so they should see the same current.

Jraef, I dont follow what you said about the wire resistance causing the upstream fuse to see less current therfore causing the fuses at the drive to blow first.

What would cause the VFD to be bad in this case? Shomething shorted on the input diode bridge

Would the motor causing blowing of the fuses? I would just expect it to trip the drive.

 

[philly]

If the drive was indeed damaged would this be a situation where the drive could be better protected by fuses located at the drive? I brought this up in another post and it seemed like the consensus that even fuses at the drive would not be quick enough to protect the drive from an internal fault

What if fuses at the drive were smaller than drive feeder fuses? Would these do a better job of protecting drive from internal fault, or by the time the drive faults internall its too late no matter what fuse size. As others mentioned maybe only a special semiconductor fuse may protect drive but maybe not.

 

[jim dungar]

If the drive was indeed damaged would this be a situation where the drive could be better protected by fuses located at the drive? I brought this up in another post and it seemed like the consensus that even fuses at the drive would not be quick enough to protect the drive from an internal fault

What if fuses at the drive were smaller than drive feeder fuses? Would these do a better job of protecting drive from internal fault, or by the time the drive faults internall its too late no matter what fuse size. As others mentioned maybe only a special semiconductor fuse may protect drive but maybe not.

Protective devices, even semi-conductor fuses, can not protect the internal components from faults, they can only minimize the amount of damaged caused by when faults occur. A fuse offers no protection unless it actually opens, which means excess current must actually flow, and an extreme amount of current is required before a fuse enters its current limiting range.

 

[Jraef]

Protective devices, even semi-conductor fuses, can not protect the internal components from faults, they can only minimize the amount of damaged caused by when faults occur. A fuse offers no protection unless it actually opens, which means excess current must actually flow, and an extreme amount of current is required before a fuse enters its current limiting range.

Perfectly stated. I'm always surprised at how many people don't think this issue through when they see their SCRs or diodes blow. They say "But the fuse should have protected it right?" But then I have them stop and ask themselves "What made the fuse blow in the first place?" The answer has to be the bad component! So how was the fuse supposed to protect a component that was already bad by the time the fuse saw it?

Somebody needs to invent a "Back to the Future" fuse that blows BEFORE an even happens in the first place.

 

[philly]

Perfectly stated. I'm always surprised at how many people don't think this issue through when they see their SCRs or diodes blow. They say "But the fuse should have protected it right?" But then I have them stop and ask themselves "What made the fuse blow in the first place?" The answer has to be the bad component! So how was the fuse supposed to protect a component that was already bad by the time the fuse saw it?

Somebody needs to invent a "Back to the Future" fuse that blows BEFORE an even happens in the first place.

Very good points.

So does having a fuse on the primary of the drive much larger than the required 125% not prevent as much damage from occuring after fault then a smaller fuse would?

I hear so many different times people say "fuses at the drive protect the drive, and the breaker at the MCC protects the cables". I see now that this is not necessarily the case since by the time the drive faults it is too late and it is ruined.

So there is really no advantage to having smaller fuses at drive and slightly larger ones used for feeder.

 

[Jraef]

I was thinking the same thing, that the fuses are in series so they should see the same current.

Jraef, I dont follow what you said about the wire resistance causing the upstream fuse to see less current therfore causing the fuses at the drive to blow first.

The voltage drop caused by the length of wire increases the amount of current seen at the 2nd fuse in series. So if the current at the load is 7A, the closest 6A fuse sees 7A, the upstream fuse sees slightly less because the voltage is a little higher there. The 2nd fuse in series blows first.

What would cause the VFD to be bad in this case? Something shorted on the input diode bridge

Yes, and it would most likely be shorted all the time, not just sometimes. So if it was a problem in the VFD, the fuses would blow immediately when you energized it again. The only caveat is moisture.

Would the motor causing blowing of the fuses? I would just expect it to trip the drive.

Yes, that's what I said, the VFD would trip if the problem were down stream.

Think of a VFD as a power source all to itself for your motor. To the VFD, the line power is just a source of raw material, the energy it needs to charge up and replenish its DC bus. What happens on the load side is the VFD's business, what happens on the line side is somewhat irrelevant as it relates to the load (other that low / no voltage on the line side). So in order for a motor-side fault to blow the fuses, the VFD would have to completely ignore what was going on and try to pull power from the line to feed the fault. Unless someone were foolish enough top disable ALL of the VFD's internal prtection features, that isn't going to happen.

That's why sometimes VFDs (and Soft Starters to a lesser extend) are what I call "Fault Finders". A problem has been going on for years in a motor or circuit, but because it happens so fast, nobody could figure out what was happening and they chalk it up to "a glitch". Then when the VFD or SS is added, suddenly it looks like the fault is much worse. That's because the VFD or SS will limit the current going into the fault and slow the reaction down to where people can perceive it.

 

[gar]

100302-2125 EST

philly:

A semiconductor device is usually a component with a small area, small mass, and little heat storage capability. Power type semiconductor devices will have explicit heat sinking materials attached to them to remove heat energy. What this means is that typically the time of overload of such a device is quite short. Maybe milliseconds to seconds. In contrast a motor may be overloaded for much greater times without failure.

Another factor is that semiconductors may be operated much nearer their abrupt failure point, and motors may have a less abrupt failure mode and be operated further further from a failure point.

Because semiconductors have short thermal time constants there are special fuses with very fast trip curves for their protection. Primarily these fuses are used to protect the semiconductor from an external overload of the semiconductor. These semiconductor fuses or any other fuse can in no way protect the semiconductor from a self failure of the semiconductor.

Back to your problem. If you have an identical fuse at both your panel and at the input to the drive, and you always blow the fuse at the panel, then with high probability the problem is between the output terminal of the fuse at the panel and the input terminal of the fuse at the drive.

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[philly]

The voltage drop caused by the length of wire increases the amount of current seen at the 2nd fuse in series. So if the current at the load is 7A, the closest 6A fuse sees 7A, the upstream fuse sees slightly less because the voltage is a little higher there. The 2nd fuse in series blows first.

I'm having a hard time following this one. If the current at the load or fault is 7A I thought this 7A was the same through all devices in the series circuit with only the voltage drops across components being different determined by resistance.

I see how there will be more voltage at upstream fuse due to voltage drop in cable, however I dont see how this increased voltage will cause upstream fuse to see less current. What am I missing?

 

[ptonsparky]

The current is the same. There may be a very small increase in heat generated at one fuse or the other but I would lay bets more on the fuse holder itself vs the distance the fuse is from the source.

 

[Besoeker]

"What made the fuse blow in the first place?" The answer has to be the bad component! .

That's not necessarily the case. A fault after the diode bridge could cause the fuse to fail. If the fuse has a lower total 1^2t rating than the diode, it will fail and should protect the diode.
This isn't just idle speculation. It is common practice with those of us involved on the design of power electronics to rate the components to ensure that to be the case.
No, it doesn't preclude the possibility of a defective component causing fuse failure, but it does mean that it isn't the only explanation nor even the most likely for fuse failure.