Fuses Blowing on VFD

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mull982

Senior Member
We have an application with a 10hp motor controlling a 10hp motor on which the VFD is protected on the line side by 20A fuses. There is an occurance happening where the motor load is getting jammed and is blowing the fuses on the VFD.

I would think that the VFD itself should trip in such an instance rather than blowing these fuses? I also belive that the NEC requires fuses protecting VFD's to be rated at 125% of VFD, in which case I would think that 20A fuses should be more than adequate for this drive, but maybe I'm wrong.

Should these fueses be blowing on extreme overload? Is there a setting in the drive that may not be correct? Do fuse sizes seem correct for drive?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
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Electrical Contractor
The drive should take care of an overload if you have the parameters set correctly. Usually the basic startup is adequate. Knowing the voltage you are working with will help us. Hopefully you are @ 460. Depending on the drive you probably have a setting that will shut it down at ___amps after "x" amount of time. Say 14.5amps after .1 seconds. These parameters are a bit deeper in the manual. What is the model & make of VFD?
 

mull982

Senior Member
The drive should take care of an overload if you have the parameters set correctly. Usually the basic startup is adequate. Knowing the voltage you are working with will help us. Hopefully you are @ 460. Depending on the drive you probably have a setting that will shut it down at ___amps after "x" amount of time. Say 14.5amps after .1 seconds. These parameters are a bit deeper in the manual. What is the model & make of VFD?

The drive is a Rockwell Powerflex 40 and is a 460V drive.

It sounds like your saying the setting might be too long, and the fuses are beating the drive?
 

Besoeker

Senior Member
Location
UK
We have an application with a 10hp motor controlling a 10hp motor on which the VFD is protected on the line side by 20A fuses. There is an occurance happening where the motor load is getting jammed and is blowing the fuses on the VFD.

I would think that the VFD itself should trip in such an instance rather than blowing these fuses? I also belive that the NEC requires fuses protecting VFD's to be rated at 125% of VFD, in which case I would think that 20A fuses should be more than adequate for this drive, but maybe I'm wrong.

Should these fueses be blowing on extreme overload? Is there a setting in the drive that may not be correct? Do fuse sizes seem correct for drive?
Is the input voltage 480V 3-ph?
If so, then I agree that 20A fuses should be adequate.
And yes, I think the drive should have tripped.
I'm looking at the overcurrent trip description on one of the units we use.
For overcurrent, the frequency converter has detected:
  • too high a current in the motor cable
  • sudden heavy load increase
  • short circuit on motor cables
  • unsuitable motor
 

mull982

Senior Member
Is the input voltage 480V 3-ph?
If so, then I agree that 20A fuses should be adequate.
And yes, I think the drive should have tripped.
I'm looking at the overcurrent trip description on one of the units we use.

Yes the input voltage is 480V 3-Phase
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
We have an application with a 10hp motor controlling a 10hp motor on which the VFD is protected on the line side by 20A fuses. Do fuse sizes seem correct for drive?

A 10hp Powerflex 40 calls for 30 amp CC type fuses. That'd be the first thing I'd replace. You can find the user manual on Rockwell's site.
 

mull982

Senior Member
A 10hp Powerflex 40 calls for 30 amp CC type fuses. That'd be the first thing I'd replace. You can find the user manual on Rockwell's site.

Is the NEC requirement of 125% only a miniumum and therfore the manufactureres recomendations should be followed?

Does anyone know what section of code this requirement is for OCPD's on VFD's? I cant seem to find it now.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I think your 125% is for motor overload protection. The drive should provide that so the fuses protecting the drive are allowed to be much larger.

Try using your output relay contacts in your control circuit, A055, 7 - Above current. Higher end drives have different methods and more detail.
You may still be able to do this with just the overload setting but I haven't time to research further at this moment.
 

mull982

Senior Member
Try 430.120... of 08 NEC

Yes it looks like I was thinking of 430.122 (A) which states that feeders must be rated at 125% of drive input current, and would therefore require the OCPD to be rated at 125%.

So I guess this fuses are to protect the feeder cables since it appears the drive is capable of protecting itself from both an overload and short circuit within the drive or on the output of the drive. These settings may need to be adjusted however the drive should protect itself.
 

Besoeker

Senior Member
Location
UK
Yes it looks like I was thinking of 430.122 (A) which states that feeders must be rated at 125% of drive input current, and would therefore require the OCPD to be rated at 125%.

So I guess this fuses are to protect the feeder cables since it appears the drive is capable of protecting itself from both an overload and short circuit within the drive or on the output of the drive. These settings may need to be adjusted however the drive should protect itself.
Drive input currents and output currents are different issues.
If you have a stalled motor, output power is zero even if the motor is drawing FLC. Input power to the motor is losses which I guess could be up to 10% of motor rated output.
The input power to the drive has to supply that 10% plus drive losses which are typically around 3% of drive rating.
I don't think a stalled or stalling motor should blow drive input fuses.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
1)You sized correctly according to min. requirements, but the mfg instructions should always be followed (put in the 30A fuses)
2)As you realized yourself, you may need to check the settings to see how many cycles does it take for the protection to react vs. how many cycles for the fuse.
3)Don't consider just the cycles but the current vs. time curves. If you can get hold of the CCT curves you can compare start up readings from your system with the curves and you may find the 20A fuse trips faster at a certain subtransient moment.

Bottom line I think you'll be ok if you use the fuse recommended by the mfg.
 

mull982

Senior Member
Drive input currents and output currents are different issues.
If you have a stalled motor, output power is zero even if the motor is drawing FLC. Input power to the motor is losses which I guess could be up to 10% of motor rated output.
The input power to the drive has to supply that 10% plus drive losses which are typically around 3% of drive rating.
I don't think a stalled or stalling motor should blow drive input fuses.

Very interesting point. So during a stalled condition we essentially will be drawing LRC. Although this LRC can be up to 6x the motor FLC most of this current is reactive current and not real current, or current contributing to "power" as you mentioned.

It is my understanding that the drive itself supplies most if not all of the reactive current, so that would only leave the line side of the drive to supply the real current which in the case of a stall may only be losses as you mentioned and may amount to very little current.

Is this where you were going with this?


So is the point of the line side fuses only to protect cable feeding drive, or to protect the drive from a certain magnitude of fault which the drive my not be able to interrupt?
 

Besoeker

Senior Member
Location
UK
Very interesting point. So during a stalled condition we essentially will be drawing LRC.
Although this LRC can be up to 6x the motor FLC most of this current is reactive current and not real current, or current contributing to "power" as you mentioned.
Yes but....
The drive, unless grossly oversized or incorrectly set up, would, or should, current limit or trip at much less than six times FLC.
 

mull982

Senior Member
Yes but....
The drive, unless grossly oversized or incorrectly set up, would, or should, current limit or trip at much less than six times FLC.

Yes I understand. I just wanted to make sure that I understood that even during a LRC condition the line side of the drive would only see the real current which would be attributed to losses, since the drive supplies all reactive power. I guess power factor for drives is quite good then.

That makes me wonder now even more why fuses would trip. I guess if we were not in a LRC but were approaching the breakdown torque and pulling more than rated current than the line side of the drive could see this excessive current. If drive is not set up correct then the fueses could trip before the drive does in such a condition.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Yes I understand. I just wanted to make sure that I understood that even during a LRC condition the line side of the drive would only see the real current which would be attributed to losses, since the drive supplies all reactive power. I guess power factor for drives is quite good then.

That makes me wonder now even more why fuses would trip. I guess if we were not in a LRC but were approaching the breakdown torque and pulling more than rated current than the line side of the drive could see this excessive current. If drive is not set up correct then the fueses could trip before the drive does in such a condition.
The input to the drive, is probably a rectifier. The output from this rectifier goes to a DC bus. The DC bus is then inverted to create an PWM/AC output.

The output of the drive, via the DC bus, rarely can deliver more than 1.5x rated current, and is typically only able to output 1.15% even if the motor is in a stalled condition.

The amount of current drawn by the input section is dependent on the loading of the DC bus and the internal power of the VFD. The input to the drive is usually a diode bridge rectifier, so there is not a lot of reactive power required, which is why most of these style drives are listed as 95% PF.
 
Drive input currents and output currents are different issues.
If you have a stalled motor, output power is zero even if the motor is drawing FLC. Input power to the motor is losses which I guess could be up to 10% of motor rated output.
The input power to the drive has to supply that 10% plus drive losses which are typically around 3% of drive rating.
I don't think a stalled or stalling motor should blow drive input fuses.

Motor 100%
safe overload(motor) +25% (indefinitie overload rating)
Drive rating could be 100-150% of or rating since drives are rated in current
Drive overlaod capability 150% for 1 minute

So based on the motor current rating the actual maximum normal input current to the drive could be:

225% and depends on the fuse T/C chareacteristics the fuse has to bear this load longer than 1 minute. (I ignored efficiencies>)
 

mull982

Senior Member
Motor 100%
safe overload(motor) +25% (indefinitie overload rating)
Drive rating could be 100-150% of or rating since drives are rated in current
Drive overlaod capability 150% for 1 minute

So based on the motor current rating the actual maximum normal input current to the drive could be:

225% and depends on the fuse T/C chareacteristics the fuse has to bear this load longer than 1 minute. (I ignored efficiencies>)

It looks like this drive has an instantaneous trip point as well. I'm assuming this will trip the drive due to large currents such as shorts on the secondary or within the drive itself.

Are these two points consisting of the drive overload capability and the instantaneous trip usually the two main points for overcurrent trips on the drive itself? Is the drive overload configurable or is it always a particular value that is not adjustable?
 

Besoeker

Senior Member
Location
UK
Yes I understand. I just wanted to make sure that I understood that even during a LRC condition the line side of the drive would only see the real current which would be attributed to losses, since the drive supplies all reactive power. I guess power factor for drives is quite good then.
Yes for displacement power factor.
Distortion pf is another matter and one that I should have looked at earlier. My apologies for that.
Input current to the drive could be discontinuous. Often, on small drives, there are no chokes either in the DC link or in the AC input so the input current might look like this:
Idiscontinuous.jpg

for a 10hp/7.5 kW drive at full load and full speed.
In this case input Irms is just under 20A.
At lower speeds it would be less and, under stall conditions, very much less.
That makes me wonder now even more why fuses would trip. I guess if we were not in a LRC but were approaching the breakdown torque and pulling more than rated current than the line side of the drive could see this excessive current. If drive is not set up correct then the fueses could trip before the drive does in such a condition.
If it happens it is obviously possible for that particular application and the drive settings. Incorrect settings would seem to be the obvious answer.
That said, a typical HRC fuse will take twice rated current for two minutes or more and 150% for more than 15 minutes.
For a sustained overload the fuses would get hot and discoloured.
That doesn't seem to match the circumstances you describe.
 
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