Fuses Blowing on VFD

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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?

Short on the output will not necessarily shut the drive down.

The drive's self protecting settings are usually not available tot he customers, but service can have access tot he settings. How clever of a hacker are ya?:D

It sounds like a nuissance trip due to improper fuse selection.
 
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?

The magnitude of LRC is voltage dependent and in most ASD control modes the voltage AND frequency are both varied.
 
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.

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.

I think we are putting the cart before the horse when we are not refering to the ASD's current rating AND setting when looking at the input protective device malfunction.
 

Besoeker

Senior Member
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I think we are putting the cart before the horse when we are not refering to the ASD's current rating AND setting when looking at the input protective device malfunction.
What I have shown is input current.
Input fuses are failing.
 

mull982

Senior Member
Ok I found the maunal for this drive and see where it lists the recommended fuse sizes. I'm going to go ahead and change fuse sized out to those that are recommended.

In this particular application the feeder cables to the drives origanate in a cabinet and are fused at this point. The feeder cable to the drive is a #10AWG and then at the drive there is another set of fuses on the line side. So should I follow the line side fuse recommendation for both sets of these fuses as long as the set in the cabinet is still capable of protecting the feeder cable? In other words should both these fuses be the same?

I guess in this case the first set of fuses would be protecting the cable, and the second set would be protecting the drive.

The understanding I've gained from this discussion about the fuses protecting the drive are that they are in place to protect the drive from current values outside which the drive is capable of protecting against. The drive has some fixed (drive protection) and some adjustable settings (load protection) that will protect the drive but there may be other cases outside of these settings that the fuse will need to interrupt. The manufactureres fuse size recommendation should be followed in these cases.

In the case where I didnt have two sets of fuses does it matter how far upstream the drive protective device can be from the drive?
 
Ok I found the maunal for this drive and see where it lists the recommended fuse sizes. I'm going to go ahead and change fuse sized out to those that are recommended.

In this particular application the feeder cables to the drives origanate in a cabinet and are fused at this point. The feeder cable to the drive is a #10AWG and then at the drive there is another set of fuses on the line side. So should I follow the line side fuse recommendation for both sets of these fuses as long as the set in the cabinet is still capable of protecting the feeder cable? In other words should both these fuses be the same?

I guess in this case the first set of fuses would be protecting the cable, and the second set would be protecting the drive.

The understanding I've gained from this discussion about the fuses protecting the drive are that they are in place to protect the drive from current values outside which the drive is capable of protecting against. The drive has some fixed (drive protection) and some adjustable settings (load protection) that will protect the drive but there may be other cases outside of these settings that the fuse will need to interrupt. The manufactureres fuse size recommendation should be followed in these cases.

In the case where I didnt have two sets of fuses does it matter how far upstream the drive protective device can be from the drive?

The drive and cable can be protected by one set of fuses, the fuses at the drive would be redundant. The fuses are to protect the drive from internal faults, such as shorted rectifier section, shorted terminals etc. Overload protection for the drive is internal, integral and electronic.
 

jim dungar

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The fuses are to protect the drive from internal faults, such as shorted rectifier section, shorted terminals etc.
Actually, the fuses do not prevent any of these items from occurring. The fuses limit additional damage to the drive, after a fault happens. Usually the fuses are there to remove a faulted drive from the 'power grid', and to prevent it from failing 'violently'.
 

ptonsparky

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I have limited experience compared to most here, but the drives that I have worked with that were blowing input fuses, you could see or smell the reason. It was not caused by the motor load.

Which set of the fuses are blowing, directly ahead of the drive or at the beginning of the cable?
 
Actually, the fuses do not prevent any of these items from occurring. The fuses limit additional damage to the drive, after a fault happens. Usually the fuses are there to remove a faulted drive from the 'power grid', and to prevent it from failing 'violently'.

So - since your reply was in response to my post - where did I say that the fuses are to 'prevent' faults from occuring?
 

jim dungar

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So - since your reply was in response to my post - where did I say that the fuses are to 'prevent' faults from occuring?
I included your exact quote in my previous response.

What other meaning did you have behind this phrase?
weressl said:
...protect the drive from internal faults...
 
I included your exact quote in my previous response.

What other meaning did you have behind this phrase?

What does OCPD stands for?

Again; I did not use the word 'prevent', so please don't attribute it to me nor should you assign meaning to it.

Protective devices are designed to limit the consequences of an event, preventive devices are designed to prohibit the occurance of the event.

Of course it would be more meaningfull to call it OCLD, but fat chance of that.:)
 

jim dungar

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I just look at the phrase "protect from" and I come up with "prevent" as having a similar meaning, maybe its just me?

My point is that very very few, if any, fuses or breakers actually protect the downstream equipment. The primary function of an OCPD is to limit the damage (magnitude and/or duration) caused upstream to a power system because of a fault.
 

mull982

Senior Member
Great discussion guys. Thanks a lot for all of the information.

So it looks like as others have stated that the line side fuses protect against any fault from within the drive, while the drive protects against any fault currents on the load side of the drive. For instance if there was a fault at the motor, the the drive would trip to remove this fault. Is this a fair summary?
 

Besoeker

Senior Member
Location
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Great discussion guys. Thanks a lot for all of the information.

So it looks like as others have stated that the line side fuses protect against any fault from within the drive, while the drive protects against any fault currents on the load side of the drive. For instance if there was a fault at the motor, the the drive would trip to remove this fault. Is this a fair summary?
I think so.
But it still doesn't explain why you have input fuse failure for an output mechanical overload.
I note the point that the manual suggests 30A fuses. At full rated output current and very lumpy input current I guess continuous rms current might get close.
But a stall condition...
Well, a 20A fuse rating should not be exceeded.
FWIW, I looked at a standard 20A HRC fuse and it can take 30A for about 17 minutes.
 

Besoeker

Senior Member
Location
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Fair enough. I don' think the drive is necessarily grossly oversized though and still trip an undersized fast fuse on an 150% 1min. overload on the output. ASD manufacturers usually require fast fuses to protect the front end power electronics.
A few points in no particular order.

An overload on the drive output won't always translate to an overload on the input. The stalled condition is a case in point.

Fuses are pretty rubbish at protecting against moderate overloads. All of those I have looked at go off the time current chart at 150% of their rating. Operating times at 150% rating are indeterminate but upwards of 10^3 seconds even for semiconductor protection fuses.

Many, maybe most, VFD manufacturers don't specify high speed fuses. I've been down this route more than a few times with consultants' specifications that are often cut and paste out of stuff from two decades prior.
For the Powerflex 40 as cited here, there is a note about recommended fuse types. One given is BS88. This a standard industrial HRC fuse.
 
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