VFD 70 Troubleshooting

Hey guys, so I have a VFD 70 controlling a fan. It's been running fine for some time now and no modifications have been made recently. It has been giving us a HW Overcurrent Fault. Before changing out the VFD i'd like to see if you could give me some tips.. here's some info

-2- wire control
-started fan while it is completely still, also have the flying start enabled.
-the fault seems to come on right AFTER a input signal from the plc
- It can sit idle no problem and the status indicator is flashing green
-I disconnected the motor leads off of the VFD to see if the fault would go away but it does not.
-There are multiple of fans fed from the same area with the exact same set up and we have no problems with those so as far as cable length and things such as that, i don't think that's an issue.

I'm about to verify the phase to phase voltage on the output side to check the IGBT's but if this isn't the case what else could i do?
Thanks for your time!

From you description it sounds like the current sense in the VFD went bad
they only look at the current when the VFD is controlling the motor in run mode
if its giving the overcurrent fault with the motor disconnected defiantly a problem with the VFD
Just replace it it's not worth the time to trouble shoot it unless it a very large fan.
and you have to consider the down time to repair it as part of the cost.

Thanks for the reply, it's not causing any issues being down so I'll be replacing the drive tomorrow night but since its there I figured I would try to learn something before doing the swap...figured that was gonna be the reply lol

GaryS said:

From you description it sounds like the current sense in the VFD went bad
they only look at the current when the VFD is controlling the motor in run mode
if its giving the overcurrent fault with the motor disconnected defiantly a problem with the VFD
Just replace it it's not worth the time to trouble shoot it unless it a very large fan.
and you have to consider the down time to repair it as part of the cost.

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It could be a bad CT or something in the drive but more commonly with the load disconnected but drive in run many times it’s a micro short from corona breakdown in the wire and very typical of its THHN or similar wire not rated for VFD’s and when installed in conduit.

If you have correct VFD rated cable with proper shielding and grounding then disregard but if not you may want to carefully check that before replacing the drive.

I have had customers replace drives and motors a couple times each on large applications and I went in when it failed again and found wire and cable faults that had been present for some time and likely the core issue all along.

Just my 2 cents.

Apologies guys, I made the assumption that the drive would still be in fault with motor leads disconnected (was told that these were all tested before hand) it turns out the motor runs just fine with motor leads disconnected. As far as proper cable we use teck on all of our VFDs (which i dont think is technically rated for vfd's?)
Another question off topic: This may be a dumb question, but I'm an electrical apprentice so I want to ask the dumb questions now before I'm a journeymen lol... rather be a dumb apprenteice over a dumb journeymen!.... with a load spinning such as a fan at high speeds, would it effect the outcome of the megger test at all?

Hehe. Motor runs fine with leads disconnected? Pretty cool!

Can you explain to use how you are megging the motor? I don’t want to assume your doing it wrong.

gouldmx01 said:

Apologies guys, I made the assumption that the drive would still be in fault with motor leads disconnected (was told that these were all tested before hand) it turns out the motor runs just fine with motor leads disconnected.

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This might be the most important thing you take away from this event. Listen to what they tell you but don't believe it as fact until you investigate it for yourself. Presuming you mean the VFD runs fine without the motor connected.

gouldmx01 said:

As far as proper cable we use teck on all of our VFDs (which i dont think is technically rated for vfd's?)

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Some are rated for VFD use It's hard to say it's not because "Teck cable" is is sometimes used as an electrical slang term.

gouldmx01 said:

with a load spinning such as a fan at high speeds, would it effect the outcome of the megger test at all?

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It can sometimes, dependent on the megger and what test you are performing. If you are performing a phase to phase ohm test it will definitely affect the reading.

If possible the rotation of the motor under test needs to be stopped. If it's not possible. You may have no choice than to disconnect the teck cable at both ends and perform a megger test to ground, and phase to phase tests on the cable preferably at twice the operating voltage. Again, presuming you cannot stop the rotation of the motor if you can, temporary connect another motor (not exposed to the air stream)

Possibly off topic, if the fan rotation is reversed due to airflow backwards through the fan are you using DC injection to stop the fan before you start?

I need to inform you of a particular type of motor failure that can be hard to detect without specialized equipment. This failure can cause high current draw from the VFD (HW over current). These are turn to turn shorts. Note: if you do happen to use the temporary motor method it should make a reasonable home brew test substitute.

P.S. You mentioned being an apprentice, and had questions about a megger test. Always disconnect the VFD from the motor when performing a megger test on the motor. You can kill the IGBT's if you expose them to voltage from the megger.

90% of the time, a hardware overcurrent fault points to a short circuit in the leads or motor and occasionally a short to ground that is so bad, it triggers before a ground fault occurs.

Since you have isolated the load, you have a bad drive. I would still recommend that you disconnect the leads at the VFD and megger test the whole motor circuit including the leads (BUT DON'T MEGGER THE VFD!). If the motor leads or motor has a serious problem you don't want to let the smoke out of a new VFD by hooking it up to a faulty motor circuit.