Defective Drive???

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jerjwillelec

jerjwillelec

Senior Member
Location
Nevada, IA
Occupation
Master Electrician
At a grain elevator, I have a 480 volt 7.5 HP motor on a VFD. This motor is running a unload pit drag. Lately, it has been intermintently tripping out the drive overloads on start up (at times under load, other times no load). A week ago I showed up and opened up the drive to find it absolutely FULL of grain dust. We blew it out, cleaned it up and it started to run just fine. Now I'm being told it is doing the same thing. I'm on my way out the door to go troubleshoot it...this time I will disconect and meg the windings to check that out, I will also isolate the drive and check out the amperage. I'm wondering if the drive is shot? Any ideas and comments are much appreciated!
 
jerjwillelec said:
Megger readings at motor @ 1000vdc for 1 minute

T1 327 Mega ohms
T2 329 Mega ohms
T3 327 Mega ohms
Click to expand...
Have you also checked continuity of the windings and current balance?
Assuming the VSD input is three phase, have you checked for current and voltage balance.
 
tripping out the drive overloads on start up
Click to expand...

Please describe what and where these overloads are. I normally don't put overloads on drives. I have a CB or fuses or a manual motor starter on the input side to protect/disconnect the branch circuit into the drive and the drive itself has settings for the motors FLA. Is that set correctly ?

Or is the drive tripping ? If so what is the trip code and what does the manual say about it?

Also, when you blew out and cleaned it up, did you actually clean the heat sink fins of all dust ? Even a small layer left over from blowing it out is a great thermal barrier.

One last thought. It's hot here in Upstate NY, what's the ambient temp. of the drive and motor, etc? Direct sun on the drive ? Get where I'm coming from ?
 
Re connected to drive and ran with no product...

L1 1.6 amp
L2 2.0 amp
L3 2.4 amp

T1 4.5 amp
T2 4.5 amp
T3 4.5 amp

Seems to run just fine for me with no product. Elevator manager says it is real inconsistent in it's failing...sometimes with no product, sometimes with product...sometimes after it's ran for a long period, sometimes on first start up

The drive is a Magnetek GPD 503...approx 20 yrs old.

I'm stumped!!!
 
Many small drives are inherently rated for variable torque loads, which means they usually do not have much starting torque.

Conveyors need to be started 'fully loaded' so they require a drive rated for constant torque loads, if not one that is oversized.
 
justdavemamm said:
Please describe what and where these overloads are. I normally don't put overloads on drives. I have a CB or fuses or a manual motor starter on the input side to protect/disconnect the branch circuit into the drive and the drive itself has settings for the motors FLA. Is that set correctly ?

Or is the drive tripping ? If so what is the trip code and what does the manual say about it?

Also, when you blew out and cleaned it up, did you actually clean the heat sink fins of all dust ? Even a small layer left over from blowing it out is a great thermal barrier.

One last thought. It's hot here in Upstate NY, what's the ambient temp. of the drive and motor, etc? Direct sun on the drive ? Get where I'm coming from ?
Click to expand...

I apologize...my first post was from memory a week ago...

The fault code indicates 'overcurrent fault'...there are no external overloads...only a breaker upstream

Drive and motor are protected from direct sun exposure but yes, this has just began in the last couple of high 90 degree days here in central Iowa
 
Temporary put a fan on it till the heat wave is over, there are also small portable cooling units you can get that just have a hose that can be rigged to push cool air onto the drive to keep it cool.

In the conditions we have across the country right now we have to make do until we get cooler weather, it's been over 100? F here for the last few days, and I have several lift stations that are PLC and VFD controlled that we had to put cooling units on them to keep them from shutting down, normally we never have a problem as we rarely get over 90? F and if we do it's only for a small peek in the middle of the day, right now its hitting over 100? and only drops to 96? way into the night, very unusual weather this year.
 
Hi,

Is it possible that the problem is mechanical and not motor / drive related ?
I am unfamiliar with a " pit drag " so I am guessing but could you have a worn bearing that binds under load or a sprocket / chain issue ?
Stinks to change the drive before your sure.
 
I'd also recommend a new drive. Now's a good time to get the proper drive as well, like mentioned above, one meant for conveyors.
 
jim dungar said:
Many small drives are inherently rated for variable torque loads, which means they usually do not have much starting torque.

Conveyors need to be started 'fully loaded' so they require a drive rated for constant torque loads, if not one that is oversized.
Click to expand...

This is interesting...thank you
 
jerjwillelec said:
At a grain elevator, I have a 480 volt 7.5 HP motor on a VFD. This motor is running a unload pit drag. Lately, it has been intermintently tripping out the drive overloads on start up (at times under load, other times no load). A week ago I showed up and opened up the drive to find it absolutely FULL of grain dust. We blew it out, cleaned it up and it started to run just fine. Now I'm being told it is doing the same thing. I'm on my way out the door to go troubleshoot it...this time I will disconect and meg the windings to check that out, I will also isolate the drive and check out the amperage. I'm wondering if the drive is shot? Any ideas and comments are much appreciated!
Click to expand...

Lets try to summarize it.

20 year old drive: I am surprised the capacitors are still alive. Perhaps time to replace those, although the drives are sooo cheap that it may not be the cost effective solution.

It was full with dust. Two issues here. It may have already cooked some of your chips and now they are pre-destruction failure mode and behaving erratically. Grain dust, when heated, becomes carbon - a conductor - and I don't care how well one thinks the 'blowing out' cleaned out the dust, the carbon tracking on the circuit broards can produce the unrepeateable intermittent faults. These may not even be visible to the casual observer as wery low voltages and currents involved to produce these 'leaks'.

Check the namaplate currents of the motor AND the drive and see if the drive manufacturer constant torque recommended ratios are met.

Distance between motor and drive, motors suitability for inverter use, cable type used, are all potential issues.

It maybe time for a new drive.
 
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