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Motor stalling. 3

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itsmoked

itsmoked

Electrical
Feb 18, 2005
19,114
US
I'm going to check out a VFD that is turning a grinding motor today. The customer is telling me, "Yeah, it comes to a complete stop sometimes".

I asked what alarms are being thrown.
None.

I told him that can happen with a VFD in V/Hz mode since the VFD has no idea what the motor is doing. I told him to switch it to 'vector mode' where it should've been all along.

He called me back to tell me, "It is in vector mode!".

Turns out he's never 'tuned it' but I'm still scratching my head about this. I can not imagine how this can happen. Can anyone give me rational for a vector driven motor to stop without alarms. I presume the drive would have to be reading the off winding in realtime and would choke instantly on uncommanded stalled rotation.

Keith Cress
kcress -
 
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Torque limit?
You have to check it yourself.
Be ready for the customer to start a sentence with;
"Ya but. I thought...."

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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The VFD is likely falling back in frequency via a torque limiting mode.

The VFD would be tripping on over-current if the motor was stalled with the VFD still running at some higher frequency.
 
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So.
I found no torque limiting turned on.
They couldn't show me the fault occur but did have it sitting on the screen for me.
The motor was labeled FLA 1A.
They had the drive set to 2A.
They run the motor only at 80Hz.
The motor is used in a wet environment. (Machine-tool)
The motors cost $12~14k a piece. (They have dozens)
They have to rebuild them ~once a year.
Rebuild/refurbish costs $4k/motor.
They run the power into the pecker-head thru an enormous blob of silicone. Eventually it leaks, mold grows, and they get shorts. They scrape off The Blob, clean the pecker-head out, re-terminate, and re-blob.
This occurred on this motor last week....
The motor runs about 30 minutes before it trips out on OVER CURRENT.

I changed their useless Freq out screen display to Load Current. Watched as the motor was turned on and saw it ramp to 1.5A for about 5 seconds and once at speed settle to 0.5A. As the grinding operation commenced the current climbed back to 0.9~1.0A and stayed there.

I left them to monitor the current closely for a day or two.

I expect it to either:
1) start climbing moderately in 20 minutes until OC occurs.
or
2) run fine for 30 minutes then trip on OC with no warning at all.

Keith Cress
kcress -
 
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So you are suspecting a failing “blob” seal and current flowing to ground then? Drive must not have GFP built-in, or someone disabled it because it was such a nuisance...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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hI jEFF; a NUSANCE LIKE CAPS LOCK. lol

It appears to be your favorite brand Teco. I don't recall seeing a GFI function.

I'm suspecting that it may have reached into the windings this time so re-doing the connections might not have been enough.

Keith Cress
kcress -
 
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Machine tools? Are they using water soluble cutting fluid? That stuff is brutal. Really hard on insulation, sealant, etc. The only thing that lasted more than a couple weeks for us was "Form-a-Gasket".
 
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Over here there are two-part gel products which are intended for use in low voltage (i.e. not HV) electrical junction boxes in wet or submersible applications where a regular junction box is doomed to short life and then failure. The gel is mixed and poured into the box where it forms a waterproof seal around the conductors. After it sets it can be easily cut through and the terminals accessed, unlike a Scotchcast resin joint.
 
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John2025; Yes! They use so much they have have a $100k recycling machine to handle it.

Scotty; Got a name for that goo?

I'm actually surprised there isn't a mechanical seal like a HEYCO cord-grip that can do this.

Keith Cress
kcress -
 
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That cutting fluid may also harm operators. Skin and lungs were affected. Today's fluids are probably a lot better than what we had 30 - 40 years ago. Then we saw an operator shortage for that kind of machines. People got sick and no one was willing to work in that mist.

For best drainage, I have used around half a meter (2' for you) of thin tubing going down from the lowest point of the pecker-box. Not too thin, it will clog. Around 5 mm inner diameter (around 200 mils for you) and thick walls. Then it was possible to use ordinary heavy duty glands with silicone compression ring.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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It might be a poor quality vector control loop in the VFD. It's possible the motor can slip back to 0 speed without drawing enough current to over current the VFD.

What is the rated speed of the motor?

Is slip compensation turned-on?

 
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Hi Keith

We like to use "Seal tight" for our ink pumps and simular wet/moist areas, it works really well but I'm not sure how it would hold up to solvents. The issues with using silicone's for sealant is due to Managements pressure for production often times it hasn't cured properly before exposure. I'm sure your aware of this product but thought I would mention it anyway. Link

Chuck
 
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Motor speed is 2 poles 120Hz running at 80Hz.

Today I found out that the poor VFD was mounted on the bottom of the ceiling of the enclosure... The motor was left spinning the stone for 16 hours and when they came in the motor was no longer spinning and there was no error indication. The VFD was ROASTING hot. Sigh.

They removed it from the ceiling but mounted it upside down on a wall and turned the fan to 'on when in run'. sigh[sup]2[/sup]



Keith Cress
kcress -
 
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LOL, I thought you posted 80rpm. Big difference between that and the speed when running at 80Hz.

Still, if the V/Hz pattern is not set correctly then the motor won't be able to create rated torque. People often get it wrong when setting a VFD up to run above 60Hz. When doing some testing, I had a run of the mill 1800rpm motor with an open shaft stall at about 90Hz when the frequency was increased but the voltage was held steady.
 
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The #1 drive killer... People not taking the environmental issues seriously.


Check out this one;
Olam_1_wgnjj8.jpg

It's in a garlic plant (not too far from you Keith). The VFDs are in MCCs which were built as NEMA 1, then they "enclosed" them into stainless steel cabinets because they put them in a hose-down area. But the VFDs and starters kept nuisance tripping because everything inside of there got roasting hot, so they took the doors off of the SS enclosures and the MCCs get wet, twice per day. That is MOLD growing on the VFD! Still works though... so far.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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Lionel; Nope not 80RPM LOL. They take this grinding wheel and hold it against the side of a 16" diameter work-piece rotating in a huge lathe chuck at about 40RPM then they drive the grinding wheel into the side of the rotating work-piece at about a 1/10,000 inch per second. They leave it doing this for hours. Meanwhile you can imagine the fun that happens to all this stuff when the grinding stone comes to an undetected stop.

Normally with the materials they process they cannot push on anything hard at all so things like 40A 3phase machining centers end up never drawing more than 5A their entire lives. Same with this grinding motor. They can barely get it up to 1A without pushing too hard so I don't think they have much problem with the fixed-voltage diminished-torque issues caused by over-freaking.


Jeff; That is disgusting! Kinda scary too, not knowing where you can touch without possibly getting lit-up. Honestly I thought that was a picture of a burn-out!

A buddies brother was put in charge of a Gilroy Garlic place and they were soo cheap and corner cutting he couldn't take it and bailed out. Maybe the same place...

Keith Cress
kcress -
 
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Probably. They called me in to help them come up with the right steel parts to repair the MCC (see rusty bits in photo). I told them the truth, that their only viable option was to move the MCCs to a dry environment, which for them meant building an annex / electrical room next to the existing plant. They rejected that and opted to leave it as is until something fails.

Back to your situation, does the motor nameplate actually say 120Hz, or are they doing the double-voltage trick to extend the speed range?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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