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packerfan

hello, i have a lot of 3 phase motors running conveyors on my bottle line at the brewery and have a quick question for you guys. i had a Danfoss drive fault out and it said the motor had a short circuit and to check the motor leads , etc. when i disconnected the cable coming in from the motor to the drive then the drive would not fault. so i go out to the motor and do preliminary checks and can't find anything wrong between the motor and the drive. i did a quick ohm check of the motor and across all 3 legs seemed fine. i proceeded to change out the 2h.p. motor and then fired everything up and all was ok then. i didn't get a chance to megger the motor later because it was thrown out. My question is that what else is the quickest sure fire way to check/test one of these motors because this is a high production output line and time is money. thanks for any input.
 
If you get the same fault again try reducing or increasing the carrier frequency before you turf the motor.

It's very difficult to test motors and get results that would indicate what the drive is seeing. Most Mega testers use a DC test voltage which isn't necessarily going to be an accurate reflection of the winding insulation at the carrier frequency of the drive output. The waveform at the drive output is also nothing like a sine wave and some motors, especially cheaper ons don't have winding insulation that's sufficiently robust to withstand being driven by a VFD. We've even had motors that have shown insulation faults on a VFD and were tracking to earth but they've run fine for years after that on a DOL 3-phase supply.
 
There’s no quick test for a motor and the supply cable. Either do it right or don’t bother.
If you can split the windings there are nine tests on the motor. Then there’s the cable to test.

I’ve worked on various packaging plants, a second failure doesn’t go down too well with the boss. I know, I was the shift production manager and I can shout very loud.
 
thanks for the reply Tony. one of our senior electricians told me to trust the message from the drive, they are usually right. even the time i took to do my preliminary checks of course is too long in the production bosses eyes. sometime i think they just want parts changers.
 
There’s no quick test for a motor and the supply cable. Either do it right or don’t bother.
If you can split the windings there are nine tests on the motor. Then there’s the cable to test.

I’ve worked on various packaging plants, a second failure doesn’t go down too well with the boss. I know, I was the shift production manager and I can shout very loud.


what are the nine tests? do you mean ohm test between L1-L2, L1-L3, L2-L3, L1-E, L2-E, L3-E, or should I be doing something else?
 
hello, i have a lot of 3 phase motors running conveyors on my bottle line at the brewery and have a quick question for you guys. i had a Danfoss drive fault out and it said the motor had a short circuit and to check the motor leads , etc. when i disconnected the cable coming in from the motor to the drive then the drive would not fault. so i go out to the motor and do preliminary checks and can't find anything wrong between the motor and the drive. i did a quick ohm check of the motor and across all 3 legs seemed fine. i proceeded to change out the 2h.p. motor and then fired everything up and all was ok then. i didn't get a chance to megger the motor later because it was thrown out. My question is that what else is the quickest sure fire way to check/test one of these motors because this is a high production output line and time is money. thanks for any input.
The output from a variable frequency drive is a PWM waveform. This is typically a pulse train at several kH and each pulse has a very fast rise time.

Here's one I did earlier:

quick motor check? {filename} | ElectriciansForums.net

Note that the time scale in in us with major divisions at 0.5us intervals.

These sharp edges can damage the motor insulation between turns. The above was measured on a site where motors were failing every few weeks because of it. It's not the sort of damage you can easily detect with a multimeter until it gets to the point of catastrophic failure. By which time.......

A couple of thoughts.
Are there any output reactors/filters in the VFD?
The motors should be VFD rated. I had one site where they were not and again they were failing regularly. Quite big motors at that. As one wag put it 800kW motors should not be treated as consumables.

The motors were rewound to make them suitable for VFD operation. And the problem went away.
 
As one wag put it 800kW motors should not be treated as consumables.

Who says?

I got the blame for blowing a 2000HP motor up 6 times after I’d been experimenting with the start up torque. No VSD, it was a slip-ring motor. It went with a hell of a bang.

There were some red faces when I tracked the problem down. Nothing to do with me.

It ended in a court case for damages.
 
Who says?

I got the blame for blowing a 2000HP motor up 6 times after I’d been experimenting with the start up torque. No VSD, it was a slip-ring motor. It went with a hell of a bang.
As a matter of interest, what starting method were you using?
 
Liquid starter 1200A @1750V.

I’ve worked on Kramer drives that start conventionally and then swap to VSD rotor control with regeneration feeding back in to the mains.
 
Liquid starter 1200A @1750V.

I’ve worked on Kramer drives that start conventionally and then swap to VSD rotor control with regeneration feeding back in to the mains.

Sometimes known as a sub-synchronous converter cascade.
I know a bit about these.
 
hello, I'm wondering what type of preventative/predictive maintenance you guy's do where motors are concerned? it seems to me that most places run to failure and then replace the motor. If you do things what are they? thanks .
 
Cheers, that will help or confuse others. But they can see what we’re talking about.

We used them on stone crushers, as a conventional starter they’ve got the grunt to get a blocked crusher going. For stone sizing the speed is then varied, a slower speed gives a larger particle size.

I had a thyristor pack fail on a 2500HP fan. When the pack fuse blew it put the fear of god up me, I was stood near it.

quick motor check? {filename} | ElectriciansForums.net


quick motor check? {filename} | ElectriciansForums.net
 
Cheers, that will help or confuse others. But they can see what we’re talking about.

We used them on stone crushers, as a conventional starter they’ve got the grunt to get a blocked crusher going. For stone sizing the speed is then varied, a slower speed gives a larger particle size.

I had a thyristor pack fail on a 2500HP fan. When the pack fuse blew it put the fear of god up me, I was stood near it.




Sounds like a quarry or a cement works.
Bad fuse selection if that happens to them.

We design and build our version of the Kramer - it has that step up chopper in the DC link that's in the diagram I posted above. They are usually upwards of 1MW but we have a few in service at about 400kW.

Apart from the merits of starting torque, they have a few other merits. Efficiency is a big plus particularly for 24/7 operation and larger powers. And that sells. I don't want to get into boring technical detail here but just a couple of figures. If you take a typical large variable frequency drive it is is likely to be 12-pulse and have to have a dedicated unit transformer. Between that and the VFD total losses are typically 4% or greater. For the Kramer it is usually under 1%. And harmonics are much lower.

Admin, for the avoidance of doubt, this is not a sales pitch.
 
I think I need to brush up on my theory, I had only come across thyristor's In small low voltage cases in electronics

if I remember right don't they only let a set voltage through and disparate the rest through heat?

(edit it's coming back to me from my 2330 at college, my lecturer really like his motors etc)
 
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I think I need to brush up on my theory, I had only come across thyristor's In small low voltage cases in electronics

if I remember right don't they only let a set voltage through and disparate the rest through heat?

(edit it's coming back to me from my 2330 at college, my lecturer really like his motors etc)
Essentially switching devices, they are either on or off.
Three terminal devices, gate, anode, and cathode.
Think of a diode. It passes current in one direction, anode to cathode, and blocks it in the other. So it will pass current in only the positive half cycle of an AC supply.
Thyristors are sort of similar in that they also can pass current in one direction, anode to cathode so again, only the positive half cycle of an AC supply.

And then only if you tickle the gate. So you can delay the point at which you turn it during that positive half cycle.
So you can vary the conduction period from zero to 180 degrees and you have a means of varying the voltage applied to the load. Often, these are in a "back to back" arrangement such as in dimmers and you can control both positive and negative half cycles.

quick motor check? {filename} | ElectriciansForums.net
 

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