# Meggering AC Motors whilst connected to a VFD Drive.



## frenchelectrican (Mar 15, 2007)

G'day John.,

Unforetally if you going to run the megger when the VFD still hooked up on load side you will fry it no question asked.

I have see someone did ruin one allready and that cost that compaine over 21 000 Euros for 3 sets of VFD's due that person goof up with short cut methold.

I am sure other guys in this fourm will chime in simauir answer it will take little more time to unhook from load side of VFD and do the right thing.

Merci,
Marc


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## Jlarson (Jun 28, 2009)

Yeah, don't do that. 


Isn't there a disconnect you can open_ somewhere_ and megger from there?


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## John Hansen (Feb 26, 2012)

frenchelectrican said:


> G'day John.,
> 
> Unforetally if you going to run the megger when the VFD still hooked up on load side you will fry it no question asked.
> 
> ...


Mr Frenchy,
Many thanks for your help. I will head your advice. I wanted to ask aorund before deciding what to do. Gues it is back to the grind of pulling things apart for very small gain. Many thanks again. John.


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## Wireman191 (Aug 28, 2011)

As Larson mentioned, Why not add a disconnect down stream from the VFD's just for this purpose? 
I'm sure they would pay for themselves within a very short time if you are doing this monthly.


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## frenchelectrican (Mar 15, 2007)

Wireman191 said:


> As Larson mentioned, Why not add a disconnect down stream from the VFD's just for this purpose?
> I'm sure they would pay for themselves within a very short time if you are doing this monthly.


If John Hansen want to add a disconnect switch near the motour then there is one more item he will have to add is aux concat ( second concats ) on the disconnect switch on the lever or camshaft of the disconnect switch so that way it will cut the command signal to the VFD to shut it off before the disconnect blades slide off the concats.

That is one of my SOP to make sure you have a aux switch in there.

Merci,
Marc


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## varmit (Apr 19, 2009)

DO NOT MEGGER ANYTHING CONNECTED TO A VFD!

I agree that adding a disconnect would be a good idea as I assume the megger test is a preventive maintenance procedure for the motor. 

Marc is correct. You would want an auxiliary contact in the disconnect, on the disconnect mechanism, to prevent the drive from having an output unless the disconnect is closed.

A disconnect for a 800 HP motor is going to be really expensive, but disconnecting and reconnecting the leads,to the motor, each month, will wear out the terminals and the wire at the terminations. What voltage is this system?


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## John Hansen (Feb 26, 2012)

*Hello Varmit. Meggering*

Hello Varmit,
The voltage is 690VAC. The drives are for drilling motors. I was thinking we could megger them at 500VAC and perhaps there would be no harm, whereas normally we megger the motors at 1000VAC. If the system operates on 690 I would think that meggering at 500 would be ok. Adding disconnects is not an option and they would be too large and with 10 motors too many. We do not have the room available. Normally we disconnect the three ABB800 drives for each motor by removing all the metal switchboard mountings and then unbolt the drives from the fuses remove the fibre optic connections and then pull the drives out fron the stabs at the back. We can then access the motor cable connections to the VFD's. Disconnecting at the motors is an even bigger job.

I appreciate your taking the time to answer my query. regards John.


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## varmit (Apr 19, 2009)

The issue would not be the megger voltage, but ANY VOLTAGE, FROM AN OUTSIDE SOURCE, CONNECTED TO THE DRIVE OUTPUT, CAN DESTROY THE DRIVE.

I would try contacting the drive manufacturer or your local representative for this brand drive and see if there is some modification kit that can be retrofitted to the drives that would lessen the effort to disconnect them from the motor. 

Are the cabinets large enough to install fusing or a breaker in the drive output? This would not be an ideal setup, but this disassembly sounds like a real ordeal on a regular basis. 

Y'all must have a healthy power system to start and run a bunch of 800 HP motors, even if they are on VFDs. Do you use really long start ramps to bring the load up gradually?

I reread you previous post. From what you wrote, there is not much chance of adding anything down stream of the drive. Is there any place that the buss bar, on the drive output , could be modified to have a bolt-in splice plate to separate the motor from the drive?


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## frenchelectrican (Mar 15, 2007)

Varmit.,

I am thinking that one of your suggest that seems will work the best especally if they have to megger the motour pretty often espcally with large conductors we know how hard to take everything apart. 

If he do have room a simple disconnect switch will work good and that will justify the cost of using a disconnect switch vs the time of labour to take it apart and back together.

Merci,
Marc


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## John Hansen (Feb 26, 2012)

*Meggering with VFD drives connected to the motor.*

Thanks to all who have helped with this problem. I have contacted the maker of the drives and spoke to an engineer and I will paste his answer here below.

When they built this rig they should have installed Assignment Contactors which would make it a lot easier but they would cost a lot of money. Guess I will have to continue dismantling the drives each time we need to megger the motors.


Hello, 
I have limited info that you enquired if motor can be megger tested while connected to ABB drive. NO 
Do NOT megger drive, you must disconnect any leads you wish to megger.








*Peter Zol*
Team Leader Drives Service
ABB Australia Pty Limited
LV Drives Service
Bapaume Road
2170, Moorebank, New South Wales, AUSTRALIA


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## varmit (Apr 19, 2009)

Thank you and welcome to the forum.

You definitely have a "pay me now or pay me later" situation.

Good luck and happiness to you.
Harry


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## JRaef (Mar 23, 2009)

Seems odd that you must dismantle the drive, all you should need to do is uncouple the motor leads. 

For those that suggest a disconnect, re-read the First post: 800Hp! That would not only be a very large disconnect, as in 1200A or more, but it would add a lot of wire bending space as well. It would likely require it's own separate cabinet. If you have room for that cabinet, it might still be worth doing. I would use a simple lock setup to avoid opening that switch under load, because you only ever need to open it when meggering which means the line disconnect must be open first anyway. So put a lock on the load disconnect and set it up so the key is only accessible if the line disconnect is open as well. No chance of opening it under load then. 

If there is no room for another cabinet, you are stuck with uncoupling the motor leads. Just to reiterate though, NEVER do it with the drive connected at any voltage level that would be useful to check the motor leads. Even at 500V, (which I am not endorsing), that does not tell you enough about the motor lead integrity at 690VAC RMS. Don't forget that at 690AC RMS output from a VFD, the peak pulse voltage from the PWM output is really the DC bus voltage at all times, which in this case would be almost 1000VDC.


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## John Valdes (May 17, 2007)

I would just stop meggering the motors each month.


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