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Megger Test On Motors With VFD or Softstarter Connected

12/14/2015 1:41 AM

Hi All,

Good day. Kindly assist to advice on the following. I need to carryout out megger test of motors which is connected to VFD and some are connected to Softstarter. I am not worry about the accuracy of the motor megger reading, all I need to know if the motor is safe to operate. megger test is done for this purpose only. With this requirement my question is to know

1. Can I check megger test using 500vdc megger without disconnecting motor from VFD or softstarter

2. This question is repeated by many but there is no conclusive answer, some say we have to disconnect, some say we can carryout test without disconnection

Testing motor insulation with VFD / Softstarter connected will lead to damage the Thyrister or SCR , is there any way we can overcome this problem and continue to test motor without disconnection.

if one or 2 motors we can disconnect and test, if 10 or more it is not practically worth disconnect and test. Some conclusive answers please

Please advice

thanks and best regards

K.Parimalam

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#1

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 2:03 AM

Sorry, but you need to disconnect the VFD before carrying out a megger test on the motor.

If you need to do this regularly, consider installing a set of removable links on the drive output cabling or, if the motor is not too large, possibly a lockable disconnect switch between the VFD and motor.

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#2
In reply to #1

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 2:25 AM

Hi Thanks. What is the reason for disconnection ?

thanks and best regards

K.Parimalam

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#3
In reply to #2

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 3:47 AM

The voltage spikes from the megger, if applied to the output stage of the VFD will most likely exceed the peak inverse voltage of the IGBTs and may destroy them instantly, or weaken them for inevitable failure at a later date. Don't take the chance.

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#7
In reply to #2

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 3:27 PM

It's so that the Megger doesn't bu99er up the VFD.

If one wants to know that the motor and the cable that feeds are is safe to operate, then one doesn't need to bu99er up the VFD in the process!

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#10
In reply to #2

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 8:14 PM

Isolation. It's the motor that you are interested to test not the drive.

Megger test line-line-ground leak, its an insulation test.

Connecting the drive and the motor, you will have 2 parallel networks and you can not tell which is giving the test reading anomaly.

Here's a guideline for insulation test. This might help.

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#4

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 7:33 AM

What voltage are the motors? What are you actually testing for?

It is normal practice to test each winding separately at 2X the working voltage so you have to disconnect anyway. On most systems 500V won't highlight any problems.

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#9
In reply to #4

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 7:29 PM

Hi Thanks,

Motors are 440V and few are 660V

the plant is remain idle

we have check schedule to record motor megger readings just to see if there no water or moisture got into the terminals are windings. if we could test witout disconnection and the value is about 20mohms we consider motor is safe. with the less manpower at this point of facility we find difficult to disconnect and connect the motors just for megger test. I am also looking for IR monitoring relay to install for continue monitoring motor insulation resistance and alert if any issue.

all that I need is to discuss with experts here and arrive easy (Lazy) and safe way.

thanks and best regards

K.Parimalam

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#12
In reply to #9

Re: Megger test on motors with VFD or softstarter connected

12/14/2015 11:09 PM

So if there's an impending internal fault, you're too lazy to find it.
When a motor you've tested goes bang, how will you explain it to your employers?

You may as well put the megger back in the cupboard as you're wasting the companies time.

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#13
In reply to #9

Re: Megger test on motors with VFD or softstarter connected

12/15/2015 5:33 AM

Easy and lazy is not compatible with safety.

The best way is the safest way;safest for the equipment and personnel.

Many fatalities and accidents result from "short cuts."

No substitute for doing it right the first time.

After all,if you can't find the time to do it right at first,when will you find the time to do it over?

If you persist, and meg the motors with the drives connected,go ahead and order a

dozen or so spare drives,or learn to repair them yourself.

Tell me your location so I can have a highly paid ($$$$$)tech nearby.

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#5

Re: Megger Test On Motors With VFD or Softstarter Connected

12/14/2015 9:02 AM

Turn on motor,

Motor turns, no smoke, OK.

Motor turns, smoke, not OK.

Motor doesn't turn or makes big noise, check for mice.

Hey, that's the way we did it on the farm.

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#6
In reply to #5

Re: Megger Test On Motors With VFD or Softstarter Connected

12/14/2015 9:41 AM

EEIEEEEI OH?

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#16
In reply to #5

Re: Megger Test On Motors With VFD or Softstarter Connected

12/18/2015 8:36 PM

thanks, expected professional answers.

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#8

Re: Megger Test On Motors With VFD or Softstarter Connected

12/14/2015 3:31 PM

Kindly follow British Standard 7671.

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#14
In reply to #8

Re: Megger Test On Motors With VFD or Softstarter Connected

12/15/2015 12:00 PM

The correct maximum leakage calculation was replaced in the 14th edition in favour of a "one size fits all" figure.

Had I followed the IET's recommendations I should have condemned 100's of motors on site and would be looking for a new job.
Something the OP should consider.

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#11

Re: Megger Test On Motors With VFD or Softstarter Connected

12/14/2015 10:41 PM

There is no need of disconnection, as Meggar applied voltage is 500Vdc.

I Have soft-starters and they always have that bypass contactor so why need to disconnect the Soft-starter.

I case of VFD there is need of disconnection as this will lead to failure of Thyrsitors

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#15
In reply to #11

Re: Megger Test On Motors With VFD or Softstarter Connected

12/18/2015 8:34 PM

Hi,

Bypass contactor provide second patch to current flow after attained satisfactory running current as per curve set. Motor connected to softstarter needed not be disconnected during megger test beacause of bypass contactor is available do not merit any value.

Motor cannot be tested using megger without disconnection from VFD is true, the same is applicable for the motor connected to softstarter either with or without bypass contactor

thanks and best regards

K.Parimalam

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