Insulation check on motor when connected to VFD 2

[golftennis]

Hi,
when a motor 440v connecter to VFD can i measure insulation resistance with 500 volt megger with out disconnecting the motor connection. what are the pros and cons please advice
Thanks

 

[Zogzog]

Pros - Easier
Cons - inaccurate readings, possible equipment damage

 

[Skogsgurra]

Many inverters have voltage measurement circuits based on differential amplifiers with high-resistance voltage dividers. There are usually at least four such circuits, some have more. Each voltage divider is typically around 1 Mohm. Eight such divider legs give you an insulation resistance equal to 125 kohms. It will be very difficult (impossible) to read a several megohms insulation resistance with 125 kohms in parallel.

Some inverters can have six more voltage measuring circuits to monitor incoming voltage, before and after fuses. The latter is used to indicate blown fuses. The resulting resistance is then quite low.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

A question for Gunnar.
Gunnar, will a 500 volt megger be adequate to check an inverter rated motor? I understood that a 480 volt inverter rated motor was insulated for about 2000 volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Zogzog]

NETA recommended min test voltage for 600V rated equipment is 1000V.

 

[Skogsgurra]

Bill, (I know you know this already)

Meggering is not supposed to replace hipot. So, I guess that 500 or 1000 V would be OK. My point is that most inverters have several resistive paths to gnd and that makes meggering with connected VFD meaningless.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[GTstartup]

Skogs, please educate me a little. Aren't the voltage dividers you refer to on the DC side? If so won't the thyristors block the test voltage? If yes, then OK the megger test may not be perfect but not meaningless

 

[waross]

Hi Gunnar;
I am thinking that almost every industrial plant has at least one megger.Many plants don't have hi-pot test sets.
It is common in many plants to megger a motor when it is taken out of storage, before installing it.
The most common meggers I see are 500 volts or 1000 volts.
In the good old days a 1000 volt megger test was adequate for 460 volt and 550 volt motors.
I am thinking motors below about 250 HP. Mostly 10 hp, 15 hp and 25 hp.
When a large motor (500 hp and over) is being commissioned, the rental of a hi-pot set can be justified, but with smaller motors that are typically swapped out be the plant electricians, if the plant does not own a hi-pot set, they will not spend the price of a small motor to rent a hi-pot set every time a small motor is changed out.
My feeling is that over 99% of the time, the megger test will pick up any faults or deterioration of the motor.
BUT, I have not worked that much with VFDs.
Am I on track here or should I at least be looking for a 2000 volt megger to test 460 volt rated inverter duty motors?
Thanks

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

GT,

There are no thyristors in normal VFDs. There are IGBTs. And they have parallel diodes that conduct DC very well.

Anyhow. Even if they werent there, the voltage measuring circuits exist both on mains side and motor side. So, either way, the megger will show isolation values below 1 Mohm. And that is not a useful value and it says nothing about the motor insulation, which usually is a lot higher.

Bill,as I said: "I guess that 500 or 1000 V would be OK" which is also what I think you say.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[burnt2x]

Many years back, we upgraded some equipment and bought VFD's to avail of the advantages these equipment bring. The plant electricians damaged two drives just by megger testing!

We then corrected the setup by using the original contactor to isolate the VFD from the motor lines, megger testing is done on the lower terminals of the contactors and all test results are representative of the motor insulation conditions from then on.