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#1 raj

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Posted 05 October 2006 - 03:40 AM

What could be possible consequences of accidently mega testing a motor if the VFD is still in circuit. Any predictable damages to the electronics in the VFD or long term failures.

#2 jraef

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Posted 05 October 2006 - 04:39 AM

Possible dV/dt damage to the transistors. What voltage did you crank up to with the megger?
"He's not dead, he's just pinin' for the fjords!"

#3 mariomaggi

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Posted 05 October 2006 - 05:58 AM

Dear raj,
it depends from the time and voltage, and from environmental situation (powders, humidity, moisture).

If the situation is perfect, and the high voltage don't arrived to control electronics, you are safe.
If control card - for example - was not clean, some mA at high voltage could be conducted on the card, over low-voltage components. You could have an hidden problem, i.e. on some pins on integrated circuits, not so used.

An high voltage peak on components reduces MTBF, if your unit is running it is expected to have a shorter MTVF compared to the previous situation.

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#4 jOmega

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Posted 05 October 2006 - 12:55 PM

What is not told is whether the Meggar outptut is a DC or AC voltage. Also not told is the magnitude of the Meggar output voltage.

JRAEF, I would offer that dV/dt (dU/dt) was not in play in this situation as the output of Meggar (AC or DC) does not change that fast.

Depending upon the magnitued of the voltage applied by the Meggar, it is most likely that the below listed components have been subjected to Overvoltage stress.
  • IGBT modules
  • capacitors across the DC Bus
  • IGBT drivers
  • snubber networks
  • possibly the DC bridge

In such a situation, it is highly recommended that you contact the Service Support people associated with the manufacturer of the VFD, and follow their recommendations.

With regard to the solid state devices, if there are doubts about their integrity then testing them with a Curver Tracer should determine if they are ok to use or require replacement. A Portable Curve Tracer could perhaps be RENTED and used at the job site. This would be preferable to removing the devices and taking them to some off-site place to be tested and then re-installing them. You could also disconnect the IGBTs from their circuitry and using an ohmmeter, check for shorted or reduced resistance in the devices. If you find devices that don't pass the ohmmeter test, I would highly recommend replacing the Driver Circuitry at the same time as you replace the Power Modules. If you find ONE power module that fails the ohmmeter test, I'd recommend replacing ALL the Power Modules at the same time.

VFD manufacturer might even recommend taking the VFD to a designated service facility for testing.

OR

You could make sure the VFD enclosure is attached and power up the unit, while standing well away from it, and waiting for the smoke to clear biggrin.gif .

#5 raj

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Posted 05 October 2006 - 10:07 PM

QUOTE(jraef @ Oct 5 2006, 04:39 PM) View Post

Possible dV/dt damage to the transistors. What voltage did you crank up to with the megger?


Well i have few new crews on board that does the testing in the maintenance season. we had quite a lot of VSD failures as we commenced operations. I would say it would be 500 - 1000v.

#6 raj

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Posted 05 October 2006 - 10:15 PM

QUOTE(raj @ Oct 6 2006, 10:07 AM) View Post

Well i have few new crews on board that does the testing in the maintenance season. we had quite a lot of VSD failures as we commenced operations. I would say it would be 500 - 1000v.

thanks omega ... I dont really know if repairing VSD's is a good option, i mean if it is economical and then reliability. Labour costs are fairly high in australia.

#7 jOmega

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Posted 06 October 2006 - 01:38 AM

QUOTE(raj @ Oct 5 2006, 05:15 PM) View Post

thanks omega ... I dont really know if repairing VSD's is a good option, i mean if it is economical and then reliability. Labour costs are fairly high in australia.


I would agree, Raj. It is better to replace the unit and have a high confidence factor than to repair it and have an unknown confidence factor.

Unfortunately, I have known— not just a few— customers who would choose to repair and take their chances that it will not fail in the near future.... than to spend the money to replace it with a NEW unit.

There is a saying......... "penny wise and pound foolish" .... Another saying would be...... "Throwing good money after bad"

Making the repairs will not be inexpensive and add to that cost .... the future cost of downtime and repair/replacement .... and it is never a good decision and an unwise investment.

That said, I think that conventional thinking would be that if the Meggar voltage is 500vac .... that perhaps no damage was done.

But if the voltage is in the 1kv or higher range then I would feel confident that something has been stressed. The application of a DC voltage can be more stressful than an AC voltage of the same magnitude.

In the past, I've had customers return such drives for Inspection, Test and Repair. In such cases, we'd Curve Trace the power semiconductor devices .... and replace all of them if we found any that didn't meet spec.

Before returning the unit to the customer, we'd burn it in at 100% load at 40°C. for 48 hours.

Even with that, the unit could not be given a 100% confidence factor.

As Dr. Phil would say— "When you choose the action.... you choose the consequences."

Cheers.


p.s. you neglected to mention whether the voltage was AC or DC ..... Also, to satisfy my curious mind, how was the Meggar applied ...... Motor Terminal to Motor Terminal ........ OR......Motor Terminal to Ground (Earth) or frame ? ? ?
Thanks ....

#8 waross

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Posted 08 October 2006 - 05:32 AM

Meggering from motor terminal to motor terminal usually results in virtually zero volts being applied, and the megger reading zero ohms. No damage will be done. If the motor is so badly damaged that there is an open circuit terminal to terminal, then there has probably been serious damage done to the VFD before the megger was connected.
yours

#9 shadymatt

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Posted 30 March 2008 - 05:40 PM

hello dear sir,

my self shady matt, from to india,

sir i am put up my immigration file in australia as a skilled indipendent visa, so i want to just know that, an australian practical test (vetassess) organization in mumbai then they will test my self, for my visa, so let me know, what they are will qes. me about my general electrician practical test, i mean that like, engineering drawing, wiring, dol starter or any kind of general electrician practical test.




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