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Why may megger test damage motor winding,while the output current is limited?

Why may megger test damage motor winding,while the output current is limited?

(OP)
Hi.

When using megger,for example,to test 220vac motor winding with 1000vdc megger test,
(I mentioned 1000v,instead the standard test of 500vdc on purpose,for the example).
why can the megger test damage the motor winding if you apply too much voltage when testing the motor,while the current that the megger provides is only max. 1mA?
You may say that the reason is because the 1000vdc,which is provided by the megger on the motor is X4 the nominal voltage of the motor.but i don't understand,the current on the motor winding will not be 1000vdc/R(winding),according to ohm law,the current is limited by the megger to only 1mA,so the current on the motor will be maximum 1mA,while the motor winding is normally operated at much much higher current than 1mA.
So,how may 1mA current damage the motor winding or any other device at that low current?

Thanks.

RE: Why may megger test damage motor winding,while the output current is limited?

The voltage damages the insulation. The current isn't the damage vector.

RE: Why may megger test damage motor winding,while the output current is limited?

It locates - or possibly creates - a weakness in the insulation system. When service voltage is applied the weakness allows the passage of a larger, more destructive fault current through the insulation weakness.

RE: Why may megger test damage motor winding,while the output current is limited?

(OP)
If the megger limits the current,for example,to 1mA and i make an insulation test
on 1000v,does it mean that:it doesn't matter what will be the insulation resistance,
the voltage on the object to be tested will always be 1000v,while only the current will be changed?
If so,how exactly does the megger limit the current to 1mA,while the voltage on the object to be tested stays 1000v?

RE: Why may megger test damage motor winding,while the output current is limited?

A megger is simply an ohm meter that happens to use 250V or 500V or 1000V as the excitation voltage. The current is likely limited by the tiny supply that's struggling to make 1000V from 4 AA batteries. Since a functioning insulation system should be over a GigaOhm ohm's law dictates V = I x R
or
I = 1000V/1,000,000,000ohms
I = 1uA or 1 microamp!

Hence the supply doesn't need to produce hardly any current.

Yes the voltage is fixed and the current is whatever the tested item's resistance allows. If the insulation is screwed up then the current gets limited to whatever the crummy little step-up power supply can provide.

A megger has a voltage regulator in it. When you set the tester to 1000V you're setting the voltage regulator to control the output voltage accurately. Then the device under test provides the resistance. The current is the variable in the system and, as stated, is simply whatever it needs to be keep the system voltage at 1000V thru a load of whatever the device under test presents - up to the current supply capability of the megger's step-up supply. No one really cares about the 1mA since if you have that bad of insulation to allow much more than a couple of microamps your device under test has a problem.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why may megger test damage motor winding,while the output current is limited?

As the insulation resistance drops, the load pulls down the voltage. The last 5 or 6 meggers that I have used show the applied voltage as well as the insulation resistance. Less resistance, less voltage.
For example if the insulation resistance is in the GigaOhm range, but the insulation will withstand only 800 Volts, the voltage will ramp up to 800 Volts. Then, when the insulation is punctured, the resistance will drop and the applied voltage will also drop.
There will be no apparent damage, but the damage will be there.
Now when you apply line voltage to the motor the line current will follow the breakdown path created by the previous over voltage and that's when the smoke and flame starts.
But on the practical side, most North American Motors are able to withstand a megger test of over 1000 Volts.
A very old rule of thumb for testing stated that a device (This rule of thumb predates semi-conductor devices, which are excepted) should be able to withstand a hi-pot test of twice working voltage plus 1000 Volts.
By this criteria a 220 Volt rated motor should withstand (220 Volts x 2)+1000 Volts = 1440 Volts AC. That is a peak voltage around 2000 Volts.
Motor insulation is much improved since this rule of thumb was current.

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

RE: Why may megger test damage motor winding,while the output current is limited?

2
I agree with everything said.

Also consider that the healthy motor insulation is somewhat like a capacitor. You charge it up by applying dc over a period of time. Then when you exceed a threshold voltage (insulation breakdown voltage), the characteristic of the insulation changes, it now may look like a local low resistance at location of the weakness. Hence a lot of that charge accumulated on the winding capacitance may be dumped to the fault in a short period of time (the current associated with discharging the motor winding capacitance can be many times higher than the current supplied by the meter).

As simple numerical example to illustrate the above principle (I don't know if numbers are realistic); if charging current is 0.1 milliamp for one minute and all of the associate charge is discharged in 0.01 seconds, then the discharge current is roughly 0.1mA*60sec/0.01sec = 600mA = 0.6A.

It may not sound like a lot, but it is likely concentrated over a small area (high current density). If there is something resembling a spark you have very high temperatures. You only need enough to locally damage the insulation (such as by carbon trail). In practice if we increase test voltage until a low voltage motor abruptly fails, I would certainly expect it to remain "failed" (as evidenced by lower insulation resistance) when retested at lower voltage.

=====================================
(2B)+(2B)' ?

RE: Why may megger test damage motor winding,while the output current is limited?

A megger ought not damage a fully compliant insulation system. However, it can reveal existing and latent defects, and users are oft to jump to erroneous conclusions about cause and effect, and test equipment.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Why may megger test damage motor winding,while the output current is limited?

(OP)
itsmoked said,that the 1000vdc is fixed whatever the insulation resistance is,
while,waross said,that the voltage is changing and drop as the insulation resistance is lower.
May you clarify this issue?what will be the voltage if the resistance will be close to zero?
BTW,if i make insulation test for a motor winding by applying it the same voltage that it normally operates with(for example:250vdc,instead of 500VDC,for a motor that operates normally at 230VAC,
will this voltage be enough to determine faults?and can it still damage the winding insulation,even that i use the same low voltage(250vdc)?

RE: Why may megger test damage motor winding,while the output current is limited?

Suppose you have a bad cable, some isolation is gone and you can see the conductor inside. If, the metal conduit where the cable is doesn't touch the exposed conductor, there is a good chance that your megger doesn't show a ground fault. To better detect this fault, you have to use a higher voltage. The more voltage you put, the more the insulation material is stressed and may be damaged.

Comment on the last post:
If you have a grounded cable, the megger won't be able to get to 1000 Vdc, because there is no resistance at the grounded point. If the cable is on the verge of failure, the megger will get to 1000Vdc, but the resistance value will be low, because some leakage current (veru low value) is able to flow through the insulation material.

RE: Why may megger test damage motor winding,while the output current is limited?

xchcui; Do you recall I said "crummy little supply"?

The supply is current limited but regulated. If during the initial charging of the device-under-test (DUT) the device's capacitance is large, the "crummy little supply's" voltage will go out of regulation for a while but will eventually reach its regulation voltage.

If on the other hand the DUT has badly damaged insulation the voltage may not ever be able to reach regulation.

If the voltage cannot reach the setting 250V, 500V or 1000V or whatever, then you cannot get the actual resistance of the insulation and either your megger is completely wrong for your DUT or it doesn't really matter as you know the DUT is completely failed in its insulation.

waross and I are in agreement.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why may megger test damage motor winding,while the output current is limited?

(OP)
Thanks for all your responses.
They were very helpful.
Thanks.thumbsup2

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