Hipot testing of 3-phase motors and equipment

[steelerr]

Hi everybody,
I have to perform Ground Bond (GB), Insulation Resistance (IR) and Dielectric Withstand (DW - a.k.a. Hipot) testing on new 3-phase industrial equipment as part of a third-party electrical safety certification.
I will soon purchase an Electrical Safety Tester (EST) to perform these tests (currently I'm renting a unit for learning purposes). Typically, I'll first do the GB test on the equipment, then on the line power circuits I'll perform the IR (Megger) test, then AC hipot, then IR again.

Testing specs as defined by my NRTL:
GB test - 25A for 1 second
IR test - 500Vdc for 1 second
Hipot test - 2 x line voltage + 1000V for 1 second (~2000V)

The hipot testers I've researched allow a minimum current value (say, 0.1mA) to be entered to ensure that the tester is actually connected to the device under test (DUT)- that is, to make sure I don't have an open test lead.

Question 1 - Does this parameter make sense? Shouldn't I expect zero current flow, even at 2000V?

Question 2 - How should I connect my 2-lead hipot tester? Do I connect the HV lead to each winding connection one at a time and the return lead to motor frame? Do I have to read across windings L1-L2, L1-L3, L2-L3? Should L1-L2-L3 be jumpered together and then the reading is taken from that connection to ground?

Thank you.

 

[edison123]

All windings will have a leakage current in an ac hipot. The amount of leakage current will depend on the type of winding, insulation materials used etc.

I own and use many ac hipot testers and none of them have this "minimum current requirement". May be you misunderstood the specs ?

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[steelerr]

Hey edison123, the minimum current spec I referred to is that leakage current you mentioned in your reply. If I have an open test lead or a bad connection, the hipot tester will know that and give a fail result.
Maybe I should disable this parameter until I get more familiar with hipot testing.
Thanks.

 

[Zogzog]

Not to offend or anything but are you qualified to do a 3rd party certification? Usually 3rd party certification requires certified people to do the certification. What certifies you to do the certification?

Overpotential testing can be a destructive test if not done properly, what equipment are you testing? I cant think of anything that would require a 500V IR test AND a hipot test. What testing standards are you following?

 

[steelerr]

My third-party NRTL is TÜV SÜD America. I am following their requirements which in this case will bring our equipment into compliance with UL, CSA, and CE electrical safety requirements.

EN 60204-1 specifies these test criteria.

I am testing 3-phase motors and associated line power wiring and devices in my company's industrial manufacturing systems.

Bob Steele

http://www.gala-industries.com

 

[Zogzog]

Are these a LV (600V) motors? What test voltage does your standard give for the hipot test? What are the min acceptable values for IR and hipot per your standard? What Hp ratings are the motors (> or < 200 hp)?

To answer part of your 1st question the current setting you see on your hipot is a high limit, not low limit. It is a safety feature. That is the leakage current that will trip the hipot off line. Lets say you test at 2000V and your motor has a IR of 20 megohms, your leakage current will be about 100 microamps + the leakage of your test leads (Which you should measure befroe and subtract from your motor test result).

Does your standard have you calculate a DAR and/or PI? IEEE/ANSI Std 43 strongly cautions against doing a hipot test on any motor with DAR <1.4 or PI <2.0.

 

[steelerr]

*Are these a LV (600V) motors?
Yes, LV motors - usually 460VAC or less
*What test voltage does your standard give for the hipot test?
(2 x line voltage)+1000V. I'll round up to 2000V for 460VAC.
*What are the min acceptable values for IR and hipot per your standard?
IR - 1M-ohm
Hipot - No threshold is defined by EN60204-1. I have set a measurement limit range to be from 0.1mA to 5.0mA, based on some references I saw while researching this subject. The 0.1mA verifies that the device under test is connected, as I understand it. I am questioning whether this is a reasonable range to test for. Is 5mA upper range too low?
*What Hp ratings are the motors (> or < 200 hp)?
Usually <100HP

I'm afraid I don't understand your reference to DAR <1.4 or PI <2.0.

Thanks for your interest. I'm looking forward to gaining some more knowledge on this subject of hipot testing.

I'm still wondering how the tester should be connected to a 3-phase motor.





Bob Steele

http://www.gala-industries.com

 

[Zogzog]

IEEE and NETA specs require at least 100 Meg for a 600V rated motor, 1 Meg is very low.

I think you are misunderstanding the measurment limit range, read my previous respomse again.

DAR is a Dielectric Absorbsion Ratio that is done on inductive equipment, PI is a Polarization index. I recommend you download and read "A Stich in Time" from the

http://www.megger.com

it is a good introduction to overpotential testing and will also help you with your connections question.

 

[steelerr]

I downloaded "A Stitch in Time" from megger.com. Thanks for the suggestion.

Looking at Page 11 of that guide, I see the statement, "For many years, maintenence professionals have used the one-megohm rule to establish the lower allowable limit for insulation resistence. The rule may be stated:

Insulation resistance should be approximately one megohm for each 1,000 volts of operating range, with a minimum value of one megohm."

So I'm confident that the one megohm that I'm testing IR to is adequate.

But this thread has drifted off-subject, I'm afraid.

I'm looking specifically for hipot testing advice for 3-phase motors and line power wiring. I'll continue to read the guide for overpotential testing and see if this applies.

Bob Steele

http://www.gala-industries.com

 

[Zogzog]

I have answered your hipot questions, you just havent listened to the advice I have given you.

 

[electricpete]

There are two components of the currents that will flow:

resistive currents and capacitive currents.

Resistive currents will typically be very small.

Capacitive currents can be much larger during an ac test.

If you want to try to roughly equate megaohm limits with current, it applies primarily to the resistive component.
Your tester has to provide enough current to handle the capacitive current as well as the resistive components.

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[waross]

Re testing motors;
The winding resistance in a motor is very low, less than 1 ohm in large motors. connect all three motor leads together to one hi-pot lead and connect the other hi-pot lead to the motor frame. Or, given the low resistance of the motor windings, you will get the same reading connecting to only one motor lead.
If testing is done from the supply end of the cable, the same advice applies.
If your testing standard requires cable quality to be reported separately from the motor quality, you will have to disconnect and test the conductors individually. You can get a pass/no-pass reading by testing all the conductors but individual testing gives you a benchmark for each cable for future evaluation of insulation quality.
For maintenance testing, a plant may establish a pass/fail value and test motors and cables together with a connection to one conductor at the supply end. If the reading is below the threshold value, the cable would be disconnected from the motor and individual testing would be done to isolate the failing component.
I think you will have to test the motor and each supply conductor individually to satisfy your standard.



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

 

[EngBM]

At what minimum voltage do we use the fomula (3VL + 1)Megaohms to calculate the minimum IR value which is acceptable.

How was the 1 Megaohm minimum derived. Is it universal for fractional horse power motors to multiple MVA motors.

 

[Zogzog]

"A stich in time "is a very good guide for theory and how to test, but it is old and outdated for some standards.

The IEEE 43 rule of kV+1000 for the min spec was removed in the 2000 version, the current IEEE 43 now states:

For most DC armature and AC winding build after 1070 (Form wound coils) the min OR is 100M

For most machines with random wound coils and form wound coils less than 1kV the min IR is 5M

For winding made before 1970, all field windings, and others not described the min IR (in MegOhms) is kV+1

So using the old statdard for your 460VAC motors the spec would be .46kV+1M=1.46M ohms

Using the updated standard it would most likely be 5M, either way 1M is low.