Predictive motor testing in the field

quote:
Originally posted by Aditya:

Which is the IEC 61557 subsection that mentions 1 mA? I went through some; could only find an upper limit of 10 mA. This (61557-8) was however for IT systems; not motors.


The publication is IEC 61557-2, "Electrical safety in low voltage distribution systems up to 1000V AC and 1500V DC - Equipment for testing, measuring, or monitoring of protective measures - Part 2: Insulation resistance." The subsection, under "Requirements", is 4.3, which states:

"The rated current shall be at least 1 mA."

quote:
How is your experience with the AT-33?


I was not impressed. I evaluated three examples of the AT-33EV, which appears to be a variant of the AT-33 IND. These units were used on dozens of motors, good and bad, by several dozen users, all with similar results.

I found the milliohmmeter function of all three examples to be especially unreliable. One of the three had supposedly been revised by All-Test Pro after they learned of my findings. However, I found that it had the same performance issues as the first two.

quote:
Did you pick up insulation faults with it?


Sometimes I could, as long as the faults were constant and significant faults. However, many of those faults were severe enough that even a conventional ohmmeter would have detected them. For intermittent faults, or insulation that had not yet failed, but was beginning to weaken, I could not trust the tool.

I also noticed that insulation resistance values were dramatically different when the tool was plugged in to its charger than when the tool was not plugged in.

quote:
Would you recommend I purchase it or not for condition monitoring of LV motors?


I can't recommend it. Had my three examples of the tool performed as advertised, I still wouldn't have been able to recommend it. As far as I can see, it doesn't do anything useful that can't be done better by cheaper tools, even on a good day.

To be fair, I should mention that the tool distributor who sold the tools, as well as All-Test Pro, did not agree with my findings. They supposedly had an independent third party evaluate several examples of the tool. The third party was Howard Penrose, who, as I understand it, used to work for All-Test Pro.

The report that I received states that Mr. Penrose tested the milliohmmeter function of the tool on a motor, and found that one AT-33, serial number E3301004, measured 38.59 milliohms on Phase 1 of the motor. Another AT-33, serial number E3301008, apparently measured 44.01 milliohms on the same phase.

44.01 millohms is about 14% higher than 38.59 milliohms. That is a hell of a difference between two examples of the same meter, in my opinion.

The report, by the way, found no issue with the two tools. Perhaps All-Test Pro will send you the report if you ask? Or post it on their website?

quote:
There are lots of techniques to test MV motors; but almost nothing for LV motors.


I realize that you already know most or all of this, but for LV windings with PM rotors, you can:

- Rotate the rotor and check for uneven rotation caused by a phase-to-phase short

- Use a milliohmmeter to check for a phase-to-phase short

- Compare phase-to-phase impedances and/or inductances to check for a turn-to-turn short

(these meters can be had for less than $400 USD)

- Use a quality megger to check for a phase-to-ground short

I haven't had any problems diagnosing low-voltage stator windings with these methods.

Jack
Hi Jack,

Thanks for the head-up on the AT-33. I was planning to buy it, will hold on now.

Shorted turns, shorted phases & ground faults aren't an issue; anyone can find these. I really need something that tells me that the ground insulation is weak. Many cases occur wherein motors have good IR but still fail. We're trying PI & Surge for LV motors now; but don't have any definitive case yet.

Regards,

Aditya
quote:
Originally posted by Aditya:

[QUOTE]I really need something that tells me that the ground insulation is weak. Many cases occur wherein motors have good IR but still fail.


What kind of megger are you using, and what is its maximum resistance measurement at, say, 500 and 1000V?
Metrel MI2077 (5 kV), Metrel MI3200 (10 kV) & Megger MIT525 (5 kV). All of them do IR, DAR, PI, SV & DD; good for MV motors (mica based insulation) but do little to qualitatively assess LV motors (enamel based insulation).

Maximum measurement at 1000 V:
MI2077 - 1 Teraohm

MI3200 - 1 Teraohm

MIT525 - Not specified; but 10 Teraohm at 5 kV

Another question often asked by clients - how to ensure the rewinder used good quality copper wire? For the enamel; someone suggested trying low voltage tan delta measurements; which is why I was thinking of the AT-33.

Regards,

Aditya
quote:
Originally posted by Aditya:
Metrel MI2077 (5 kV), Metrel MI3200 (10 kV) & Megger MIT525 (5 kV). All of them do IR, DAR, PI, SV & DD; good for MV motors (mica based insulation) but do little to qualitatively assess LV motors (enamel based insulation).


I haven't used the models you have, but they all have decent specs, so it looks like you are covered there.

I've seen some low-voltage windings that had not yet failed, but had insulation that — although technically meeting specifications — had weak insulation which could be seen with any decent megger by simply watching the bar graph for an uneven climb over, say,a five-minute period. Unfortunately, such cases are the exceptions, rather than the rule.

quote:
Another question often asked by clients - how to ensure the rewinder used good quality copper wire? For the enamel; someone suggested trying low voltage tan delta measurements; which is why I was thinking of the AT-33.


Are they especially known for their tan delta measurements? Reputation is the first thing I look for. On the other hand, it might be worth a gamble if you're willing to buy a tool that may, unfortunately, be useless. Sometimes that is the only way to find out.

its a great to be part of this discussion. great ideas and information shared here.

I will like a personal instructor on Electric motor reliability. i cannot afford any training at the moment.

orugbosamuel@gmail.com. 

Thanks you all for your contribution.

 

Add Reply

Likes (0)
×
×
×
×