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.