Any Motor Men Out There?

[conny]

Got a bit of a puzzle in work with a submersible pump stator.
Have tested it using a piece of equipment commonly called a 'Baker' Tester which checks for earth leakage from the windings to the stator body. In this test we ramp the voltage up to around the 2kV mark and hold it at this level for 1 full minute. The machine then gives a reading of upto 20,000MΩ, anything above 1MΩ is acceptable though we wouldn't let a motor go if it was less then 500MΩ. That test was fine as the reading was 20,000MΩ meaning there was no detectable earth leakage.
We then do a 'Surge' test on the windings and the results are shown as a sinusodial wave form for each winding phase. If there are no shorted turns on the coils and if each coil phase is insulated from each other then all three wave forms will lay exactly on top of each other. Voltage for this test is around the 1800/1900 volts.
On doing this test the result showed up as perfect. Then the client arrived to witness the tests being carried out. Ran through them all, perfect results. Then he asked if we could just 'megger' between phases with a standard 1,000volt megger. No problem I said.
I was getting a reading of0.07Ω between 2 of the phases! Checked all the leads were separated and the star point was secure but could not get shut of this 'short'. Put it back on the 'Baker' and got perfect result. Anyway, by the end of the day I had done every test I could think of to reproduce this short with every different peice of equipment I could use.
3 different meggers, 2 Baker test units, (these cost upwards of 20k each by the way), 2 ducter units, 1 motorised 5kV megger, (which showed the short), and even fed 120A through the windings to check if the currents were balanced. They were all spot on the same.
Anyone got any ideas why a 1,000volt megger (3 different ones actually and all recently calibrated and checked by an outside company), should show this fault but more 'sophisticated' equipment says it is alright?

 

[Spark123]

What were you testing between? Phase to phase will just show the winding resistance, phase to earth will show the insulation resistance.

 

[Adam_151]

Checked all the leads were separated and the star point was secure but

Not sure what you mean about star point being secure, because surely you are eliminating the star point when you are disconnecting the ends of the windings for meggering?

Or have I mis-understood?

 

[RF Lighting]

How are you testing between windings? i.e. did you put the wires from the end of the windings straight into the tester croc clip, or did you just put the meter ends onto the input studs?

Were the star / delta links removed?

<edit> Pah! far too slow there

 

[JACKC]

Some of the submersible pumps at my old empolyer were 2 speed with the windings linked internally for each speed. Caught many of us out on the first time of meggering during fault finding. How many wires are coming into the terminal box?, did you disconnect the star links in the terminal box? A picture would be nice. Check the data plate to see if it is multi speed.

 

[LoveRocket]

Star point!

 

[conny]

Sorry I haven't been back on, had to go out.
Surprised at so many responses, let alone the speed of them!
Thank you all.
Easy questions first, it is a single speed 185kW, 8 pole machine, (720rpm), FLC is 205A @415V, phase resistance 0.8/0.9&#937;
6 leads into the box, (3 starts and 3 finishes as we call them, 1 of each per phase obviously), no data plate on the machine but we have obtained a manufacturers 'spec' booklet. Using the 'Baker' it has to be connected in Star, the machine sends a 'pulse' down lead 1 through the star point and returns down lead 2. This gives you a datum wave. If their are 'shorted' turns in the coil the waveform tends to 'spike' at the high and low points of the wave, if there are no shorts you get a diminishing wave form. Switching to lead 2 it repeats the pulse down lead 2, through the star point and returns along lead 3. This wave should overlay the first one exactly if there are no shorted turns. You then turn to lead 3 and repeat the process, this time down lead 3, through the star point and return along lead 1.
If all coils in each phase are correct turns, correctly internally connected and insulated from the other 2 phases then all waveforms will overlay each other with no differences. This is known as a 'Surge' test and our tests on 2 different machines gave exactly the same readout. (Should mention that the second machine was from another branch and was a later model than the one we have).
The 'Hi-Pot' test is basically an insulation resistance test of all windings to earth, basically a 'glorified' megger test! The reading for this was 20,000M&#937;+ (scale only goes to 20,000M&#937 after 0ne minute.
So, according to these machines the stator is perfect.
Client, out of the blue asked if I could open the star point and 'megger' between phases using a 'Megger BT322' hand held megger. 3 ranges on it, 250v,500v and 1,000v. So now I have 6 leads marked as follows;
U1-U2 = Red phase,
V1-V2 = Yellow phase,
W1-W2 = Blue phase

Now to megger between phases you only need to megger between U1,V1,W1. First test;

U1 to V1 = 999M&#937; (Pass)
U1 to W1 = 999M&#937; (Pass)
Second test;
V1 to W1 = 0.07&#937; (Fail, it seems we have an almost dead short)

Made sure all leads were seperated and tried again, same result.
Tried a number of times, re-arranging leads, placing on insulated slot liners, changing to 2 different meggers etc all to no avail. It was showing as a short between Yelow phase and Blue phase. Set up the 'Baker' again and got perfect results. Client decides to call it a day and leave us to sort it out, (thankfully!). So I decided to connect up the star point and feed 150Amps through the windings, if there were any shorts it would show up on the Klippon as different readings for the faulty phase(s). Blue phase was out by about 7amps lower so swapped the Blue feed to the Red, Red was now 7amps lower so I figured it was the supply not the windings causing the drop.
(Also carried out a test which is frowned upon by managers as it can go wrong and hurt someone, what you do is to take a small bearing, the whole bearing not just one of the balls, and 'flick' it round the core of the stator. If the windings are ok and you flick it right the bearing will run round the core till you switch off just as a rotor would. If there is a fault with the windings it will either stop where the fault is or run jerkily past the fault till it skews off course and flies out! It ran round like a perpetual hamster for 5 minutes till I switched off)

Final note, the manufacturers 'spec' states the resistance for each winding should be between 0.8 and 0.9&#937;, our readings were, 0.85&#937;, 0.86&#937; and 0.86&#937; so that was correct as well.

I am at a complete loss as to why this is happening.
If there is anything else you need to know just ask.

 

[LoveRocket]

Right mate, thanks for your description of how the posh tester works, it was quite interesting (if you are an electrical geek! )

I reckon you have a short between V1 and W1 - the tester won't have picked this up because these are connected together anyway at the star point.
To confirm this, continuity test (rather than IR) V1 - W2 (or V2 - W1), if I am right then you'll see approx 0.94ohms as you are in effect measuring the winding resistance via the fault.

I bet if you make the star point U2-V2-W2 you will get some faults on the Baker test.

 

[conny]

Hi LR,
thanks for the reply. Sorry for the long winded description but thought it best to explain how it works so people might understand better.

I reckon you have a short between V1 and W1 - the tester won't have picked this up because these are connected together anyway at the star point.

Yes they are connected at the star point. However, with the star point open all resistances are balanced. If there was a short between yellow and blue phases the resistance would be different to the red phase. (Would not class 0.01&#937; as a significant difference on any motor never mind one this big). Also if 2 phases were shorted the wave forms would differ from the datum wave form, we proved this to the client by testing a smaller motor with such a fault on it).

To confirm this, continuity test (rather than IR) V1 - W2 (or V2 - W1), if I am right then you'll see approx 0.94ohms as you are in effect measuring the winding resistance via the fault.

By this do you mean with something like a 'Fluke' multi tester?
If so, I did that and got '0' meaning open circuit between V1 and W1. Also tried using a low ohm meter and that too showed no circuit between these phases.
Pretty certain if you suggest a test then I've more than likely done it, but please, keep suggesting. There may be one simple thing that I am missing. (And my colleagues as well! )

BTW, the star point was made up with U2,V2,W2

 

[LoveRocket]

Hmmmmmmmmmmm........................!

So if you use a low-reading ohmmeter (con tester) between V1-W1 it shows open circuit, but with a high-reading ohmmeter (IR tester / 'Megger') it shows 0.07ohms? Do you get the same when IR testing at 250 or 500v?

I don't get what you are saying about the resistances being balanced - assuming you are measuring U1-U2, V1-V2 and W1-W2, then these readings would not be affected by a fault between windings, and the waveforms would not differ if the fault was in parallel to the star point connection.

You say that V1 and W1 are connected at the star point, but then say that the star point is U2-V2-W2 - is that a typo as it's confused me!

 

[NotHimAgain]

Have you considered the instrument! The '1000volt Megger' is probably the type designed for use on electrical installations rather than on machines. As such its capabilities will doubtless be limited to that field of use.

The reason your specialist instruments cost so much is that they require a more sophisticated measuring system to produce an accurate result. The required degree of accuracy is likely to be much higher than that produced by an instrument and measurement method designed for electrical installations (+/- 30 %).

I would be looking at the output of each type of instrument - you might find that the specialist devices produce a steady (true DC output) whereas the Megger might be producing a square way. This could have implications due to the relatively high capacitance of the windings

 

[Spark123]

Bog standard Meggers deliver DC for test purposes, this negates the need to correct for capacitive reactance.
We use meggers on motors a fair bit, never had a problem with using that instrument as all a motor winding consists of is wire.

 

[ban-all-sheds]

Bog standard Meggers deliver DC for test purposes

Don't they deliver it in a pulse? i.e. with rising and falling edges which are going to be able to induce current in other conductors?

But in any event - even if that were an effect here, the test would show the same misleading results between any pairs....

 

[ColJack]

are you certain that the ends are marked up right?

V1, W1 might be the missmarked ends of one winding.. ?

 

[flameport]

I was getting a reading of 0.07&#937;

Are you sure it was 0.07 ohms, rather than 0.07M&#937; ?
Very unlikely that an instrument which would be designed to provide readings in the M&#937; range would display results to 0.01 ohms

If this was 0.07M&#937;, which is 70000 ohms then the answer is there is a fault between V1 and W1 (or V2 and W2, depending on whether you are connecting U1V1W1 or U2V2W2 as the star point when doing the other tests).

The low ohms meter would show o/c as that would typically only go up to a few hundred ohms. It is also possible that the 70k fault only shows up at higher voltages.

The other machine won't show this as a fault as it is right at the star point. Probably damaged insulation either in the terminal box or just behind it.