Introduction
This is an update to my original review of the MetraHit Coil motor testing multimeter from Gossen Metrawatt.
MetraHit Coil Motor Testing Multimeter Blog 1
My primary interest in this instrument is the coil test function, that is for detecting inter-turn faults within motor stator windings. On testing a number of motors available to me, I had obtained a mixed bag of test results. As no support was obtained from the manufacturer, I decided to build up different versions of the the coil test adapter to see if I could obtain results on motors where the original coil test adapter had produced an 'OL' reading.
The biggest culprit to this was a 22 kW motor that I could not obtain a reading for under any winding configuration, a number of the motors in star winding configuration and finally a 1 MW, 3300 V motor that I could not obtain a reading from at all. Initially, I tested the new coil test adapters against the four reels of wire I had used previously.
Coil Test Adapter Build and Initial Tests
I opted for smaller sized capacitance to see if I could bring the reading under the 250 us limit of the MetraHit Coil. Instead of modifying the existing coil test adapter, I decided to make up some new ones courtesy of some parts from Farnell.
You can see the parts required can be obtained for a reasonable cost. You will also note a case I purchased for the meter, that will be shown further on in the blog.
For the first design, I reduced the capacitance down to 0.33 uF and tested on the coils of wire I had used previously. This dropped the reading down to 77 %, so I opted for a dual design with a 0.22 uF and 0.15 uF capacitor to carry out some further testing.
A 0.22 uF dropped the reading to 62 % of the original and the 0.15 uF dropped it to around 51 %. You will also see a set of test results for a 0.1 uF capacitor. This one came later after I had tested some motors and did not really see much of an improvement with the 0.33 uF, so modified that adapter for the 0.1 uF capacitor. I also created a 0.075 uF adapter, but could not get any readings with this either on any of the motors or the wire coils I have been testing, this has therefore been omitted from these results. Interestingly though, the 0.075 uF device produced the same 'OL' reading on screen as the other capacitors when it failed.
Uh-oh moment
Initially after building up the initial coil adapter, I could not get any readings on the MetraHit Coil, even using the original coil adapter. This was traced to a component on the PCB that I must have dislodged when I desoldered the jacks from the PCB during the teardown.
This was in the output circuit and close to one of the output jack connections, just hidden by the wire wound resistor in the picture above. It looks like I unknowingly heated one of the pads and then moved the component and broke it away from its other pad. Soldered back into position, thankfully the MetraHit Coil started to work again.
Motor Tests
The following tests cover the different motors tested with different winding configurations and with a range of different capacitors within the coil test adapter. I have listed the tests in order of motor kW size.
As can be seen from the results below, the original test only recorded values for the winding on the 15 kW motor configured in delta. When utilising the 0.33 uF and 0.22 uF units, values could be recorded for both the delta and open winding configurations. The 0.15 uF and 0.10 uF units obtained readings for all three of the winding configurations. Both the units produced stable readings across all three phases and I considered the testing of this motor to be a success.
On the 18.5 kW motor, the original test adapter failed only when the motor was in star winding configuration.
Using any of the four coil adapters I made up, I could obtain a reading for all of the winding configurations, as shown below. There readings were reasonably consistent and stable across the range of tests.
The 22 kW motor was one of the motors that I could not obtain any reading for with the original adapter. However, no matter what winding configuration or capacitor size I tried, I could not get a coil test value from this motor. This motor has significantly higher inductance in the windings than any of the other motors that I have tested, and I presume this is the factor that is causing the issue. In the delta configuration though, the inductance is around 16 mH and in certain configurations, both the 15 kW and 18.5 kW motors have inductance values close to 16 mH.
The motor is actually a special wind as the frame size is a 200L. This frame size relates to the physical dimension of the centreline of the motor shaft to the bottom of its mounting feet. The following motor frame size table is from the ABB L motor guide.
It can be seen that a 22 kW motor would generally be in a 180 frame size, unless it was a 6 pole motor. A look at the motor label below, tells me that the motor being tested is a 1470 rpm, so it is a 4 pole motor, therefore it would generally be in an 180 frame as per the table above. This indicates that having a larger than normal frame size would indicate hat the winding / stator core could be physically larger than expected in a 180 frame and I am assuming that is affecting the inductance of the motor, and therefore the manner in which the coil test function is working.
Luckily enough a second hand pump has ben transferred to the stores that is actually a standard 22 kW motor in a 180 frame, I therefore proceed to carry out coil tests on this for a comparison.
You can see that a reading was obtained for all the winding configurations and all of the coil adapters used. There was some instability with some of the readings, but this appeared to be consistent when looking at the specific winding and coil test adapter arrangement. You will also notice a 0.37 uF capacitor arrangement being tested. During the testing of the dual adapter box, I experimented and found that I could parallel up the two capacitors due to the common terminal arrangement and still obtain successful readings.
This test arrangement was tried on this motor and produced a reading, which therefore gives me more versatility with the dual adapter box of actually three test configurations. The single 0.15 uF capacitor at around 50% of the original, the 0.22 uF capacitor giving about 60% of the original, and the 0.15 uF and 0.22 uF in parallel to give 0.37 uF which equates to around 75% of the original test results.
The next two motors are 30 kW and 37 kW. Originally, Had no issues when testing the 30 kW motor as I could obtain a reading in all winding configurations. This was proven to be the same with all the capacitor arrangements I tested on this motor. The results were constant and stable across all the tests and this has shown to be an almost 'ideal' test motor for the MetraHit Coil.
The 37 kW motor is somewhat different. On the original tests a reading in star winding configuration could not be obtained and there was a little sporadic behaviour with the other winding configurations. There was some corrosion evident on the motor terminals and I did consider if this could be causing the slightly erratic readings.
For the star configuration, both the 0.37 and 0.33 uF capacitor adapters failed to produce readings. I did get the W-U phase test to produce a result with the 0.22 uF arrangement. At 241 us, this value is close to the 250 us limit of the instrument, which could explain why the other two phases failed to produce readings. However, on all the other winding configurations, the other two phases always read lower than the W-U reading, so there appears to be another issue at play.
With a 0.15 uF capacitor, the U-V and V-W readings are obtained in star but now the W-U reading could not be obtained. The readings have dropped down to the 200 us range, so there should be no reason as to why the W-U test should have worked. At 0.10 uF test arrangement a reading is successfully obtained for all winding and capacitor test arrangements. Consistently the U-V test reads lower than the other two phases. Looking at a summary of the test results for the delta configuration of the winding and the one star configuration that produced a value on all three phases shows the irregularity of the tests.
The coil differences trend from 3.43% up to 18.77%. With Gossen Metrawatt advising that an inter-turn fault is likely to be present at reading above a 10% differential, the motor switches between displaying a fault and no fault in the windings. No trend is observed across the results, such as increasing differential with an increase in capacitor size. The results on this motor are a little disappointing, and even more disappointing is the fact that I cannot seem to pinpoint the problem.
The 55 kW motor seems to fair a little better.
In star configuration a reading could not be obtained for the original adapter with a 0.56 uF capacitor and the new adapters with the 0.33 uF and 0.22 uF capacitors. A full set of readings was obtained for both the 0.15 and 0.10 uF capacitors. A similar pattern was seen for the results for the new adapters, which seems to show the opposite for the U and W phases with the original adapter. It may be that I have inadvertently crossed over the U and W phase connections when doing the tests. They were carried out on different days and the terminals on this particular motor are not marked.
The erratic behaviour seemed to return for the 90 kW motor.
A full set of readings for all the winding configurations could only be obtained for the adapter with the 0.22 uF capacitor. The U Phase always seems to produce a higher reading, that was matched by the W Phase in star configuration, but was in delta and open winding, the W Phase reading was closer to V Phase. In star configuration a reading was obtained for U Phase that was close to the 250 us limit, but as this phase was always highest, I would have expected a reading for the other two phases as they were usually lower. For the 0.15 and 0.10 uF capacitor based adapters, odd readings were missed despite the values for the other phases being well below the 250 us limit.
The final tests were conducted on the 1MW 3300V motor, that I could not obtain a reading for with the original coil test adapter. I only tested this motor with the 0.33, 0.22 and 0.15 uF adapters.
As can be seen from the table above, a reading was obtained for all three of the new adapters for both delta and open winding configurations. The results were all stable and balanced out well indicating that the motor was in good condition.
The following table provides a summary of the tests carried out.
It does actually show that as the test capacitance is decreased the number of successful readings increased. The 0.10 uF capacitor seemed to be the most successful, so over time I think I will look to replace the 0.15 uF in the dual test adapter with a 0.10 uF to give a better test range with just the one extra adapter.
A Case for the New Adapter
I never purchased a case for the MetraHit Coil meter and really just left it in its original cardboard box. Two hard cases are available from Gossen Metrawatt, one for a single instrument at £45.00 and the other for two instruments at £69.00. Two single instrument soft cases are also available that would not take the original adapter with the meter, never mind adding in my new adapter. I decided to order an empty plastic case and utilise shadow foam I have, to build up my own case for the MetraHit Coil.
| {gallery} MetraHit Coil Case |
|---|
Original card board box with new adapter |
All the contents do squeeze in with the lid closed |
New moulded plastic case |
Shadow foam cut-out added to case |
The shadow foam provides a nice neat solution to fitting the meter and all of its accessories into the case to prevent damage in transit. If I want, I still have the option to add in another foam insert to the top of the case to store the communication lead and crocodile clips if I want to go down that route. This is also cheaper than purchasing either of the two hard cases available from the manufacturer.
Conclusions
The MetraHit Coil continues to be a bit of a mixed bag for me. Improvements have been made with new coil test adapters being built and I have been able to obtain readings for some of the motors that would not work with the original coil test adapter. There are however, some motors that still produce erratic behaviour and a 22 kW motor that still defies having any reading taken. I am not sure how much I can progress with the tester now without input from the manufacturer, which does not seem to be forth coming.
I will look to replace the 0.15 uF capacitor with a 0.10 uF capacitor to extend the test range of the coil test adapter. If I can find the right size capacitor with a suitable voltage rating, I will try a setting somewhere between the 0.10 uF that works and the 0.075 uF that I found did not work.
Over time I may approach some local motor rewind shops, to see if I can expand my test results further and potentially even find a known faulty motor with shorted turns to test.
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