In the previous blog some motors that had been refitted after maintenance were tested from their respective control panels I found that the MIT420/2MIT420/2 had a few quirks whilst using it but nothing that became a major issue or prevented the instrument from being used In the next blog I am off to the other end of the cable and test out some motors at the actual terminal box Actually I am cheating a bit as I have access to motor stores so I tested a new 4kW motor a refurbished 90kW motor and two 132kW motors removed for utilisation at another plant In general testing at the motor terminals is much easier than testing at the motor control centre so I am not expecting any issues with the use of the MIT420/2MIT420/2
The first motor is a 4kW star configured motor. This is easily verified by the shorting strips seen across the three terminals at the top of the block. This means that I will connect the tester to the terminals at the bottom of the block.
The same test procedure used during the test from the panel are used. A phase balance test across each pair of phases and then a single insulation resistance test at 500V. As the motor is new and the store is heated, I am not expecting any issues.
As the table above shows, the phase were well balanced and the 1 minute insulation reading was high with a dielectric absorption ration above the 1.2 minimum. With such a high 1 minute insulation value and the fact that the motor is relatively small, cheap and random wound, I would not progress onto carrying out a polarisation index test on this motor.
So onto the next motor. This a much larger 90kW motor. Looking at its terminals, it can be seen that the shorting bars are now going across the block indicating that this motor is actually connected up in a delta winding configuration. This is typical for larger motors that are predicted to have long running cycles as the motor runs more efficiently, but takes more power to start. The testing principle is still the same with the phase balance measured across either the top set or bottom set of terminals and then a single insulation resistance test to earth.
The insulation test was carried out as both a DAR test and then a PI test to record both values.
As this motor is a much larger motor a much lower phase balance reading is obtained unfortunately I got caught out with the retention of the lead null and the initial readings were 0.01Ohms The lead null was reset and then the readings repeated with 0.04Ohms now being obtained to verify the accuracy a phase balance reading was also taken with the Keysight U1461AU1461A which came out as an average of 0.053 ohms with the lead resistance subtracted Given that this is such a low resistance reading with only a two wire method the results seem comparable to me
Again a good 1 minute insulation value was achieved, however, the DAR value was a little low if going by recommended guidelines. Reviewing the PI ratio result, also indicates a low value, which ideally should have been above 2.0. However, as the 1 minute insulation value is high, the DAR and PI values are not as important and this motor is still acceptable for installation should it be required. This motor is also random wound design and may also be impacting on the PI ratio value.
The insulation readings were recorded manually at 15 second intervals for the first minute and then at 60 second intervals for the remaining 9 minutes to produce the insulation resistance plot seen next to the results table. This shows a rapid rise of insulation resistance over the first minute that then flattens out and shows very little rise.
Music Credit: 'Roadtrip' by Nicolai Heidlas Music
The final pair of motors are part of an old vacuum pumping system that has been decommissioned and removed from one site to another for potential installation. The preservation testing of these motors are more critical, as they would have seen deterioration from use due to thermal, electrical and mechanical stresses during operation, with also the potential to have suffered from dirt ingress over the years. These are also in an non-heated store and therefore have the potential for lower resistance values.
At 132kW rating, this is a much larger motor would be powered from a star / delta starter and not a direct-on-line starter. As a consequence no shorting bars are installed across the motor terminals, meaning that each phase can be tested individually. There is also the added bonus of an anti-condensation heater, that can also be tested.
An expected resistance value for the anti-condensation heater can be calculated based on the rating on the nameplate which was 487.5 Ohms, this can be compared to the measured value and was found to be a good match for both motors. The insulation resistance of the anti-condensation heater is then measured at 250V with all three phases of the motor shorted to earth, as the heater is actually a cable taped to the windings so could short direct to earth, or could short to one of the motor windings. Therefore, shorting the motor windings to earth saves testing the heater against each phase individually.
The motor winding resistance is measured across diagonal terminals for each individual phase and was found to be balanced on both motors, but with motor A showing a slightly higher resistance value. In hindsight though, as with the 90kW motor, I may have been caught out by the instrument retaining the lead null setting from previous tests.
For me, the retention of the lead null settings is proving a bit troublesome. It is the only meter that retains the setting have changing functions or switching of the instrument. I am not sure what the value of this function is.
The dielectric absorption ratio tests were all initially conducted at 250V as the condition of the motors is unknown, so a lower test voltage is used initially to reduce the likelihood of damage to the winding if it is in a poor condition. As can be seen in the tables below, all the motors had relatively low insulation values and poor DAR.
As W Phase on motor B had the best insulation value, it was decided to increase the test voltage on this phase to see if the insulation value reduced significantly which would be a further sign of deteriorating insulation. Not much of a reduction was seen, 60MOhms at 500V in comparison to 61MOhms at 250V.
As the motors displayed no DAR ratio it was pointless proceeding to a PI ratio test and it was decided that the best course of action would be to power up the anti-condensation heater on one of the motors and leave it for a couple of weeks before returning to carry out another test and see if an improvement in insulation resistance is obtained I plan to return later November December to recheck the motors to see if the insulation resistance has improved I will also take the Keysight U1461AU1461A with me to verify the phase resistance readings
This concludes the blog on testing at the motor terminals There were no major concerns found with the use of the MIT420/2MIT420/2 all tests were conducted safely and effectively It was found that whilst clipping to the motor case to prove the earth connections the crocodile clip would sometime be at its maximum opening and didn't grip very well and I did get caught out with forgetting to reset the lead null function
In the next blog I will move on to carrying out insulation tests on air circuit breakers.