The bench tests and desktop review have all gone well for the MIT420/2 Insulation Tester. In the next blog, I carried out some phase balance and insulation resistance tests from a low voltage (400V) motor control centre (MCC) for a gas turbine. The motor control centre is a large panel with two main feeders and then split down into multiple direct-on-line starters for various pumps and fans that the gas turbine requires to operate. The motors had been removed for maintenance work and then refitted and reconnected, so a quick test of the motors was required to show that they were safe to reinstate ready for a direction of rotation check.
In all three motors were tested;
- A 415V, 30kW Exhaust Ventilation Fan Motor
- A 415V, 37kW Cool Air Fan Motor
- A 415V, 45kW Exhaust Frame Cooling Fan Motor
Each motor has two tests carried out;
- A phase balance test using the resistance / continuity function of the MIT420/2
- A 1 minute insulation resistance test using the 500V range on the MIT420/2
The objective of the phase balance is to ensure that all three phases of the motor have been connected correctly and there is not cable damage or loose terminations.
Obviously the insulation test is done to ensure that there are no earth faults on either the cable or the electric motor.
As the motors are all fed from direct on line starters, the phase balance test is carried out first to ensure the motor connections are good and then a single insulation resistance test is done, as the phase balance test has shown that the phases are all interconnected.
The first motor control centre is a swing-out direct-on-line starter used for smaller motors. All of the controls are mounted onto the door frame and I gain access to the motor contractor by opening up an inner door. This makes measuring the phase resistance a little bit of a challenge as the probes have to be held in one hand across the contractor terminals whilst pushing the MCC apart to gain access. Mean while the instrument is held in the other hand. The meter will read the phase resistance automatically, but pressing the save button was more of a challenge as the meter is held in my right hand, I cannot reach across to the save button. As the panel is 5 foot from ground level I cannot rest the instrument on the ground. I would have really benefited from a magnetic style loop accessory so I could have left the meter hooked up onto the next panel to release my right hand to help take the measurements and operate the save function when required. The instrument looks like it it has a fixture on the back for such an accessory, but I have not been able to find one yet.
The 1 minute insulation test went much easier as one of the crocodile clips could be used to clip to the metal chassis of the MCC. As the instrument has the built in dielectric absorption ratio timed function that could be set up before applying the leads, it was an easy one handed operation to operate the instrument. On insulation testing, the reading is also stored on screen at the end of the test, so, unlike the phase balance test, the save button could be pressed without the probes connected. Had I wanted to carry out a polarisation index test that would have required manually writing down the readings every minute, I would have had a very hard time without having an assistant or utilising some other form of connection for the leads.
The test results can be seen in the table below. The phase balance was stable and equal across all three phases. The one minute insulation value was 5.8GOhms which is excellent for a combined motor and cable test of all three phases together.
The dielectric absorption ratio was 1.36 which is above the minimum threshold value of 1.2.
The tests show that motor was connected correctly, with no earth faults and was ready for a run to prove the direction of rotation.
The next MCC panel was a fixed starter panel and slightly easier to work in than the swing-out starter. This type of panels are for slightly larger motors with direct-on-line starters and the largest motors with star-delta starters. The same test procedure is utilised, however ad the contractors in this type of starter have unshrouded connections, the crocodile clips can be utilised to make the connections which removes all of the fumbling around when trying to use the probes in one hand and operate the instrument in the other.
The phase balance test results from this motor were unsteady. This sometimes happens when fan motors are tested and the fan is free-wheeling in the air flow turning the motor. This results in small voltages being induced into the motor stator windings that can prevent an ohmmeter from making reliable readings and is not a problem with the instrument. This can be seen in the high U-V reading compared to the other two phases. A bad connection would have given a steady high reading and not a reading cycling from low to high.
As with the other motor, the insulation test readings were very high and the dielectric absorption ration above the minimum accepted value.
The final motor to be tested was a 45kW motor, this was also a fixed starter type panel and the leftist motor of the three to be tested. For this motor I also decided to carry out a polarisation index test to test this aspect of the MIT420/2 that had not been tested on the other two motors.
For this test I tried to make a video, but I apologise for the audio quality, as it was drowned out by the switchroom and panel fans, so I have tried to do some voice over which doesn't explain things quite as well.
There were no problems carrying out the tests using the MIT420/2 as the crocodile clips could easily be used with the open terminal style contactor installed. At the end of the PI test, it was noted that the instrument had gone over the 100GOhm resistance limit for the 500V range, so internally no PI ratio was recorded. For the purposes of the data table and the insulation resistance plot, I used the 100GOhm figure.
Despite going up into high GOhm resistance, the test results clearly show DAR and PI ratios, this is likely to be due to testing from the MCC that includes the motor and the cable which the latter would generally exhibit a capacitance trait when connected to a DC supply.
This completes this blog, the sensible thing now seems to go to the other end of the cable and test out some motors on their own.
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