A new inline ventilation motor operating on a VFD with bypass tripped on startup with the VFD and instantaneously tripped when tested across the line. Smoke was noted in the drive cabinet and MotorDoc was contacted to evaluate the electric motor and work with the VFD installer following recommendations by repair shops to remove the motor for evaluation. The motor located on the 59th floor would be prohibitively expensive to remove for basic testing and would have delayed facility commissioning.
The above information was provided for evaluating the electric motor, a 200 hp, air-air over, 1190 RPM, VFD motor. The inspection of the VFD discovered smoke and the bypass contactor inspected and contacts were found to be welded together.
One of the conditions of note was that the drive disconnect did not trip in bypass, but a main breaker further up the circuit did trip. When circuit breaker coordination is set up correctly, this will often indicate a direct short circuit without the motor winding involved (slower rise-time fault). This prompted us to open the motor connection box as a common defect for this type of problem is a short to ground in the connection box.
Once the defect was identified, an ALL-TEST PRO 5 was used to test from above the VFD circuit out to the motor to verify the insulation to ground and winding circuit to verify the condition of the electric motor winding.
As noted above, the resistance is balanced (the values are lower than the minimum 0.2 Ohms available on a multi-meter), the phase angle and I/F are closely balanced (+/-1 from average) and show a similar ‘medium-low-high’ and the impedance and inductance show a similar ‘medium-high-low’ pattern and are not extreme. The contamination is <5% (frequency-based power factor) and the insulation resistance is greater than 999 MegOhms. The winding was tested at a 400 Hz output. This means that the winding is in good condition and was able to run after correcting the splices and the faulted contactor.
Overall, this saved pulling the motor and transporting it to a repair facility, having it torn down, cleaned, bearings replaced and tested, which would have cost $10s of thousands and delayed opening the facility. Time to test was less than 2 hours (from arrival to departure) and parts on-site the next day meaning the system was up and running for a fraction of the remove to test off-site option. Otherwise, if the motor had faulted, we would have been aware of the severity and could have had planning in-place to return the ventilation to service quickly.
For more information on MotorDoc field services, training and consulting, contact us at info@motordoc.com
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