Plant Services - June 2008 - (Page 48)

reliability Motors The results Test date Voltage 1-2 (Fund RMS) Voltage 2-3 (Fund RMS) Voltage 1-3 (Fund RMS) Average imbalance Ph-Ph NEMA derating (%) Voltage 1-2 (THD) Voltage 2-3 (THD) Voltage 1-3 (THD) Voltage 1 (Fund RMS) Voltage 2 (Fund RMS) Voltage 1 (Fund RMS) Voltage imbalance Ph-N Current 1 (RMS) Current 2 (RMS) Current 3 (RMS) Average current (amps) % of full-load amps % current imbalance Current 1 (THD) Current 2 (THD) Current 3 (THD) Self-impedance imbalance Idle 475.53 475.68 475.12 475.44 100 0.94 0.77 0.93 274.36 274.68 274.45 0.07 154.86 155.45 161.10 157.14 105.46 2.52 1.62 1.16 0.97 2.13 Running 454.28 454.29 451.72 452.43 100 7.47 7.56 8.09 261.29 262.78 261.30 .038 162.90 162.75 158.95 161.53 108.41 1.60 5.96 5.60 6.22 2.33 Table 1. Test results with the winder down and with it running. taminated with dirt, carbon dust and the like, the PIP will display significant spiking throughout the test. Table 1, when the winder wasn’t running, current harmonics (THD) were less than 2%, voltage harmonics were less than 1%, full load current was 105% of rated current, and the system voltage was 475 volts. With the winder running, current harmonics (THD) were greater than 5%, voltage harmonics exceeded 7%, full load current was 108% of rated current and a system voltage drop was 21 volts. These power-quality problems were reported to the appropriate department managers and then filed away. A new microprocessor-based dock lock system for their eight-bay loading dock had been purchased and the electrical supervisor’s crew had installed a new power feed for the controllers. The vendor installed the lock system and it was under warranty. It worked fine most of the time, but sometimes the locks would open and close spontaneously. The controller manufacturer’s field service group (including their design engineer) had made several trips to investigate. They replaced several electronic cards and two complete controllers. The supervisor’s crew took the power for the new feed from the motor control center in question. The electrical supervisor remembered the power quality report and wondered if the power quality issues might still be on the supply from the motor control center. After reviewing the report, the electrical crew installed line filters on the new controllers. The dock lock controller problem disappeared. There are three tips to remember. First, a battery of online and offline testing can detect broken rotor bars. Online testing includes demodulated current and current signature analysis. Offline testing includes a rotor influence check, which graphically depicts broken rotor bars. Second, a modification to the standard polarization index test can provide a better analysis of the health of insulation systems. Third, power quality can be used to analyze a power system and correct the harmonic problem that motor drives can cause. David L. McKinnon is a project manager at PdMA Corp. Contact him at mckinnon@pdma.com and (813) 621-6463 ext. 125. Power quality Initial test data on a another motor indicated the voltage harmonics were greater than 5% and full load amps were at 107%. We researched the power distribution system to the motor control center to try to identify what might be causing the harmonics. A 13.8-KV, 2,500-KVA transformer was the feed for the motor control center’s 460-volt service handling eight motor lineups and five DC drives. The other DC drive power transformers in the plant supply power only to the DC drives and aren’t used in combination with AC motors. This suggested the DC drives might be generating the power-quality problems. We ran further testing to compare the results when the winder wasn’t running and when it was. As shown in 48 More resources at www.PlantServices.com Motor repair – “Analyzing premature winding failures – part 1” Motor repair – “Analyzing premature winding failures – part 2” Motor insulation – “Inverter-duty motors battle high-voltage spikes” Power quality – “Unbalanced currents” Drive harmonics – “Bad vibes – part 1” Drive harmonics – “Bad vibes – part 2” Applying IEEE Std 43-2000 – “Testing the insulation resistance of rotating machinery” For more, search www.PlantServices.com using the keywords harmonics, polarization and rotor. www.PLANTSERVICES.com June 2008 http://www.PLANTSERVICES.com http://www.PLANTSERVICES.com http://www.PLANTSERVICES.com

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Plant Services - June 2008

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