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Case Study: Energized Motor Testing Reveals Improper VFD Settings

Load fluctuations can occur when a VFD asks a motor to continuously change speed, even when a change in speed is not really required for the motor to perform its intended function.

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Easy Tool is an Italian Condition Monitoring Solutions Provider with service and support representatives in Italy. Easy Tool, located in the town of Fabriano, in the province of Ancona, represents ALL-TEST Pro (Old Saybrook, CT) condition monitoring instruments and performs regular condition monitoring services for a local business that gravure prints food packaging materials. This gravure printing is a type of ‘intaglio’ process that uses specialized printing presses to engrave designs into plastic food packaging materials.

This food packaging printing company has a condition monitoring professional from SPM Instrument Italy come on-site to their facility 3 times each year to test their equipment, trend data, and support their reliability program. In November 2015, Ettore Di Pasquale, Field Engineer, was performing a dynamic test on a 55-kilowatt, 400-volt motor at the food packaging printing facility when he discovered an abnormal current waveform.

The Application

This 55-kilowatt, 400-volt motor is controlled by a variable frequency drive (VFD). This motor drives the centrifugal fan that sends air into a natural gas combustion burner. The gas burner, which needs the fan for the proper combustion air dosage, is required by the steam generator to create heat for the drying section of the printing machine. The heat generated dries the ink on the plastic packaging.

The Discovery

Di Pasquale had connected the ALL-TEST PRO On-Line II energized motor testing instrument to the electrical panel in order to conduct the condition monitoring tests at a safe distance from the operating motor. He quickly discovered there was an abnormal current waveform (see graph below).

“While there was no visible cause for alarm, the motor testing showed evidence of strong load fluctuations,” explains Di Pasquale. “Load fluctuations can occur when a VFD asks a motor to continuously change speed, even when a change in speed is not really required for the motor to perform its intended function.”

Di Pasquale discussed the issue with the managers at the food packaging printing plant. He explained that when a VFD asks a motor to continuously change speed, that this activity can put a lot of stress on the motor mechanics and the motor windings. If the VFD continued to vary the motor speed in this way, over time the motor could have eventually exhibited rotor problems or winding failure.

The Solution

To prevent motor failure, Di Pasquale suggested the VFD be checked. Trusting Di Pasquale’s recommendation, the maintenance managers at the food packaging printing plant contacted the VFD supplier. The VFD supplier went on-site to re-program the VFD, adjusting it to correctly drive the motor.

A Second Round of Energized Motor Testing

After the VFD supplier re-programmed the VFD, Di Pasquale went back to the food packaging printing facility to perform a follow-up energized motor test. He wanted to make sure there were no more load fluctuations or abnormal waveforms. He again brought the ATPOL II to confirm that the problem had been corrected. The second round of energized motor testing with the ATPOL II showed the load performance had been greatly improved after the VFD reconfiguration / PID correction (see below). Testing proved the VFD supplier correctly re-programmed the VFD to drive the motor.

Lessons Learned

1.  Variable frequency drives need to be set correctly. Improper programming of VFDs can reduce motor life through incorrect operation over time.

2.  Consistent condition monitoring is critical for preventing equipment failure. This food packaging printing facility essentially prevented motor failure by having their highly-skilled condition monitoring technicians perform regular tests and look for ‘red flags’ indicative of problems.

3.  There are easy-to-use portable instruments, like the ATPOL II, that can help you check the health of your motors. Make sure you have the proper tools to support your condition monitoring program.

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