To ensure the reliability of an RV’s electrical devices, especially electric motors, campers must know the service voltage of the hookup their RV is using. Teaching consumers to check that before they plug in the vehicle could save them many headaches.

The effects of excessive temperature on motor performance are notorious. After moisture, they are the greatest contributor to bearing and winding failures. Understanding the source of increased temperature is key to correcting the problem and improving the reliability of your facility’s motor fleet.

The three-phase squirrel cage induction motor is a very robust machine that can operate over a wide range of voltage conditions; it is the workhorse of the industry. However, for optimum performance and life expectancy, the voltage supply should be a balanced three-phase, sine-wave voltage, and its magnitude should be as close to the nominal voltage stated on the motor nameplate as practical. 

Any deviations from these conditions may cause a depreciation in the motor performance and expected life, and increase energy costs. Benchmarking the service entry voltage and the motor performance may be a useful tool to assist in knowing when reasonable limits have been exceeded and that corrective action needs to be taken. 

The following detrimental conditions may alone or in combination exist at various times or continuously: 

1. Voltage variation (+/- nominal)
2. Voltage Unbalance
3. Transient voltage (surges)
4. Voltage waveform distortion (harmonics)

What is voltage variation? Voltage variation is the deviation of voltage from the rated voltage; NEMA MG1 Section 12 allows a plus or minus 10% variation from rated voltage. That rating assumes balanced voltages and acknowledges that motor performance will not necessarily be the same as at rated voltage. "Table 1: Typical effects of voltage change" explains numerically the relationships between various motor performance parameters and a dynamic power supply subject to variation. It was featured in a paper by P. Pillay at an IEEE Petroleum and Chemical Industry Committee (PCIC) conference in 1995 and is shown on Page 24 in EASA's Electrical Engineering Pocket Handbook. The table was a subject of discussion at the recent meeting of the EASA Technical Services Committee in St. Louis. One purpose of this article is to better utilize the information in this table.

There is usually no problem with frequency variation, unless we get into operating at 50 Hz, but that would be an article all by itself, so the focus here will be on voltage variation only. Even though NEMA states that motors are designed to operate within 10 percent of rated voltage, that doesn't give an accurate picture of what will happen when a motor is operated with high or low voltage. Any change from the rated voltage and frequency will affect the performance of the motor, sometimes dramatically.