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Motor Operation Efficiency Under Abnormal Conditions

Motor Operation Efficiency Under Abnormal Conditions

Operation under unusual service conditions may result in efficiency losses and the consumption of additional energy. Both standard and energy-efficient motors can have their efficiency and useful life reduced by a poorly maintained electrical system. Monitoring voltage is important for maintaining high-efficiency operation and correcting potential problems before failures occur.

Preventative maintenance personnel should periodically measure and log the voltage at a motor’s terminals while the machine is fully loaded.

Motors must be properly selected according to known service conditions. Usual service conditions, defined in NEMA Standards Publication MG1-1987, Motors and Generators, include:

  • Exposure to an ambient temperature between 0°C and 40°C
  • Installation in areas or enclosures that do not seri- ously interfere with the ventilation of the machine
  • Operation within a tolerance of ± 10 percent of rated voltage
  • Operation from a sine wave voltage source (not to ex- ceed 10 percent deviation factor)
  • Operation within a tolerance of ± 5 percent of rated frequency
  • Operation with a voltage unbalance of 1 percent or less

Over Voltage

As the voltage is increased, the magnetizing current increases by an exponential function. At some point, depending upon design of the motor, saturation of the core iron will increase and overheating will occur. At about 10 to 15 percent over voltage both efficiency and power factor significantly decrease while the full-load slip decreases. The starting current, starting torque, and breakdown torque all significantly increase with over voltage conditions.
A voltage that is at the high end of tolerance limits frequently indicates that a transformer tap has been moved in the wrong direction. An overload relay will not recognize this overvoltage situation and, if the voltage is more than 10 percent high, the motor can over-heat. Over voltage operation with VAR currents above acceptable limits for extended periods of time may accelerate deterioration of a motor’s insulation.

Under Voltage

If a motor is operated at reduced voltage, even within the allowable 10 percent limit, the motor will draw in- creased current to produce the torque requirements imposed by the load. This causes an increase in both stator and rotor I²R losses. Low voltages can also prevent the motor from developing an adequate starting torque. The effects on motor efficiency, power factor, RPM, and current from operating outside nominal design voltage are indicated in the diagram below.

Voltage Variation Effects on Motor Performance

Voltage Variation Effects on Motor Performance


Reduced operating efficiency because of low voltages at the motor terminals is generally due to excessive voltage drops in the supply system. If the motor is at the end of a long feeder, reconfiguration may be necessary. The system voltage can also be modified by:

  • Adjusting the transformer tap settings
  • Installing automatic tap-changing equipment if sys- tem loads vary considerably over the course of a day
  • Installing power factor correction capacitors that raise the system voltage while correcting for power factor

Since motor efficiency and operating life are degraded by voltage variations, only motors with compatible voltage nameplate ratings should be specified for a system.

For example, three-phase motors are available with voltage ratings of 440, 460, 480, and 575 volts. The use of a motor designed for 460-volt service in a 480-volt system results in reduced efficiency, increased heating, and reduced motor life. A 440-volt motor would be even more seriously affected.

Phase Voltage Imbalance

A voltage imbalance occurs when there are unequal voltages on the lines to a polyphase induction motor. This imbalance in phase voltages also causes the line currents to be out of balance. The unbalanced currents cause torque pulsations, vibrations, increased mechanical stress on the motor, and overheating of one and possibly two phase windings. This results in a dramatic increase in motor losses and heat generation, which both decrease the efficiency of the motor and shorten its life.
Voltage imbalance is defined by NEMA as 100 times the maximum deviation of the line voltage from the average voltage on a three-phase system divided by the average voltage. For example, if the measured line voltages are 462, 463, and 455 volts, the average is 460 volts. The voltage imbalance is:

voltage imbalanceA voltage unbalance of only 3.5 percent can increase motor losses by approximately 20 percent. Imbalances over 5 percent indicate a serious problem. Imbalances over 1 percent require derating of the motor, and will void most manufacturers’ warranties. Per NEMA MG1-14.35, a voltage imbalance of 2.5 percent would require a derate factor of 0.925 to be applied to the motor rating. Derating factors due to unbalanced voltage for integral horsepower motors are given in the diagram below. The NEMA derating factors apply to all motors. There is no distinction between standard and energy-efficient motors when selecting a derate factor for operation under voltage unbalance conditions.

Motor Derating due to Voltage Unbalance

Motor Derating due to Voltage Unbalance

Common causes of voltage unbalance include:

  • Faulty operation of automatic power factor connection equipment
  • Unbalanced or unstable utility supply
  • Unbalanced transformer bank supplying a three-phase load that is too large for the bank
  • Unevenly distributed single-phase loads on the same power system
  • Unidentified single-phase to ground faults
  • An open circuit on the distribution system primary

The following steps will ensure proper system balancing:

  • Check your electrical system single-line diagram to verify that single-phase loads are uniformly distributed
  • Regularly monitor voltages on all phases to verify that a minimum variation exists
  • Install required ground fault indicators
  • Perform annual thermographic inspections


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