Ecmweb 7027 Unbalanced Voltage Effectpromo
Ecmweb 7027 Unbalanced Voltage Effectpromo
Ecmweb 7027 Unbalanced Voltage Effectpromo
Ecmweb 7027 Unbalanced Voltage Effectpromo
Ecmweb 7027 Unbalanced Voltage Effectpromo

Correcting Unbalanced Voltage Situations

Dec. 18, 2014
Learn how to find and correct out of voltage conditions for electric motors  

In the real world, it is impossible to balance line-to-line voltages perfectly in a 3-phase circuit. In fact, line voltages typically differ by a few volts or more. But a difference that exceeds 1% can lead to serious trouble on the plant floor. To maintain peak energy efficiency and thwart premature failure of 3-phase motors, it makes sense to install adequate protective devices and periodically check for voltage unbalance at the motor terminals.

The unequal heating of these motor coils was due to an unbalanced voltage situation.

Simply stated, voltage unbalance describes the condition when not all line voltages in a 3-phase circuit are equal. How this will affect motors (Photo) and other devices in the circuit depends directly upon the percent of unbalance that is present. The National Electrical Manufacturers Association (NEMA) defines percent voltage unbalance as follows in its standards publication MG 1-2011: Motors and Generators, Part 14.36:

% voltage unbalance = 100 x [(maximum voltage deviation from average voltage) ÷ (average voltage)]

With line-to-line voltages of 460V, 467V and 450V, for example, the average voltage is 459V, and the maximum deviation from average is 9V. Therefore, the percent unbalance is calculated as follows:

100 x [9V ÷ 459V] = 1.96%

Acknowledging possible differences in performance, NEMA MG 1-2011, Part 12.45, calls for 3-phase motors to “operate successfully” at rated load if voltage unbalance at the motor terminals is 1% or less. For reliable motor operation, be sure to keep this limiting value in mind. (Note that the 1.96% unbalance in the above example exceeds the NEMA standard.)

Redistributing and reconnecting single-phase loads can reduce voltage unbalance caused by excessively unequal load distribution among phases. The most prevalent culprits among heavy, single-phase loads are lighting equipment and occasionally welders. A blown fuse on a bank of 3-phase power factor improvement capacitors could also cause the problem.

About the Author

Tom Bishop, P.E. | Senior Technical Support Specialist

Bishop, P.E., joined the staff of the Electrical Apparatus Service Association (EASA) following more than 30 years of engineering experience at electrical machinery manufacturers and electrical apparatus service firms. Holding a BS in electrical engineering and a professional engineer’s license, Bishop has authored dozens of technical articles and papers as well as presented numerous seminars on electric motor application, maintenance, and repair. He is chairman of EASA’s Technical Services Committee and a principal member of the National Fire Protection Association Electrical Equipment Maintenance Committee (NFPA 70B).

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