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Tech Tip 17

Understanding Voltage Unbalance


SYMPTOMS

  • Motors overheating
  • Motor fuses blowing
  • Motor unbalance protection operates
  • Unbalanced motor currents

CAUSES

  • Motor problems
  • Unbalanced voltages

SOLUTIONS

  • Use higher rated motors
  • Adjust protection circuits
  • Reduce voltage and load unbalance

BACKGROUND

New technology, high efficiency motors, and competitive pressures to reduce motor costs cause more problems with voltage unbalance. Some adjustable frequency drives are often far more sensitive to voltage unbalance than a standard induction motor. Designs for high efficiency motors tend to be more sensitive to voltage unbalance. Motors manufactured with less tolerance for unbalance can sometimes have a slightly lower selling price.

The increased sensitivity to unbalance increases the number of frustrating events between electric utilities and customers. Voltage unbalance that meets national and international standards may be considered unacceptable to the motor or to the adjustable speed drive.

VOLTAGE UNBALANCE

Voltage unbalance normally results from uneven loading or uneven impedance in the electric supply system. This causes a different voltage drop on each of the three phases. This difference between phase to phase voltages is called voltage unbalance.

The correct way to calculate percent unbalance is to use phase to phase voltages in the equation below.

 

For example, phase-to-phase voltages of 470, 479, and 482 average to 477. The maximum deviation from average is the phase with 470 volts. The deviation between 470 and 477 is 7 volts. The percent unbalance is

MOTOR AND DRIVE RESPONSE TO UNBALANCE

Induction motors and adjustable frequency drives respond to the voltage unbalance with much greater current unbalance. For example, a 1% voltage unbalance can easily translate to 5% to 9% current unbalance on induction motors. High efficiency motors are usually closer to the 9% current unbalance for 1% voltage unbalance. Current unbalance in an adjustable frequency drive can exceed 15 times the voltage unbalance.

Higher levels of current unbalance may cause nuisance fuse blowing, motor overheating, and a variety of other problems. People who use motor saving protection circuits are often encouraged to set the current unbalance level at no more than 5%. As little as 1% voltage unbalance can cause motor protection circuits to trip.

RECOMMENDATION

Where practical, improve voltage balance by balancing other single phase loads. Remember that voltage and current unbalance normally can be tolerated so long as motor currents stay below safe levels. Increase the unbalance trip setting, but keep phase current overload settings at a safe level. Consider a slightly oversized motor or reduce load a few percent to tolerate the unbalance. For adjustable speed drives, line reactors will help current unbalance and provide many other benefits.

Some motor and equipment manufacturers want less than 1% voltage unbalance. International standards suggest 2% voltage unbalance is acceptable. The National Equipment Manufacturers Association and many other organizations recently approved the following language in the appendix to ANSI Std. C84.1. "Electric supply systems should be designed and operated to limit the maximum voltage unbalance to 3 percent when measured at the electric-utility revenue meter under no-load conditions."

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.