Voltage Imbalance: How to Deal With This Power System Fault

In industrial plants, electrical power issues are pretty common. Current harmonics, voltage unbalance, and current unbalance are some of them.  These issues lead to abnormal functioning of the electric power systems. All of them, the unbalanced condition of voltage is the most hazardous. Let’s get into details of voltage imbalance issues and how you can prevent them.

What Is Voltage Imbalance?

There is no particular definition of voltage imbalance, and it is simply the voltage difference between different phases. 

In general, in three-phase motors or polyphase systems, the phase voltage between different phases should be equal or nearly equal. However, due to some issues, three-phase voltages become unequal, which results in the flow of negative or zero sequence currents.

Major Effects of Voltage Imbalance

Sometimes, there are extensive voltage imbalances. As a result, it impacts polyphase motors and other electrical loads. 

Unbalanced voltage mainly causes motor failure due to extreme heat. Because the voltage unbalances produce high unbalance currents, and these currents produce heat and lead to an increase in winding temperature. As a result, it can damage motor insulation. 

Also, a severe imbalance in voltage can lead to the overheating of components in the motor, and there can be severe or permanent damage to the motor. Motor failures, in turn, lead to user facility downtime.

Voltage imbalance also creates negative sequence voltage, and this negative voltage produces opposite torque. As a result, there is vibration and noise in the motor. 

Sometimes, imbalances in power systems also lead to transformer failure, and relay malfunction is also one of its adverse effects.

motor winding

Image: motor winding

Causes and Sources

Several factors affect voltage imbalance in a distribution line, and these are either general or motor-related.


  • Unequal distribution of single-phase loads
  • Overloading in feeders due to electrical faults
  • Faulty equipment
  • Unbalanced power source voltage


  • Wrong tapping in transformer
  • Unbalanced load in three phases
  • An Unequal impedance of the three-phase distribution system
  • Unbalanced loading of capacitors

Image: overload

Voltage unbalance standards

Specific standards decide the limit of voltage unbalance, and ANSI recommends a 3 percent unbalance in voltage for electrical systems. You must take this percentage under no-load conditions. However, according to Pacific Gas and Electric, this voltage imbalance percentage should not be more than 2.5. According to NEMA MG-1-1998, there is just a 1 percent unbalance limit, and this rule is the strictest. NEMA is an association that represents motor manufacturers.

As per the NEMA rule of 1% voltage imbalance, a current unbalance is 6-10%. On the other hand, some makers fix the current imbalance value to less than 5%. It is essential to get a valid warranty, which means the makers’ requirements are stricter than NEMA MG-1.

At times, disputes arise between customers and makers due to this difference. Thus, you need to check the service guidelines of the utility at a specific location.

Testing for Voltage Unbalance

To test the voltage unbalance, you must measure phase-to-phase voltage. The 3-phase system has a connection across phases. Thus, do not measure phase-to-neutral voltages. Take the phase-to-phase voltage readings with a voltmeter. 

According to IEEE, it is a ratio of the positive sequence component to the negative sequence component. Now, use this formula to calculate the percentage of voltage unbalance.

Voltage unbalance percent = 100* (maximum voltage deviation/average voltage)

The average voltage is the average of voltages across all three phases. 

This formula identifies the voltage unbalance magnitude present in the system. If there is any, you must determine the problem source. The unbalanced situation can be due to the motor or the power.

Follow these steps to know the source of unbalance:

  • Firstly, measure and note down the current through each load
  • Secondly, rotate all power lines (three) by one position. However, do not change the order as it will change the motor’s rotation.
  • Now, again measure the current across all leads in this new position.
  • Now, again rotate all power lines by one more position.
  • Again, record the current across all lines in the new position
  • For every three rotations, calculate the average value of current. Observe the power line/motor lead combination that shows the maximum deviation from the average current.
  • Finally, compare all three power lines with the most current deviations. If the combination always has the same motor lead, the problem is with the motor. On the other hand, the same power line in combination indicates a problem with the power supply.
electrician testing industrial machine

Image: electrician testing industrial machine

Voltage Unbalance Mitigation

The issues of power quality are obvious in distribution networks. You cannot make voltage imbalance as Zero in a distribution system because of three reasons:

  • Firstly, connection and termination of single-phase loads are random
  • Secondly, due to the uneven distribution of loads in the three-phase system
  • Finally, due to asymmetry of the power system

However, you can mitigate it after a thorough voltage imbalance study. To reduce the effects of voltage unbalance, you can use:

Utility level methods.

  • Redistribute single-phase loads across all phases.
  • Reduce unequal impedance due to transformers and lines
  • Decrease single-phase regulators to correct the imbalance. However, you must use them carefully.
  • Use active and passive electronic systems to correct voltage imbalances.

Plant level methods:

  • Do load balancing.
  • Avoid connecting sensitive equipment to systems with single-phase loads
  • Make sure that you size AC side and DC link reactors properly. It reduces the effect of voltage imbalance in speed drives.
  • Have passive networks.


The impacts of Voltage imbalance are harmful to motors. Thus, you must adequately find and correct the problem. When you balance voltage, the life cycle of equipment becomes better. As a result, you save time, energy, and maintenance costs. Thus, you must do proper testing of the electrical equipment. We can help you in managing your electrical systems. We deal in premium quality cable assemblies.

Hey, I am John, General manager of Cloom and OurPCB.

I am a responsible, intelligent and experienced business professional with an extensive background in the electronics industry.

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