In a balanced three-phase system, the phase voltages should be equal or very close to equal. Unbalance is a measurement of the inequality of the phase voltages. Voltage unbalance is the measure of voltage differences between the phases of a three-phase system. It degrades the performance and shortens the life of three-phase motors.
Voltage unbalance can cause three-phase motors and other three-phase loads to experience poor performance or premature failure because of the following:
Voltage unbalance at the motor terminals causes high current unbalance, which can be six to 10 times as large as the voltage unbalance. Unbalanced currents lead to torque pulsation, increased vibration and mechanical stress, increased losses, and motor overheating. Voltage and current unbalances could also indicate maintenance issues such as loose connections and worn contacts.
Unbalance can occur at any point throughout the distribution system. Loads should be equally divided across each phase of a panelboard. Should one phase become too heavily loaded in comparison to others, voltage will be lower on that phase. Transformers and three-phase motors fed from that panel may run hotter, be unusually noisy, vibrate excessively, and even suffer premature failure.
You can make some basic phase-to-phase voltage unbalance measurements using a high-quality DMM and phase-to-phase current unbalance using a high-quality clamp meter. Accurate, real-time unbalance measurements need a three-phase power quality analyzer to enable solving unbalance problems. Open circuits and single-phase to ground faults are easier to correct than load balancing, which typically requires corrective system-level design changes..
In reality, voltage differences between phases vary as loads operate. However, motor or transformer overheating, or excessive noise or vibration, can merit troubleshooting for voltage unbalance. Monitoring over time is the key to capturing unbalance. In a three-phase system, the maximum variation in voltage between phases should be no more than 2 percent (the Vneg % value on the analyzer), or significant equipment damage can occur.