How to measure voltage imbalance in motors

Electrical power issues such as transients, voltage sags, swells, harmonics, current unbalance, and voltage unbalance frequently affect industrial plants.

The health of your motor is proportionate to the total stress level it experiences. You can examine stressor levels individually to diagnose standard motor health. For example, vibration analysis detects mechanical stress, and thermography measures thermal stress. The stress levels can indicate an issue long before the initial symptoms of a problem.

Energy-based condition monitoring tools provide distinct advantages over other standard condition-monitoring techniques. Energy-based tools are interval based as it is difficult and expensive to monitor diagnostics like vibration and ultrasound remotely. These diagnostics can identify a current issue and determine whether your motor is enjoying regular operation. Energy-based condition monitoring detects motor-damaging electrical stressors before these stressors damage your motor.

A motor operating with one percent unbalance has half the life expectancy of the same motor operating at nominal, balanced voltages. Similarly, a two percent unbalance yields one-quarter the life expectancy, three percent one-eighth, and so on. Life expectancy may change depending on whether the voltage unbalances in the leading or lagging phase. These changes increase if a motor operates with varying levels and phases of voltage unbalance or with sagged or swelled voltage.

The phase voltages in a balanced three-phase system should be equal or almost equal. Unbalance or imbalance measures the inequality of the phase voltages. Voltage imbalance measures the voltage differences between the phases of a three-phase system. It shortens the life of three-phase motors and degrades their performance.

Figure 1:  Unbalanced phases

Figure 2:  Balanced phases

Transients can severely impact motor operation. Motor winding insulation can break down, leading to costly premature motor failure and unplanned downtime.

What causes unbalanced voltage?

An unbalanced three-phase system can cause three-phase loads and three-phase motors to experience poor performance or premature failure because of the following:

• The motor's mechanical stress is lower than normal torque output
• Greater than the normal current in motors and three-phase rectifiers
• Unbalanced current will flow in neutral conductors in three-phase wye systems

Voltage unbalances at the motor terminals cause high current to unbalance, which can be six to 10 times bigger than the voltage unbalance. The unbalanced currents lead to torque pulsation, increased vibration and mechanical stress, increased losses, and motor overheating. Voltage and current unbalance may also indicate maintenance issues like worn contacts and loose connections.

Unbalance may occur at any point throughout the distribution system. There must be an equal division of loads across each phase of a panelboard. If one phase becomes too heavily loaded in comparison to others, the voltage will be lower in that phase. The three-phase motors and transformers fed from that panel may run hotter, be unusually noisy, vibrate excessively, and even suffer premature failure.

Implementation of power quality monitoring to test transient voltage in motors

The transient voltages—temporary unwanted spikes or blips of voltage in an electrical circuit—can arise from any number of sources, either inside or outside an industrial plant. Power factor correction capacitor banks, adjacent loads turning on or off, or even distant weather may produce transient voltages on distribution systems. These transients, varying in amplitude and frequency, can erode or cause insulation breakdown in motor windings.

The frequency of such occurrences may make finding the source of these transients challenging. These symptoms can also appear differently. For example, a transient can appear on control cables that do not necessarily cause direct equipment damage but may disrupt operations.

An excellent technique to identify and measure transients is to cccc a three-phase power quality analyzer, as shown in the below figure, with a transient function. The meter's transient function is set to greater than 50V above the standard voltage. The display will consequently show the potentially problematic voltage above 50V – the transients.

Figure 3:  Three-phase power quality utilize with transient function

If an initial measurement reveals an absence of transients, it is an excellent practice to log and measure the power quality over time with an advanced industrial power quality logger.

How to calculate voltage unbalance

You can determine voltage unbalance using a simple calculation. The result is the percentage of unbalance that you can use to troubleshoot motor issues. The calculation involves three steps:

1. Determine the voltage or current average
2. Calculate the largest voltage or current deviation
3. Divide the maximum deviation by the average voltage or current and multiply by 100 per cent unbalance = (Max deviation from average V or I/average V or I) x 100
   Example: Suppose in a 480V nominal system, the phase-to-phase voltages are 463V, 482V, and 474V
   The average voltage is
   (463+482+474)/3=473V
   The maximum deviation from average voltage is
   473V-463V=10V
   Therefore, the percentage unbalance = (Max. deviation from average voltage / average voltage) × 100
   = 10V/473V×100

The voltage unbalance is 2.1%. So here, the voltage unbalance is within the limit as per NEMA Standard.
A manual unbalance calculation is a point-in-time determination of voltage or current unbalance. A motor drive analyze like the Fluke 438-II will show voltage or current unbalance in real-time, including any variations in unbalance.