The Journal | Drives and Motors

How to Troubleshoot Three-Phase Motors

A motor drive analyzer can show voltage or current unbalance in real time to help prevent poor performance or premature failure and avoid costly downtime.

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By Sean Silvey, product application specialist, Fluke

The electrical power issues that most often affect industrial plants include voltage sags and swells, harmonics, transients, and voltage and current unbalance.

In a balanced three-phase system, the phase voltages should be equal or very close to equal. Unbalance or imbalance is a measurement of the inequality of the phase voltages. Voltage imbalance 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.

The impact of the transients on motors can be severe. Motor winding insulation can break down, which can then lead to costly early motor failure and unplanned downtime.

Testing for Transient Voltage in Motors

Transient voltages — temporary unwanted spikes or blips of voltage in an electrical circuit — can come from any number of sources either inside or outside of an industrial plant.

Adjacent loads turning on or off, power-factor correction capacitor banks or even distant weather can generate transient voltages on distribution systems. These transients, which vary in amplitude and frequency, can erode or cause insulation breakdown in motor windings.

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Finding the source of these transients can be difficult because of the frequency of the occurrences, and because the symptoms can present themselves in different ways. For example, a transient might appear on control cables that don’t necessarily cause equipment damage directly but may disrupt operations.

What Causes Unbalanced Voltage?

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

  • Mechanical stresses in motors due to lower than normal torque output.
  • Higher than normal current in motors and three-phase rectifiers.
  • Unbalance current will flow in neutral conductors in three-phase wye systems.

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 also could indicate maintenance issues such as loose connections and worn contacts.

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A good way to identify and measure transients is to use a three-phase power quality analyzer with a transient function. The meter’s transient function is set to greater than 50V above the normal voltage. The meter’s display then will show the potentially problematic voltage above 50V — the transients.

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.

How to Calculate Voltage Unbalance

The calculation for determining voltage unbalance is straightforward. The result is the percentage unbalance and can be used to determine next steps in troubleshooting motor issues. The calculation has three steps:

Determine the voltage or current average.

Calculate the largest voltage or current deviation.

Divide the maximum deviation by the average voltage or current and multiply by 100.

% unbalance = (Max deviation from average V or I/average V or I) x 100

A manual unbalance calculation is a point-in-time determination of voltage or current unbalance. A motor drive analyzer will show voltage or current unbalance in real time, including any variations in unbalance.

Fluke Corp. is a participating EncompassTM Product Partner in the Rockwell Automation PartnerNetworkTM program. Based in Everett, Washington, Fluke Corp. manufactures, distributes and services electronic test tools.



The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.

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