Transients and voltage unbalance

The electric control issues that most recurrently affect industrial plants contain voltage sags and swells, harmonics, transients, and voltage and current unbalance.

In a balanced three-phase system, the phase voltages must be equivalent or very close to equal. Unbalance or imbalance is a measurement of the difference of the phase voltages. Voltage imbalance is the measure of voltage differences between the phases of a three-phase system. It worsens 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 are temporary unwanted spikes or blips of voltage in an electrical circuit that 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

Discovering the source of these transients can be challenging because of the frequency of the occurrences and the fact that the symptoms can present themselves in different ways. For example, a transient may appear on control cables that do not necessarily cause equipment impairment directly but may interrupt operations.

The best opportunity to classify and measure transients is to use a three-phase power quality analyzer with a transient function, such as the Fluke 438-II Power Quality and Motor Analyzer. The transient function on the meter is set to great than 50V above the normal voltage. The meter’s display will then show the potentially problematic voltage above 50V – the transients.

If there are no transients found in an initial measurement, it is good training to measure and log the power quality over time with an advanced industrial power quality logger, like the Fluke 1750 Three-Phase Power Quality Logger.

 

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 the normal current in motors and three-phase rectifiers
  • Unbalance current will flow in neutral conductors in three-phase wye systems

Voltage unbalances at the motor terminals causes high current to unbalance, which can be six to 10 times as large as the voltage unbalance. Unbalanced currents turn to torque pulsation, increased vibration and mechanical stress, increased losses, and motor overheating. Voltage and current unbalance could also specify maintenance problems such as unfastened connections and worn contacts.

Unbalance can occur at any point throughout the distribution system. Loads should be equally divided across each phase of a panel board. Should one phase become too heavily loaded in comparison to others, the 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 defining voltage unbalance is straight forward. The result is the percentage unbalance and can be used to determine the next steps in troubleshooting motor issues. There are three steps in the calculation:

  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 % 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 like the Fluke 438-II will show voltage or current unbalance in real-time, including any variations in unbalance.

Get to know more of the product: Fluke 438 – II Three-Phase Power Quality and Motor Analyzer – Presidium PH

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