May 9, 2021 | Motors, transmissions, pumps, compressors, troubleshooting
The electrical power issues that most commonly affect industrial plants include voltage sag and sag, harmonics, transients, and voltage and current imbalances.
In a balanced three-phase system, the voltages of the individual phases should be the same or very close to the same. Unbalance is a measure of the degree to which the phase voltage is not equal. Voltage imbalance is an indicator of the voltage difference between the phases in a three-phase system. Imbalance can affect the performance of a three-phase motor and shorten its life.
The impact of transients on the motor can be very severe. Insulation in the motor windings can break down, leading to premature failure of expensive motors and unplanned downtime.
Test the transient voltage in the motor
Transient voltages, which are transient and harmful voltage spikes that occur in electrical circuits, can have a variety of origins, both inside or outside the plant.
Cut-in or cut-out of adjacent loads, power factor correction capacitor banks, and even distant weather factors can create transient voltages on the distribution system. These transient voltages vary greatly in amplitude and frequency, which can cause the insulation performance of the motor windings to deteriorate or be broken down.
However, finding these sources of transients is very difficult due to the low frequency of occurrence and the different ways in which the phenomena occur. For example, transients can occur on control cables, which do not necessarily cause equipment failure directly, but can affect normal operation
One of the best ways to identify and measure transients is to use a three-phase power quality analyzer with transient capabilities, such as the Fluke 438-II power quality and motor analyzer. The transient function on the meter can be set to be more than 50V above the normal voltage. The screen of the meter can then display a potential problem with voltages above 50V – transients.
If transients are detected during the initial measurement, a good course of action is to measure and record the change in power quality over time, using an advanced industrial power quality recorder such as the Fluke 1750 three-phase power quality recorder.
What are the causes of unbalanced voltage?
Unbalanced three-phase systems can cause degraded performance or premature failure of three-phase motors and other three-phase loads due to:
As the torque output is lower than normal, mechanical stress is created on the motor.
The current in the motor and three-phase rectifier is higher than normal.
The unbalanced current passes through the neutral of the Y-type three-phase system.
The voltage imbalance on the motor terminals results in a higher current imbalance, which can be 6 to 10 times that of the former. Unbalanced currents cause torque ripples and mechanical stress, increased losses, and motor overheating. Voltage and current imbalances also indicate maintenance issues such as loose connections and worn contacts.
Imbalances can occur at any point in the distribution system. The load on each phase of the distribution box should be evenly distributed. If the load on one phase is much higher than the other phases, the voltage will be lower than the voltage of the other phases. Transformers and three-phase motors connected to the distribution box can operate at high temperatures, significantly increase noise, experience severe vibrations, and even cause permanent failures.
How to calculate voltage imbalance
The calculation of voltage imbalance is simple. The result is a percentage imbalance that can be used to determine the next diagnostic action. The calculation is divided into three steps:
Determine voltage and current averages
Calculate the maximum voltage or current deviation
The maximum deviation is divided by the average voltage or current, then multiplied by 100 %: unbalanced = (maximum deviation calculated from the average voltage or current/average voltage or current) x 100
Manually calculating an imbalance can only determine a voltage or current imbalance at a certain point in time. Motor drive analyzers such as the Fluke 438-II show real-time voltage or current imbalances, including any form of imbalance.
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