A common question we receive is: ‘what is the impact of voltage unbalance on temperature?’ With incoming power to facilities having challenges in many parts of the United States (and elsewhere) from aging grid and the rapid application of power electronics, this becomes a significant issue.
Voltage unbalance is the variation from the average voltage to the furthest value from that average. For instance, if I have a 480 Volts, 60 Hz system and we read 479 Volts, 495 Volts, and 498 Volts, the unbalance would be: (479 + 495 + 498) Volts/3 = 490.7 Volts. This would result in ((490.7 V – 479 V)/490.7 V) * 100% = 2.4%.
According to Hofmann and Pillay, (‘Derating of Induction Motors Operating with a Combination of Unbalanced Voltages and Over-or Undervoltages,” IEEE Transactions on Energy Conversion, Vol. 17, No. 4, Dec. 2002, pp 485-491) the relationship is exponential and approximately increases by twice the square of the percent of voltage unbalance.
At 2.4%, the increase in temperature rise would be about 15%. This means that a motor operating in a 40C (104F) environment at 100% load with a rise of 60 C and an insulation class F (155 C) would have an operating temperature of 109C, almost 10C warmer. This would result in a reduction to half the thermal life of the machine.
If the unbalance increases to 5%, then the operating temperature would be 130 degrees, or a life reduction to approximately 1/9th of the expected thermal life of the machine. Is this possible? In the Ram Review article, Part IV, More Thoughts on Electrical Reliability (Specifically, Is Your Electricity Clean?) | THE RAM REVIEW, we noted voltage unbalances at the utility of over 7% (Figure 5 of the article) over over an hour each day. With known ambient temperatures of 22 C in the facility near these systems, and an average rise of 50 C with an average insulation class F (155 C), the increase to the average operating temperature would be 121 C, or +49 C, which would be a thermal life reduction of 1/25th of the potential thermal life of the machine.
The challenge here is a gradual reduction in the resistance to failure of a machine and random failures of machines due to a combination of the voltage unbalance thermal rise and other electrical stresses from the unbalance. Additionally, voltage unbalance increases the stresses associated with harmonics in the transformers and motors within a facility.
Electric motor de-rating is set by NEMA MG-1 for unbalance and can be found here. The maximum voltage unbalance that a motor or transformer should see is 5%.
Identifying voltage unbalance in a facility with ESA provides an opportunity to mitigate the condition well in advance of early equipment failure with the added benefit of energy improvements, which we will cover in a future post.
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