MONITORING MOTOR SUPPLY PAYS OFF
Colin Kinsey, Power Systems Specialist
Supply voltage phase imbalance and harmonics can have a significant effect on the life of a three-phase motor, making the case for supply monitoring and filtering to improve efficiency and reduce downtime.
T
he life of three-phase motors is often cut short because of voltage imbalance and voltage harmonics. It is sometimes argued that the use of a variable speed drive removes the effect of problems with the supply lines, but this is usually a false argument because of problems that occur on the input converter and DC link when there is voltage imbalance and voltage harmonics. There is a case for monitoring of supply conditions in particular when critical motor applications are involved, but monitoring by itself is not enough. However, monitoring with programmable critical limits of imbalance and harmonics, etc, combined with logic functions as inputs to PLCs, relays and active filters, has significant advantages. No doubt a lot depends on the application as to the cost-effectiveness but monitoring and control can provide a smooth running mechanical plant. Think of high inertia loads that must be brought up to speed reliably — and within specified time limits — when supply conditions are less than ideal.
38 WHAT'S NEW IN PROCESS TECHNOLOGY - NOVEMBER 2016
Why voltage imbalance is bad for motors In short, unless derated, motor life is badly affected by line voltage imbalance. The NEMA derating curve shows that under a 4% imbalance, 20% derating is required. Somehow 4% imbalance sounds small but the effects are magnified. Apart from a reduction in torque as a result of negative sequence components, there is the imperilling of insulation because of excessive current. In addition there can be problems associated with harmonics, particularly the 5th, causing large counter torque. The solution is not found by increasing the rating of motors because apart from the capital cost, the result is excessive use of electrical energy as a result of lower efficiency.
Imbalance: the new norm Network providers might take issue with the bald statement that voltage imbalance is the new norm. However, the growth of dis-
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