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Abstract

Misalignment of a direct drive motor-load arrangement results in an identifiable power demand characteristic with variation in power computed each power system cycle. The cycle-by-cycle calculation lends a beating pattern to the variations which are detected in automated analysis. The strength of the indication increases over time as alignment deteriorates.

A new Delta configured induction motor, installed on a critical circulating water pump, exhibits higher than expected current unbalance (26.4%) while voltage is well balanced (0.6%). Measured phase current levels suggest a high impedance winding. The cause of the unbalance is resolved and corrected. Current unbalance is reduced to a respectable 1.7%.

A 200HP, 220 amp multi-stage vertical pump uses a 2-stage autotransformer soft starter. Observed starting profile indicates excessive current flow for a prolonged period (>700 amps for >7 seconds). Adjusted timing of Stage 1 of the starting profile is able to reduce starting stress with no negative impact on plant distribution voltage starting sag. I2t heating during the start cycle is reduced by 83% (75.64 to 13.22 Amp2-sec).

A 250HP induction motor experiences occasional overload conditions. Continuous monitoring and trending allow the overloads to be identified within the daily work cycles. This time synchronized analysis aids the operator in identifying and correcting the causes of the overloads.

In this case study, an induction motor is close coupled to a variable displacement swashplate hydraulic pump. The motor is started using a variable frequency drive, but the start profile indicates high starting currents and high winding I2t heating early in each start cycle.

All active production plants experience momentary voltage sag as large loads are started. Activity outside the plant, on the utility grid, also affect the motor. The response of induction motors to changes in the internal distribution voltage is examined, and differences between in-plant process load starts and switching transients on the utility grid are compared.

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