Motors & Drives

Replacing Oversized

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When a motor has a significantly higher horsepower rating than the load it is driving, the efficiency of the motor is notably reduced. There are substantial numbers of motors presently operating in this fashion. There are several common reasons given by plant managers for this:

To ensure against motor failures in critical processes.
Plant staff did not have the correct size on hand and used the next larger size.
Plant staff did not know the actual load and selected one they knew would do the job.
To ensure capability to accommodate future increases in production.
Original production requirements were reduced, leaving a grossly oversized motor in place.

Cost penalties associated with using an oversized motor can be considerable. These include:

higher purchase price,
more costly electrical supply equipment,
increased energy costs due to lower efficiency, and
higher costs due to lower power factor.

Because efficiencies of most motors, especially smaller horsepower sizes, decline rapidly at less than 50% loading, it is generally a good idea to evaluate downsizing any motor that is less than 50 percent loaded.

Replacing grossly oversized motors with accurately sized energy-efficient motors can yield considerable energy cost savings.

And Furthermore . . .

Specifications
kW Saved
Energy Costs Savings

Payback
Operational Life
Amortize

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