Frequency Change on Pump or Fan

Changing the frequency from 50 to 60 Hz on a pump or fan increases the operating speed, and consequently increases the load on the motor and driven equipment. That is because fan and pump loads vary by the cube of the speed. A 50 Hz motor operating on 60 Hz power will attempt to rotate 20% faster. The load therefore becomes 1.23 (1.2 x 1.2 x 1.2) or 1.73 times (173%) greater than on the original frequency. Re-designing a motor for that much of an increase in horsepower is simply not realistic: the motor's magnetic material would be driven into saturation long before reaching the 73% increased horsepower level. Very few motors can be redesigned for a horsepower increase of 30% or greater.

The typical solution when increasing speed due to a frequency change is to modify the driven equipment so that the increased horsepower requirement falls within the motor's capability. For example, the diameter of a fan wheel or a pump impeller can be trimmed to provide the same performance at 60 Hz as the unit had at 50 Hz. Before making such a modification, however, always consult the original equipment manufacturer. Not all pumps and fans behave exactly in accordance with the cube rule; thus the OEM may specify a diameter that differs from the rule.

For fans and blowers, the diameter change follows a 5th power rule. The discussion is one that usually takes place between the end user and the OEM -- i.e., the motor rebuilder seldom modifies the driven equipment.

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