What fraction of its power is a centrifugal pump using when operating at half its rated speed with a VFD motor?

To understand the fraction of power used by a centrifugal pump operating at half its rated speed with a Variable Frequency Drive (VFD) motor, it's important to consider the principles of pump performance and the affinity laws.

According to the affinity laws for pumps, when the speed of a pump is reduced, the flow rate, head (or pressure), and power consumption change in a predictable way. Specifically, the power needed by a centrifugal pump is proportional to the cube of the speed (RPM) at which it operates. This relationship can be represented mathematically as follows:

Power ∝ (Speed)³

If a centrifugal pump is operating at half its rated speed, we can express this as:

Power at half speed ∝ (1/2)³

This simplifies to:

Power at half speed ∝ 1/8

Thus, when the pump is run at half of its rated speed, it consumes only one-eighth of the power compared to its full-speed operation. Therefore, the correct answer indicates that the pump uses one-eighth of its power when operating at this reduced speed, making it C, or 1/8.

Understanding this concept is vital for optimizing pump operations and energy efficiency in refrigeration and air conditioning systems.