Question

If a 75-kW fan motor has a VSD installed that slows down the fan by 20% what is the effect on air flow rate and the kW savings?

20% CFM reduction, and 36.6 kW savings
80% CFM reduction, and 51.2 kW savings
20% CFM reduction, and 20 kW savings
20% CFM reduction, and 37.4 kW savings

Answer 1

VSD, Variable Speed Drive control the speed of the motor. If the VSD slows down the fan by 20%, there will be a reduction in airflow of 20%.

VSDs can save energy because they reduces the fan speed. According to the cube law relation between speed and power,

here the fan's speed is reduced to 80%. the fan now consumes only (0.8³ x 75)KW

that is,

the power consumed for 80% flow/speed = 0.8 x 0.8 x 0.8 x 75 = 38.4 KW

energy saved = 75-38.4 = 36.6 KW

Correct Answer : 20% CFM reduction, and 36.6 kW savings.

Hope the concept is clear.

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