The performance of variable-frequency drives (VFDs) can be compared with that of other methods of volume control quickly and simply. All that you need are:

  • Motor horsepower.

  • The alternative to VFD control (inlet guide vane, outlet damper, “ride the fan curve,” bypass damper, discharge valve, bypass valve, no control).

  • The total hours of operation per year (8,760 maximum).

  • The cost per kilowatt-hour of electricity.

Step 1. Convert motor horsepower to kilowatts:
__ hp × 0.746 = ____ kw1

Step 2. Multiply VFD power ratio (Table 1) by the product of Step 1:
___ ratio × ____ kw1 = ____ kw2 (using VFD)

Step 3. Multiply the power ratio of the alternative control method (Table 1) by the product of Step 1:
___ ratio × ____ kw1 = ____ kw3 (using alternative method)

Step 4. Subtract the product of Step 2 from the product of Step 3:
____ kw3 - ____ kw2 = ____ kw4 (savings using VFD)

Step 5. Multiply the product of Step 4 by the total hours of operation per year by the cost per kilowatt-hour of electricity:
____ kw4 × ____ hr × $___ per kwh = $_____ (annual savings using VFD)1

EXAMPLE

A 60-hp fan motor operates 24 hr a day, or 8,760 hr a year, “riding the fan curve” for variable-volume control. The local utility charges 12 cents per kilowatt-hour.

  • Step 1: 60 hp × 0.746 = 44.76 kw1

  • Step 2: 0.28 ratio × 44.76 kw1 = 12.53 kw2

  • Step 3: 0.88 ratio × 44.76 kw1 = 39.39 kw3

  • Step 4: 39.39 kw3 - 12.53 kw2 = 26.86 kw4

  • Step 5: 26.86 kw4 × 8,760 hr × $0.12 per kwh = $28,235 (annual savings)

NOTE

1) This is only an estimate based on averages and assumptions. Additional savings, such as power-factor improvement to 0.98 and reduced demand charges, also should be considered.


Yaskawa Electric America Inc., headquartered in Waukegan, Ill., is a manufacturer of HVAC variable-frequency drives and configured packages.