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.

TABLE 1: Power ratios of various flow-control methods.

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.