The performance of variablefrequency 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 kilowatthour of electricity.
Step 1. Convert motor horsepower to kilowatts:
__ hp × 0.746 = ____ kw_{1}
Step 2. Multiply VFD power ratio (Table 1) by the product of Step 1:
___ ratio × ____ kw_{1} = ____ kw_{2} (using VFD)
Step 3. Multiply the power ratio of the alternative control method (Table 1) by the product of Step 1:
___ ratio × ____ kw_{1} = ____ kw_{3} (using alternative method)
Step 4. Subtract the product of Step 2 from the product of Step 3:
____ kw_{3}  ____ kw_{2} = ____ kw_{4} (savings using VFD)
Step 5. Multiply the product of Step 4 by the total hours of operation per year by the cost per kilowatthour of electricity:
____ kw_{4} × ____ hr × $___ per kwh = $_____ (annual savings using VFD)^{1}
EXAMPLE
A 60hp fan motor operates 24 hr a day, or 8,760 hr a year, “riding the fan curve” for variablevolume control. The local utility charges 12 cents per kilowatthour.

Step 1: 60 hp × 0.746 = 44.76 kw_{1}

Step 2: 0.28 ratio × 44.76 kw_{1} = 12.53 kw_{2}

Step 3: 0.88 ratio × 44.76 kw_{1} = 39.39 kw_{3}

Step 4: 39.39 kw_{3}  12.53 kw_{2} = 26.86 kw_{4}

Step 5: 26.86 kw_{4} × 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 powerfactor 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 variablefrequency drives and configured packages.