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
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)
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.