Estimate your potential energy savings

5-step savings estimator from Yaskawa Electric shows how to compare variable frequency drive performance savings to other methods of motor volume control. Now includes downloadable calculator!
By David Greenfield July 28, 2009
Industrial Calculator

Much has been written in the pages of Control Engineering about the use of variable frequency drives to save energy . This content has often focused on how VFDs enable energy savings via their namesake partial operating modes versus regular drives’ all-on or totally-off method of operation.

What’s been missing from much of this discussion is how to figure out how much money the use of a VFD could possible save your particular operation. Now, with the help of a simple-to-follow,

All you need to perform the calculations is your current motor:

  • Horsepower;

  • Cost per kWh of electricity;

  • Total hours of operation per year (8,760 maximum); and

  • Thealternative to VFD control (inlet guide vane, outlet damper, ride thefan curve, discharge valve, bypass damper, bypass valve or no control).

Here are the 5 steps: Annual VFD Estimated Savings Calculator

(Theresult here is your estimated annual savings using a VFD. This resultis only an estimate based on averages and assumptions. You should alsoconsider additional savings to be gained from VFD application over andabove the electricity rate, such as power factor improvement to 0.98and reduced demand charges. )
Step 1. Convert motor hp to kW:
___________HP × .746 = _______ kW1
Step 2. Multiply the Variable Frequency Drive Power Ratio (from Table below) times kW1 from Step 1:
______Ratio × ______kW1 = ______ kW2 (using VFD)
Step 3. Multiply the power ratio of the control method now being used (see table below) times kW1 from Step 1:
______ Ratio × ______ kW1 = ______ kW3 (method now being used)
Step 4. Subtract Step2 kW2 from Step 3 kW3:
______ kW3 minus ______ kW2 = ______ kW4 (savings using VFD)
Step 5. Multiply Step 4 kW4 savings times hours per year of operation times cost per kWh of electricity:
____ kW4 × ______ Hrs × $______/kWh = $______

Pumps at 70% of maximum flow*

Ratio Flow control method
.41 Variable Frequency Drive
.83 Discharge valve
1.0 Bypass value
1.0 No Control

Fans at 60% of maximum flow*

Ratio Flow control method
.32 Variable Frequency Drive
.69 Inlet Guide vane
.94 Outlet Damper
.94 Ride the Fan Curve
1.0 Bypass Damper

* The power ratio data in the above tables is a conservative assumption based on HVAC applications which have shown that fans and pumps operate, on average, at 60% and 70% of maximum flow rate, respectively.

To help illustrate how this savings estimator works before you conduct your first test, see this example:

Basedon a 60 hp fan motor operating 24 hours per day (or 8,760 hours peryear) riding the fan curve for variable volume control and the localutility charges $0.12 per KwHr:

  • Step 1.) 60 hp × .746 = 44.76 kW1

  • Step 2.) .32 Ratio × 44.76 kW1 = 14.32 kW2

  • Step 3.) .94 Ratio × 44.76 kW1 = 39.39 kW3

  • Step 4.) 42.07 kW3 – 14.32 kW2 = 27.75 kW4

  • Step 5.) 27.75 kW4 × 8,760 hrs × $0.12/kWh = $29.172 annual savings…for one motor!

Nowyou can download an Excel version of this calculator to perform thesecalculations more quickly and easily by clicking on this link: VFD Savings Calculator .Fields that require input include Excel notes as reminders of what datashould be placed in the field. Our thanks to Jerry Archer of JacobsEngineering for providing us with this helpful resource.


– By David Greenfield , editorial director, with data from Yaskawa Electric America
Control Engineering Sustainable Engineering
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