Calculator Inputs
Example Data Table
| Input | Example value | Purpose |
|---|---|---|
| Full speed power | 50 kW | Sets the full speed electrical baseline. |
| Load exponent | 3 | Applies the fan and pump affinity law. |
| Speed schedule | 45% to 95% | Models operation at many duty points. |
| Energy rate | $0.14/kWh | Converts saved energy into money. |
| Demand charge | $8/kW-month | Estimates reduced monthly demand cost. |
Formula Used
- Full speed input kW: input kW = shaft kW ÷ motor efficiency. If measured input power is selected, the entered kW is used directly.
- Variable speed kW: VFD kW = full speed input kW × (speed ÷ 100)load exponent ÷ drive efficiency.
- Existing kWh: existing kWh = existing kW × yearly hours.
- VFD kWh: VFD kWh = VFD kW × yearly hours.
- Energy savings: saved kWh = existing kWh − VFD kWh.
- Cost savings: yearly savings = saved kWh × energy rate + demand savings + maintenance savings.
- Payback: simple payback = installed project cost ÷ total annual savings.
How to Use This Calculator
Enter the full speed motor power first. Choose whether the value is measured electrical input or shaft output power. Add motor and drive efficiencies. Use a load exponent of 3 for most centrifugal fans and pumps. Use another exponent when measured field data supports it.
Next, enter each duty point. A duty point is one speed level and its operating hours. Set existing power percent to 100 when the old system runs at full speed with a damper or valve. Lower it when you have measured baseline power. Submit the form. The result appears above the form.
Why Speed Control Changes Energy Use
A variable frequency drive changes motor speed. That change can cut energy sharply. Fans and centrifugal pumps follow affinity rules. Flow changes almost directly with speed. Pressure changes with speed squared. Power often changes with speed cubed. This is why a small speed reduction can make a large energy reduction.
Duty Points Matter
Many systems do not run at one condition. A fan may run at low flow overnight. It may run near full speed during production. A pump may follow a process schedule. This calculator lets you enter several speed points. Each point has its own hours. The yearly result then reflects real operation.
Baseline Power Needs Care
The existing power percent is important. Some old systems run at full motor power while a valve or damper wastes pressure. Other systems already have staging or inlet guide control. Use measured kilowatts when possible. If measured data is not available, use a conservative baseline. This prevents exaggerated savings.
Efficiency and Losses
A drive is efficient, but it is not perfect. Motor efficiency also matters. The calculator includes both values. It also lets you choose input power or shaft power. This avoids double counting. It keeps the estimate clear for audits, reports, and project screening.
Costs and Payback
Energy savings are only one part of the decision. Demand charges can add more value. Maintenance savings may also matter. Soft starting can reduce belt wear. Lower speed can reduce vibration. The calculator combines these yearly benefits. It then estimates simple payback and return on investment.
Best Use Cases
This tool works best for centrifugal loads. Examples include air handlers, cooling tower fans, booster pumps, and chilled water pumps. It is less accurate for constant torque machines. Conveyors, crushers, and positive displacement pumps may not follow the cube rule. For those loads, use a measured exponent or field testing.
Practical Notes
Check minimum flow limits before slowing any pump. Verify cooling and lubrication needs. Motors may need inverter duty ratings. Harmonics may need review on sensitive sites. The calculator gives a strong planning estimate. Final designs should still use equipment data, controls logic, and site measurements.
Measurement Tips
Good inputs make the estimate stronger. Measure kilowatts at several stable operating points. Record valve position, damper position, flow, pressure, and speed. Use the same production period when comparing before and after data. Weather and process changes can shift loads. Keep notes about bypasses and manual overrides. These details help explain savings and improve future tuning. They also support better maintenance planning. Use them across similar equipment groups sitewide later too.
Interpreting Results
A high savings percent is common when long hours occur at low speed. A short payback suggests a strong project. Negative savings means the entered baseline is already lower than the modeled drive load. In that case, review the baseline percent, efficiency values, and speed schedule.
FAQs
What does this calculator estimate?
It estimates yearly energy, cost, demand, and emission savings from running a fan or pump at different speeds with a variable frequency drive.
Why is the load exponent set to 3?
Centrifugal fans and pumps often follow the cube law. Power changes with the cube of speed. Field data can justify another exponent.
Can I use horsepower instead of kilowatts?
Yes. Select hp as the power unit. The calculator converts horsepower into kilowatts before running the savings formulas.
What is existing power percent?
It is the present input power at that duty point as a percent of full speed input power. Use 100 for full speed throttled operation.
Should I choose input power or shaft power?
Choose input power when you have measured line kilowatts. Choose shaft power when using motor output rating or calculated mechanical load.
Does the result include demand savings?
Yes. It compares the highest existing kilowatt value with the highest drive kilowatt value, then applies the monthly demand charge.
Can the savings be negative?
Yes. Negative savings can appear when baseline power is low, speed is high, or efficiency losses exceed modeled reductions. Recheck inputs.
Is this accurate for conveyors?
Not usually. Conveyors are often constant torque loads. Use measured power data or a different exponent for those applications.
Why include drive efficiency?
Drives have small electrical losses. Including efficiency gives a more realistic estimate and prevents overstating the final savings.
What does simple payback mean?
Simple payback is installed project cost divided by yearly savings. It shows how many years the savings need to recover the cost.
Can I export the results?
Yes. Use the CSV button for spreadsheet data. Use the PDF button to print or save the result as a document.