Calculators
Calculate the saving from speed control
On centrifugal pumps and fans, power goes with the cube of speed. Twenty percent less speed means almost half the power. That is exactly what makes throttle control so expensive. It keeps the speed up and destroys the energy in the valve.
Result
- Power at controlled speedIncluding the losses of the drive.
- —kW
- Annual saving compared with throttling
- —€
- Power under throttle control
- —kW
- Energy saved per year
- —kWh
- Saving in percent
- —%
Please fill all fields with valid numbers.
A guide, not a design to standard. We give no warranty for the correctness of the results. The applicable standards and case-by-case verification govern.
The grey values are an example. Simply enter your own.
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Calculating is the easy part.
A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.
Method and questions
Speed control instead of throttling
What a variable frequency drive saves on pumps and fans compared with a throttle.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- Power drawn at full speed
- 30 kW
- Operating speed
- 80 %
- Power under throttle control
- 93 %
- Operating hours per year at this speed
- 6,000 h
- Electricity price
- 25 ct/kWh
- Efficiency of the drive
- 97 %
Result
- Power at controlled speed
- 15.8 kW
- Annual saving compared with throttling
- 18,097 €
- Power under throttle control
- 27.9 kW
- Energy saved per year
- 72,390 kWh
- Saving in percent
- 43.2 %
Formula and standards
Formula
- Flow Q ∝ n
- Pressure H ∝ n²
- Power P ∝ n³
- P_drive = P · (n/n_N)³ / η_drive
Assumptions and standards
- The affinity laws hold for centrifugal pumps and fans whose curve passes through the origin. On a system with static head, such as a pump actually lifting water, they hold only in part, and the saving is smaller.
- They do not hold for positive displacement pumps, nor for drives with constant load torque such as conveyors or piston compressors.
- The comparison figure for throttle control is entered here. It comes from the system curve, not from a formula.
- The efficiency of the drive is accounted for. Additional motor losses caused by the switching frequency are not.
Why is throttling so much worse?
Because it sets the flow through a resistance rather than through the power. The pump keeps running at nearly full speed and pushes against a half-closed valve. There the surplus energy is turned into heat and noise.
Does this apply to conveyors?
No. A conveyor has constant load torque, so power goes linearly with speed, not cubically. A drive may still make sense there for other reasons, such as gentle starting, positioning or matching the cycle, but not for this saving.
Why does even a small reduction achieve so much?
Because of the cube. Ten percent less speed means twenty-seven percent less power, and twenty percent less speed almost half. That is why even a modest reduction pays on these drives.
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