FISEKON GmbH – Fischer Elektrokonstruktion

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22 — 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.

Input

As a percentage of full speed. For pumps and fans it equals the flow rate.

The comparison case: the same flow set with a throttle. From the pump or fan curve; at 80 % flow it is often still 90 to 95 %.

A variable frequency drive consumes something itself. Usual values are 96 to 98 percent.

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
—%

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.

Worked 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

  • 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 — 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.

Frequently asked

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 — the surplus energy is turned into heat and noise there.

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 — gentle starting, positioning, 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; twenty percent less speed almost half. That is why even a modest reduction pays on these drives.

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.

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