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"We fitted a drive and the bill dropped" is not true for every application. The gain depends on the load characteristic. On variable-torque loads (centrifugal pumps, fans, blowers) reducing speed cuts power by the cube; on constant-torque loads (conveyors, extruders, hoists) there is no such gain.
The affinity laws
- Flow is proportional to speed: Q ∝ n
- Pressure varies with the square of speed: H ∝ n²
- Power varies with the cube of speed: P ∝ n³
In practice: run a centrifugal pump at 80% speed and flow drops 20%, while power falls to roughly 49%. With throttling, the motor still runs at full speed and the surplus energy turns into heat at the valve.
| Application | Load type | Expected benefit from a drive |
|---|---|---|
| Centrifugal pump | Variable torque | High — the cubic law applies |
| Fan / extractor | Variable torque | High |
| Conveyor | Constant torque | Low — the gain is in process control |
| Hoist / crane | Constant torque | Control and safety rather than energy |
| Extruder | Constant torque | Product quality and ramp control |
Benefits beyond energy
- Soft starting: less mechanical wear on belts, couplings and bearings.
- Lower inrush current, easing the impact on the grid and the transformer.
- Process control: flow, pressure or speed can be held in closed loop.
- Maintenance planning: current and torque data from the drive gives early warning.
Frequently asked questions
How long is the payback on a drive?
On variable-flow pump and fan applications with high running hours, payback usually falls within one to two years. On constant-torque loads the justification is process control and mechanical life rather than energy. Only measurement gives a firm figure.
Do drives cause harmonic problems?
Drives inject harmonic currents into the grid. Individual small units are not an issue; when total drive power is high relative to the plant, line reactors, passive or active filters should be considered. Detuned reactors in the correction panel may also be required.
How far can a drive slow a motor down?
Electrically the speed can go very low, but on self-ventilated motors cooling falls with speed. Applications running long periods at low speed need a force-ventilated motor or a torque limit.
What is the difference between a drive and a soft starter?
A soft starter only smooths starting and stopping and does not change running speed, so it saves no energy. A drive controls speed continuously, so on variable-flow duties savings are only possible with a drive.