Motor efficiency & drives
A motor consumes its own purchase price in electricity every few weeks, so efficiency class, loading and transmission losses dominate its lifetime cost. These are the working figures for motor arithmetic.
Last reviewed July 2026. Figures are indicative working values for first-pass estimates; confirm anything compliance- or design-critical against the cited source.
IE efficiency classes
Representative full-load efficiency for a medium motor (~22–37 kW). UK ecodesign rules require at least IE3 for most new three-phase motors (0.75–1,000 kW) and IE4 for 75–200 kW.
| Class | Full-load efficiency |
|---|---|
| IE1 (standard) | 89% |
| IE2 (high) | 91% |
| IE3 (premium) | 93% |
| IE4 (super premium) | 95% |
Efficiency at part load
A standard induction motor holds its efficiency down to about half load, then falls away steeply. A motor drawing under 40% of its rated current is a right-sizing candidate.
| Load | Efficiency |
|---|---|
| 100% | 93% |
| 75% | 93% |
| 50% | 91% |
| 40% | 89% |
| 30% | 84% |
| 25% | 80% |
| 20% | 74% |
Belt-drive transmission efficiency
Slipping and worn belts lose energy continuously; cogged and synchronous belts recover most of it.
| Belt type | Efficiency |
|---|---|
| Worn V-belt | 93% |
| Good V-belt | 95% |
| Cogged V-belt | 97% |
| Synchronous (toothed) belt | 98% |
The affinity laws (pumps & fans)
For centrifugal machines, power scales with the cube of speed. This is the whole variable-speed-drive case on variable flows.
| Speed | Power |
|---|---|
| 100% | 100% |
| 90% | 73% |
| 80% | 51% |
| 70% | 34% |
| 60% | 22% |
Power factor: tan φ
Capacitor kVAr to correct from pf₁ to pf₂ = kW × (tan φ₁ − tan φ₂).
| Power factor (cos φ) | tan φ |
|---|---|
| 0.70 | 1.020 |
| 0.78 | 0.802 |
| 0.85 | 0.620 |
| 0.90 | 0.484 |
| 0.95 | 0.329 |
| 0.98 | 0.203 |