Electric motors and drives: Science and limits
Why motors beat any engine, and small ones lag big ones
What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.An electric motor uses magnetic forces between currents and magnetic fields to turn electrical energy into rotation or straight-line motion. Motor-driven systems used 13,462 TWh in 2023, about 53% of the world's electricity.
The conversion, step by step
Variable speed drive (optional)Electrical (fixed-frequency AC) → Electrical (variable-frequency AC) · 95-97% at full load for 5-200 hp drives, falling at very light load4Stator windings create a rotating magnetic field
Electrical → Magnetic field energy · Stator copper (I2R) loss is usually the largest single loss: 28-42% of total losses in DOE's typical 25-100 hp motors (1,800 rpm, open drip-proof)3Rotor is pulled round by the field
Magnetic → Mechanical (shaft torque) · Whole motor 79.6-96.3% (IE2 to IE4, 0.75 kW to 110 kW, 4-pole, 50 Hz)1Driven machine (pump, fan, compressor, conveyor)
Mechanical (shaft) → Fluid flow, pressure or movement · Varies widely; throttling with valves and dampers wastes much of it, which is why speed control matters4
The science
A wire carrying current in a magnetic field feels a sideways force (the Lorentz force). A motor arranges many such wires so the forces add up into torque. In an AC induction motor, three-phase current in the stator windings makes a magnetic field that rotates; it induces currents in a cage of bars in the rotor, and those currents are pushed along by the field, so the rotor follows slightly slower than the field (slip). A synchronous motor locks onto the field instead, using permanent magnets or a rotor shaped so the field pulls on iron (reluctance). Losses are mostly heat: resistance in copper and aluminium conductors, eddy currents and hysteresis in the iron core, bearing friction and air drag, and stray losses. For pumps and fans, flow scales with speed but power scales with the cube of speed, so running 20% slower can cut input power by about half.
Limits set by science
| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| No heat-engine (Carnot) limit | Not applicable1 | A motor converts one ordered form of energy (electrical) into another (mechanical), so the second law does not cap it the way it caps engines that run on heat. Every loss is a resistance, magnetic or friction effect that better materials and design can shrink. | IE4 motors reach 96.3% at 110 kW and 96.7% at 355-400 kW (4-pole, 50 Hz). |
| Small-motor penalty | IE4 minimum 85.7% at 0.75 kW versus 96.3% at 110 kW1 | Losses such as magnetising current and friction do not shrink in proportion to size, so small motors are always less efficient than large ones. | Permanent-magnet and synchronous reluctance designs narrow the gap at small sizes. |
| Loss reduction per efficiency class | About 20% lower losses per step (IE3 to IE4 to IE5 to IE6)2 | Each step needs more active material (more copper, better steel) or a different rotor, so returns diminish: going from 95% to 96% removes a fifth of the remaining loss. | ABB reports a magnet-free synchronous reluctance motor at IE6, with 20% lower losses than a comparable IE5 motor (company claim, 2025). |
| Magnet materials | China makes about 94% of sintered permanent magnets (2024) and about 91% of refined rare earths5 | The most efficient compact motors use neodymium-iron-boron magnets. The limit here is supply concentration and magnet heat tolerance rather than physics of conversion. | Magnet-free reluctance and copper-rotor induction motors avoid the dependency. |
Motor systems used 13,462 TWh in 2023 (53% of world electricity): 72% of electricity in industry, 36% in buildings, 87% in agriculture and 86% in transport (source 9).
Sources
- Technical note: IEC 60034-30-1 standard on efficiency classes for low voltage AC motors ABB, 2018
- SynRM motors with unprecedented efficiency ABB, 2025
- Premium Efficiency Motor Selection and Application Guide: A Handbook for Industry US Department of Energy, Advanced Manufacturing Office, 2014
- Adjustable Speed Drive Part-Load Efficiency, Motor Systems Tip Sheet #11 US Department of Energy, 2012
- Rare earth elements: executive summary International Energy Agency, 2026