Electric motors and drives: Engineering methods

Induction, magnet and magnet-free motors compared

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Motors differ in how the rotor gets its magnetism (induced currents, permanent magnets, iron shape or brushes) and whether they turn or push in a straight line.

Comparison

Method	Efficiency	Scale	Economics	Limitations	Reliability
AC induction motor¹³	IE1-IE4 minimums 87.6-93.3% at 11 kW, 93.3-96.3% at 110 kW (4-pole, 50 Hz)	0.12 kW to many MW; the workhorse of industry	Cheapest per kW and widely stocked	Rotor losses from induced current; efficiency drops at light load	Very robust; main wear parts are bearings and winding insulation
Permanent-magnet synchronous and brushless DC⁵	Among the most efficient types, because the rotor carries no current; used in EVs, compressors and appliance motors (no class figure in the sources used)	Watts to hundreds of kW	Needs a drive and rare-earth magnets, so higher cost and supply risk	Magnets can demagnetise if overheated; supply concentrated in China	High; electronics are the usual failure point
Synchronous reluctance²	IE5 and, in one 2025 product, IE6	Industrial pumps and fans, kW to hundreds of kW	Uses familiar materials; drive cost is extra	Needs a drive to start and run	Lower winding and bearing temperatures than equivalent induction motors
Brushed DC motor	Lower than modern AC types (no standard figure found)	Toys, car accessories, legacy industrial drives	Very cheap at small sizes, simple to control	Brush wear, sparking, dust	Brushes need replacing
Stepper motor	Low, because current flows even when holding still	Printers, 3D printers, small actuators	Cheap precise positioning	Can lose steps under overload; noisy	Few wearing parts
Linear motors, maglev and solenoids	Linear motors lose more through the air gap than rotary ones; solenoids are on-off actuators	Machine tools, maglev lines, valves and locks	Track-length windings make maglev expensive per km	Large air gaps, heat in held solenoids	No contact wear in maglev
Variable speed drive on pumps, fans and compressors⁴	Drive itself 95-97% at full load; system savings large because power falls with the cube of speed	Any AC motor	Pays back fastest on variable-flow fans and pumps	Little benefit for constant-load or high static-lift systems; drive losses at very light load	Electronics and cooling fans need maintenance

Induction motors remain the default because they are cheap, tough and run straight off the grid. Permanent-magnet machines are most efficient and compact, so they win in vehicles and appliances, at the cost of magnet supply risk. Synchronous reluctance motors reach top efficiency classes without magnets but need a drive. Across all types the biggest saving is usually at system level: matching speed to load with a drive and sizing the motor correctly matters more than one efficiency class.

How each method works

AC induction motor

Three-phase stator currents create a rotating field that induces current in a rotor cage of aluminium or copper bars; the rotor follows the field with a few per cent slip. No brushes or magnets.

Permanent-magnet synchronous and brushless DC

Magnets on the rotor follow a stator field switched by electronics, with no slip and no rotor current, so rotor losses nearly vanish.

Synchronous reluctance

A rotor of shaped steel with air slots aligns itself with the stator field, producing torque without magnets or rotor cage. Always runs from a drive.

Brushed DC motor

Brushes and a commutator reverse current in the rotor coils each half turn so torque stays in one direction.

Stepper motor

The rotor moves in fixed angular steps as electronics energise stator coils in sequence, giving position control without a sensor.

Linear motors, maglev and solenoids

An unrolled motor pushes a carriage or train along a track; maglev also levitates the vehicle magnetically. A solenoid is a coil that pulls an iron plunger a short distance.

Variable speed drive on pumps, fans and compressors

Power electronics change motor speed to match demand instead of throttling flow with valves or dampers.

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