Electric motors and generators
Motors turn electrical energy into motion, and generators turn motion into electricity. They consume a large share of the world's electricity, running everything from fans to electric cars.
Motors and generators
Motors and generators are the same kind of machine run in opposite directions. In a motor, current in conductors within a magnetic field produces force (F = BIL) and torque. In a generator, mechanical rotation moves conductors through a field, inducing voltage by Faraday's law. Electric motors account for roughly 40% or more of global electricity consumption, according to the International Energy Agency.
DC motors
A brushed DC motor has a rotating armature coil in a stationary magnetic field from permanent magnets or field windings. A commutator and brushes reverse the coil current every half-turn so the torque keeps the same direction. Speed rises with supply voltage, and torque with current. Brushed motors are simple and cheap but their brushes wear, and the motor's back EMF, proportional to speed, limits current as it speeds up.
Brushless DC motors
Brushless DC (BLDC) motors put permanent magnets on the rotor and windings on the stator, with electronics switching the stator currents in sequence based on rotor position. With no brushes to wear, they are efficient, long-lived, and controllable, powering drones, computer fans, power tools, and many electric vehicles.
Induction motors
Three-phase stator windings create a magnetic field rotating at synchronous speed: 3,000 rpm for a two-pole motor on 50 Hz (3,600 rpm on 60 Hz). In an induction motor, patented by Nikola Tesla in 1888, the rotating field induces currents in a rotor of bars (a squirrel cage), producing torque. The rotor runs slightly slower than the field, the difference called slip, typically a few percent. Induction motors are rugged, cheap, and the workhorses of industry.
Synchronous machines
In a synchronous machine, the rotor carries a DC field winding or permanent magnets and turns exactly at synchronous speed, locked to the supply frequency. Almost all large power-station generators are synchronous machines; their rotation speed determines grid frequency. Permanent-magnet synchronous motors are highly efficient and common in electric vehicles.
Speed and torque
Mechanical power equals torque times angular speed: \(P = T\omega\). A motor delivering 10 N m at 3,000 rpm (about 314 rad/s) produces about 3.1 kW. Stepper motors move in precise angular steps for printers and 3D printers; servo motors use feedback for accurate positioning in robots; and gearboxes trade speed for torque.
Efficiency standards
Motor efficiency classes, such as the IEC's IE1 to IE4 and higher, define minimum efficiencies, and many countries require premium-efficiency motors. Because motors run for years, the electricity they consume costs far more than their purchase price, so even a few percentage points of efficiency save substantial energy and money over a motor's life.
Worked example
A four-pole induction motor on a 50 Hz supply has synchronous speed \(n_s = \frac{120 f}{p} = \frac{120 \times 50}{4} = 1{,}500\) rpm. If the rotor turns at 1,440 rpm, the slip is \(\frac{1{,}500 - 1{,}440}{1{,}500} = 4\%\). At full load it delivers 7.5 kW of mechanical power; at 90% efficiency it draws about \(\frac{7.5}{0.9} \approx 8.3\) kW of electrical power.
Key takeaways
Motors and generators are the same machines: current in a field makes torque, and motion in a field induces voltage. Motors use roughly 40% or more of world electricity. Brushed DC motors use commutators and brushes; speed follows voltage and torque follows current. Brushless DC motors switch stator currents electronically and power drones, tools, and EVs. Three-phase stators create a rotating field: induction motors (Tesla, 1888) run with a few percent slip, and synchronous machines lock to the supply, forming most power-station generators. P = T omega links torque and speed; steppers and servos give precise motion. Efficiency classes such as IE1 to IE4 matter because lifetime energy costs dwarf purchase prices.
Flashcard
Force on current in a magnetic field
Check your understanding
What is the synchronous speed of a two-pole motor on a 60 Hz supply?