Types of Electric Motor: AC vs DC, Induction vs Synchronous, and Which One Fits the Job

Open up a CNC machining centre and you may find three motor families in one machine: a plain three-phase induction motor driving the chip conveyor, servo motors positioning the axes to within fractions of a millimetre, and a small motor turning the coolant pump. All of them turn electrical energy into rotation, yet none can stand in for another. Knowing how the types of electric motor differ turns the vague question of which motor to buy into a sharper one: which family does this job belong to?

The first split: AC or DC?

Motors are first grouped by the kind of current they use.

• DC motors run on direct current. Because their speed is easy to control through voltage, they long dominated variable-speed drives, cranes and rolling mills. Brushed types have a commutator and brushes that wear and need regular maintenance. In industry they have largely given way to AC motors on frequency converters, but they remain common in battery vehicles, forklifts and small devices.
• AC motors can run straight from the alternating-current mains. With no brushes or commutator, they make up the great majority of industrial motors.

Within AC: induction and synchronous

What separates AC motors is whether the rotor turns at the same speed as the rotating magnetic field.

Induction (asynchronous) motor

The stator's rotating field induces current in the rotor and drags it round. The rotor lags slightly behind the field; that lag is called slip. There are two rotor designs:

• Squirrel cage: the rotor carries aluminium or copper bars shorted at both ends, with no external connection, brushes or slip rings. It is simple, rugged and inexpensive, and it drives most pumps, fans, compressors, conveyors and crushers.
• Slip-ring (wound rotor): the rotor windings are brought out through slip rings and brushes so resistance can be added at start. Once used for very heavy starts on large drives, it is now rare in new projects as drives and soft starters have spread.

Synchronous motor

The rotor turns in exact step with the field. Permanent magnet (PM) synchronous motors offer high efficiency and usually run on a drive. Synchronous reluctance motors achieve synchronous speed without magnets, through the magnetic shape of the rotor. Both generally cannot be connected straight to the mains without a converter, so motor and drive are chosen together.

An induction motor's speed depends on its pole count: on 50 Hz, roughly 3000, 1500, 1000 and 750 rpm for 2, 4, 6 and 8 poles. The details are in our pole and speed table.

By supply: single-phase and three-phase

Induction motors also split by how they connect to the network. Single-phase motors run from a 220 V single-phase line. Since they cannot produce starting torque on their own, they use an auxiliary winding and a capacitor. They suit small pumps, compressors and workshop machines where only a domestic supply exists; the HEM Motor single-phase series covers 0.18–5.5 kW. Three-phase motors run on the industrial three-phase supply, reach far higher outputs, produce smoother torque and are smaller for the same power. We compare the two in single-phase or three-phase.

Special motors for positioning and precise motion

Some motors are built not to spin continuously but to move to a given angle and hold it:

• Servo motor: works with a feedback device that reads shaft position (an encoder or resolver) and its own drive. Used for CNC axes, robots and fast, accurate moves in packaging machines.
• Stepper motor: advances a fixed angle with each pulse. Because it can run without feedback, it is common in simple positioning tasks, 3D printers and dosing devices. At high speed and load it can lose steps.
• Brushless DC (BLDC): electronically commutated, compact and efficient. Often found in fans, small pumps and portable equipment.

These motors are sold as systems with their drives and do not replace an industrial induction motor, nor the other way round.

Variants of the industrial induction motor

Deciding on a three-phase squirrel-cage motor does not end the choice; it moves it to other axes. The same basic motor appears in a catalogue in versions that differ by:

• Efficiency class: IE3, IE4 and IE5. Losses differ at the same output; for a 7.5 kW 4-pole motor, the minimum efficiencies are 90.4%, 92.6% and 94.0% respectively. How the price difference pays back is worked through in IE3 or IE4.
• Frame material: cast iron for heavy industry and harsh environments, aluminium for lighter weight in selected small ratings.
• Mounting: foot (B3), flange (B5, B14) or both (B35, B34), plus vertical positions such as V1 and V5.
• Enclosure rating: IP55 as standard, with IP44, IP56 or IP65 depending on the environment.
• Application versions: heavy-duty motors with high starting torque for crushers and mills, vibration motors for screens and feeders, and geared motors where low output speed is needed.

Starting behaviour is another dividing line. Two cage motors of the same rating can deliver different starting torque depending on rotor bar design, as explained in our article on torque design classes.

Which job, which motor family?

Treat this as a first orientation rather than a rule:

• Constant-speed continuous load (pump, fan, compressor, conveyor): three-phase squirrel-cage induction motor.
• Frequently changing speed (flow-controlled pump, variable conveyor): three-phase induction motor on a frequency converter, or a PM or reluctance motor with its drive where efficiency is the priority.
• Heavy, shock-loaded starting (crusher, mill): heavy-duty induction motor with high starting torque.
• Slow, high-torque motion (mixer, drum): geared motor or a separate motor and gearbox.
• Precise position and rapid stop-start (robot, CNC axis): servo system.
• Single-phase supply only (small workshop, farm): single-phase motor.

Conclusion

The range of electric motor types can look crowded, but most industrial jobs land on the three-phase squirrel-cage induction motor; the real choice is among its power, speed, efficiency, frame and mounting options. Positioning motors and drive-dependent designs should be treated as separate systems. You can look for the version that fits your application on our three-phase electric motors page.

Frequently Asked Questions

Which electric motor is most widely used in industry?

The three-phase squirrel-cage induction motor is by far the most common. It has no wearing parts such as brushes or slip rings, needs little maintenance, and drives most pumps, fans, compressors and conveyors.

What is the difference between induction and synchronous motors?

In an induction motor the rotor turns slightly slower than the rotating field, and that slip is what produces torque. In a synchronous motor the rotor turns at exactly field speed, and such motors usually run with a frequency converter.

Can an induction motor replace a servo motor?

For continuous rotation without positional accuracy, an induction motor is enough. Axes that must move quickly to a set angle and stop there need the feedback and drive of a servo system, which an induction motor cannot match.

Are DC motors still used?

In industrial variable-speed drives they have largely been replaced by AC motors with frequency converters. They remain widespread wherever a DC source is on hand, such as battery vehicles, forklifts and small devices.