The rotor poles of a three-phase induction motor align with the rotating stator poles.

　　 What is a synchronous motor?

　　A three-phase synchronous motor operates at synchronous speed, with the rotor poles aligned with the rotating stator poles. When the poles are aligned, the motor is said to be running at synchronous speed. This type of motor always runs at synchronous speed, which means that a synchronous motor cannot vary its speed.

　　Synchronous motors require a secondary DC power supply to energize and magnetize the rotor; the DC current is fed to the motor via slip rings. Permanent magnets can also be used on the rotor. Because the motor must operate at synchronous speed to function, it typically necessitates a starting mechanism to bring the rotor up to operating speed or near‑operating speed before it can run normally.

　　The rotor’s DC power supply is typically derived from the main AC power source, which is rectified to provide the rotor with DC. Thus, although two distinct supply voltages are required, both are sourced from the input three-phase AC supply.

　　 What is an induction motor?

　　 The operating principle of an induction motor differs from that of a synchronous motor.

　　Induction is the process by which a current is induced in a conductor when it is placed near or within a changing magnetic field. Induction can occur only in AC circuits, because as the current alternates between states, the resulting changes in the magnetic field give rise to an induced current.

　　Induction motors exploit this principle by generating mechanical torque through the time-varying magnetic field produced by alternating current in the windings. Because the stator is supplied with three-phase power, it creates a rotating magnetic field that continuously changes relative to the rotor.

　　Since the magnetic field’s rotation is defined relative to the rotor, the rotor is constantly attempting to “catch up” with the rotating magnetic field. Consequently, the induction motor’s rotational speed always remains lower than that of the rotating magnetic field.

　　 The rotor of a synchronous motor requires a DC power supply.

　　The rotor of a synchronous motor requires a DC power supply.

　　 Synchronous motor

　　Let us examine synchronous motors in detail. Since a synchronous motor is not an induction motor, it requires the rotor to be connected to a power source. The power supplied to the rotor must be direct current in order to properly energize the rotor windings.

　　In addition to a DC power supply, a synchronous motor requires a starting mechanism to bring the rotor up to synchronous speed before it can operate under normal running conditions.

　　Synchronous motors can adjust their power factor—rendering it lagging, unity, or leading—by varying the excitation in the rotor. Induction motors, by contrast, always operate at a lagging power factor.

　　Because synchronous motors are generally more complex and require more components, they tend to be more expensive. Although their upfront cost may be higher, their efficiency is often greater than that of induction motors, allowing the initial price difference to be offset over time.

　　 Induction motor type

　　Now let us examine two types of induction motors.

　　Squirrel-cage motor

　　A squirrel-cage motor is an induction motor whose name derives from the shape and configuration of its internal rotor. The rotor of a squirrel-cage motor is not connected to any external power source; it generates its own electromotive force solely through electromagnetic induction.

　　These motors are extremely simple, which significantly reduces both maintenance and manufacturing costs. They require no brushes and do not need any direct current to be introduced. Robust and durable, they are well suited for harsh environments.

　　Squirrel-cage motors are efficient, compact electric motors; however, when used as large, high‑kilowatt drives, they suffer from excessive starting torque, which is a drawback. Consequently, they exhibit lower efficiency in applications requiring high‑torque starts, though in other applications they generally outperform wound‑rotor motors. Moreover, since the rotor contains no adjustable resistance, squirrel‑cage motors lack speed‑control capability.