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About Synchronous Motors - What They are And How They Work

Apr. 21, 2021

Most people know that electric motors use electricity to produce motion, but few know how many different ways there are to accomplish this task. There seems to be no need to create new ways to accomplish the same task, but engineers have good reason to do so. Some motors are driven by DC current, some by AC, and some by a combination of the two, with specific energy transfer methods that are unique to each motor. As a result, there are many types of DC and AC Motors, each with its own advantages in certain applications.

synchronous motor

synchronous motor

What is a synchronous motor?

synchronous motors are considered to be a type of AC motor specifically designed to address the limitations of induction motors, another prolific class of AC motor. Induction motors, as their name implies, use electromagnetic induction to generate mechanical energy; however, their main drawback is the "slip" phenomenon that occurs. This "slip" is the difference between the AC frequency of oscillation and the frequency of rotation, and is a direct result of using induction effects to produce rotation. Although not particularly important for most applications, because of this slip, ordinary induction motors cannot be used for precisely timed applications and are therefore called "asynchronous" motors.

Synchronous motors, on the other hand, are made so that the output rotational frequency is exactly equal to the input AC frequency. They can be used in clocks, rolling mills, and even record players because their speed is exactly proportional to the AC current supplying the motor. Although not as powerful or widely varied as induction motors, synchronous motors have an important place in any project that requires precise timing and accurate speed.

How does a synchronous motor work?

Like other Induction Motor, synchronous motors consist of an external stator and an internal rotor, which produce output torque by magnetic action. Just like other AC motors, synchronous motors can be powered by a single-phase input, or a multi-phase input, depending on size and application.

Synchronous motors have the same stator as other induction motors, with copper/aluminum coils running through a laminated metal plate. These coils carry AC current and produce a rotating magnetic field. They differ most in their rotors, which contain a permanent magnetic field, generated either by an actual magnet or by a DC power source passing through the rotor coils. This permanent magnetic field has its own set of north and south poles that will eventually align with the poles of the RMF, resulting in a precisely rotating output proportional to the stator frequency. These poles can either protrude from the rotor surface or be contained in slots on the rotor; these are called protruding and non-protruding pole rotors, respectively. There must be some excitation to start though, as the speed difference between the stationary rotor and the fast RMF will not allow their poles to lock at start-up. This is achieved in a different way and therefore synchronous motors are divided into non-excited synchronous motors and current-excited synchronous motors.

Types of synchronous motors

Synchronous motors can be distinguished according to the way their rotors are excited to synchronous speed, with non-excited synchronous motors and current-excited synchronous motors.

Non-excited synchronous motors

These synchronous motors do not require an excitation voltage to start and use ferromagnetic material in their rotors to interact with the stator. They are available in three main designs: hysteresis motors, synchronous reluctance motors and permanent magnet motors, each of which will be briefly described below.

Hysteresis motors use a rotor shaft contained in some non-magnetic material which has a layer of ferromagnetic material wrapped around it, forming a "hysteresis ring". The stator RMF induces two poles in this ring, but due to some loss of hysteresis - or loss of energy due to the hysteresis between the magnetization of the ferromagnet and the changing flux - the rotor flux will lag behind the stator flux. This hysteresis causes an angular separation between the rotor magnetic field and the stator magnetic field, resulting in torque. These are relatively noiseless motors and are best suited for record players, tape recorders and other audio equipment.

Magnetoresistive motors use magnetic attraction and magnetoresistance phenomena to generate motion. They are similar in design to stepper motors and induction motors, where the stator consists of prominent poles of a coil that produces a magnetic field. The rotor is made of ferromagnetic metal and is shaped as a modified squirrel cage. The rotor has recesses, barriers or slots that align with the magnetic field lines of the stator when the rotor and stator poles are matched. When mismatched, the magnetic field takes a longer path through the rotor and causes the magnetoresistance - a magnetic force similar to resistance - to increase. This creates a reluctance torque on the motor as the rotor wants to reach some lower reluctance, or return to its aligned position. This allows the rotor to be "pulled in" to synchronous speeds in some designs, providing precise rotational output.

Unsurprisingly, permanent magnet motors use permanent magnets in their rotors to produce a constant magnetic flux. This interacts with the stator RMF pole and results in a rotating output. These motors must be controlled with a variable frequency drive because the only way to change their speed and torque is to change the AC frequency of the stator.

Current-excited synchronous motors

The only major current-excited synchronous motor is the DC-excited synchronous motor, which requires a DC input and an AC input. DC power enters the rotor, which contains windings similar to those of the stator, and these windings will produce a constant magnetic field induced by the DC power supply. This will excite the motor and align its poles with the RMF of the stator, causing synchronization. These motors are usually found at >1hp and are often referred to as synchronous motors because of the prevalence of this rotor design.

It may sound like synchronous motors are the next best thing to induction motors, but without them we would not have clocks, record players, windshield wipers, hard drives, signaling equipment, recording instruments, microwave boards, or any other timing devices. Again, the efficiency of these motors helps correct the inefficiencies of induction motors and provides a way to correct distribution losses. They are invaluable to industry both in their ability to correct power and in their accuracy, and although more expensive and complex than induction motors, synchronous motors are another capable machine that designers can utilize.


This article has described what Synchronous Motors are and how they work. For more information on related products, please feel free to contact us or visit

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