What is Synchronous Motors? A Complete Guide to Their Functionality

Most people understand that electric motors convert electricity into mechanical motion, yet few are familiar with the diverse motor designs that deliver this function. While it may seem unnecessary to develop multiple solutions for the same goal, each motor variant is engineered for distinct working conditions. Some motors run on DC power, others on AC power, and a few support mixed‑current operation, with unique energy‑transfer mechanisms for every design. As a result, there exists a wide range of DC and AC motor products, each bringing specific advantages to targeted industrial scenarios. So what is synchronous motors, and what sets it apart from other AC motor options?

What is synchronous motors: A synchronous motor is a category of AC motor engineered to overcome performance limitations of induction motors, which are another widely‑used AC motor family. Induction motors generate mechanical output through electromagnetic induction, but they suffer from an inherent “slip” effect. Slip refers to the speed gap between the AC supply oscillation frequency and actual rotor rotation speed, a natural by‑product of induction‑driven rotation. For many general‑purpose applications slip is negligible; nevertheless, it disqualifies standard induction motors from precision‑timing tasks, hence they are named asynchronous motors.

In contrast, the output rotating frequency of a synchronous motor perfectly matches the input AC supply frequency. Its rotational speed maintains a strict proportional relationship with input AC power, making it suitable for clocks, rolling mills, record players and other precision‑demanding equipment. Though synchronous motors cannot match induction motors in total power output or model diversity, they occupy an irreplaceable position in projects requiring accurate timing and stable speed control. If you are sourcing synchronous motors, feel free to get in touch with our team.

Key Components of a Synchronous Motor

To fully grasp what is synchronous motors, it is necessary to learn about its core assemblies that cooperate to realize stable, high‑efficiency operation. Major components and their functions are listed below:

1. Stator
   The stator forms the stationary part of the motor. It is stacked from silicon‑steel laminations with inner slots for winding installation. Its primary mission is to generate the rotating magnetic field that drives motor operation.
2. Stator Winding
   Manufactured from heavy‑duty super‑enamelled copper wire, the stator winding adopts three‑phase star or delta connection and fits inside stator slots. Energized by three‑phase AC input, it produces the rotating magnetic field.
3. Rotor
   The rotor is the rotating assembly of the synchronous motor. This cylindrical component carries magnetic poles on its outer surface. Identical to the stator, it is built with silicon‑steel stampings to optimize magnetic performance and cut energy losses.
4. Rotor Winding
   Enamelled copper wire is wound onto rotor poles to form the rotor winding. Supplied with DC excitation current from the exciter, it builds up a fixed magnetic field so that the rotor can magnetically lock with the stator rotating magnetic field.
5. Exciter
   Mounted on the same shaft as the rotor, the exciter is a small‑capacity DC shunt generator. During motor running, it produces DC excitation current for rotor windings, forming a self‑sustaining excitation supply loop.
6. Slip‑Rings and Brushes
   Two phosphor‑bronze slip‑rings are fitted on the rotor shaft. Carbon brushes keep continuous physical contact with slip‑rings, transferring DC excitation power from the exciter to rotor windings for smooth power transmission.

The Main Characteristics of Synchronous Motors

Knowing what is synchronous motors, we can summarize its typical operating features:

1. Synchronous motors have no inherent self‑starting capability. Auxiliary external driving force must accelerate the rotor to near‑synchronous speed before magnetic synchronisation can take effect.
2. Its operating speed is locked to supply frequency. Under fixed‑frequency power supply, it runs at constant speed regardless of load fluctuation.
3. It features adjustable power‑factor performance. This special property enables synchronous motors to improve overall system electrical power factor.

Applications of Synchronous Motors

Based on its unique traits, synchronous motors serve multiple practical scenarios:

1. Capable of running at leading or lagging power factors, synchronous motors deliver power‑factor correction. Operating with leading power factor under no‑load status, they can be deployed in power systems where static capacitors cannot work.
2. They are widely adopted for low‑speed, high‑power working conditions, including mills, chippers, agitators, large pumps, compressors and other heavy‑duty industrial equipment.

How Does a Synchronous Motor Work?

Similar to slip‑ring induction motors, a synchronous motor consists of an outer stator and inner rotor, generating output torque through magnetic interaction. Subject to equipment size and application requirements, it can accept single‑phase or multi‑phase AC input.

Its stator shares the same structure as induction motors, with copper or aluminium coils embedded in laminated metal cores. AC current passing through these coils generates a rotating magnetic field. The most obvious difference lies in the rotor: the rotor carries a permanent magnetic field, created either by built‑in permanent magnets or DC‑powered rotor windings. With independent north‑south magnetic poles, the rotor magnetic field will align with poles of the stator rotating magnetic field, producing rotation speed strictly proportional to stator supply frequency. Rotor poles can protrude outward or sit inside embedded slots, corresponding to salient‑pole and non‑salient‑pole rotor structures respectively.

However, direct startup is impossible. At standstill, the static rotor cannot catch up with the fast‑spinning rotating magnetic field, so pole locking cannot occur upon power‑on. According to different excitation approaches, synchronous motors fall into two categories: non‑excited synchronous motors and current‑excited synchronous motors.

Working Principle of Synchronous Motor

A synchronous motor counts as a double‑excitation machine, receiving two separate electrical inputs. Its stator winding takes three‑phase AC power while its rotor winding receives DC power. Three‑phase current flowing through stator windings yields a rotating three‑phase magnetic flux. Meanwhile, DC power fed to the rotor creates steady constant magnetic flux.

At one instant, rotor and stator magnetic poles may carry identical polarity and generate repulsion; in the next moment, opposite‑polarity poles bring attraction. Due to rotor mechanical inertia, neither repulsive nor attractive force can drive the stationary rotor to spin. This explains why synchronous motors cannot start on their own.

External mechanical driving equipment is required to spin the rotor toward the same direction as the rotating magnetic field until rotor speed approaches synchronous speed. Once synchronous speed is achieved, magnetic locking engages. Even after removing the auxiliary driving source, the motor will keep rotating steadily.

Types of Synchronous Motors

Classified by rotor excitation modes for achieving synchronous rotation, synchronous motors are divided into non‑excited synchronous motors and current‑excited synchronous motors.

Non‑excited Synchronous Motors

Non‑excited synchronous motors do not require external excitation voltage for startup. Their rotors use ferromagnetic materials to interact with the stator magnetic field. Three mainstream sub‑types are hysteresis motors, synchronous reluctance motors and permanent‑magnet motors.

• Hysteresis motors: The rotor shaft is wrapped with a ferromagnetic hysteresis ring supported by non‑magnetic materials. The stator rotating magnetic field induces magnetic poles on this ring. Hysteresis loss creates an angular offset between rotor flux and stator flux, which generates driving torque. These motors run quietly and are mainly used for record players, tape recorders and other audio devices.
• Reluctance motors: They produce motion leveraging magnetic attraction and reluctance effects. Stator design resembles stepper and induction motors, with coil‑wound prominent poles that build magnetic fields. The ferromagnetic rotor features recesses, barriers or slots. When rotor and stator poles align, magnetic flux travels through low‑reluctance paths. When misaligned, magnetic reluctance rises. The rotor will be pulled toward the aligned low‑reluctance position and pulled into synchronous speed, delivering precise rotary output.
• Permanent‑magnet motors: Permanent magnets mounted on the rotor supply constant magnetic flux, interacting with the stator rotating magnetic field to produce rotation. Variable‑frequency drives are mandatory for these motors, as speed and torque adjustment can only be realised by modifying stator AC supply frequency.

Current‑excited Synchronous Motors

DC‑excited synchronous motor represents the major current‑excited type. It accepts both AC and DC inputs. DC power feeds into rotor windings comparable to stator coils, generating steady rotor magnetic fields. This excitation pulls rotor poles to align with the stator rotating magnetic field and completes synchronisation. Most units of this category exceed 1 horsepower, and this rotor configuration defines what people commonly refer to as synchronous motors.

Summary

After exploring what is synchronous motors, its construction, working theory, classification and real‑world uses, we can conclude that synchronous motors are irreplaceable industrial components. Without them, timing devices such as clocks, record players, windshield wipers, hard disks and signal instruments would not exist. Beyond precise speed control, synchronous motors help offset induction motor efficiency losses and mitigate power‑distribution losses. Though higher in cost and complexity compared with induction motors, they bring great value for power‑factor correction and high‑precision motion control.