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Synchronous motor is one of the most efficient motors. The ability to control their power factor makes it very demandable especially for low speed drives. This post will discuss Synchronous motor, its construction, working principle, types, characteristics, starting methods, applications, model/ phasor diagram, advantages and disadvantages.

What is Synchronous Motor

A synchronous motor is an AC motor wherein, at steady state, the rotation of the shaft is synchronized with the frequency of the supplied current; the rotation period is exactly equal to an integral number of AC cycles.

Synchronous Motor

Fig. 1 – Synchronous Motor

These motors contain multi-phase AC electromagnets on the stator of the motor that create a magnetic field that rotates in time with the oscillations of the line current. A synchronous-motor is doubly fed if it is supplied with independent excited multi-phase AC electromagnets on both the rotor and stator.

Construction of Synchronous Motor

The structure is same as of other motors. Stator and rotor are the main parts of a synchronous motor while a frame is the cover and both stator and rotor make up the electric and magnetic circuitry of the Synchronous motors. The main components of the motor are:

  • Stator
  • Rotor
  • Exciter
  • Frame

Components of Synchronous Motor

Fig. 2 – Components of Synchronous Motor


Stator is the stationary part of the motor. It has a cylindrical frame which has slots to carry winding circuitry. The Stator consists of the core, which is generally made up of steel. This core is insulated to prevent the flow of eddy currents.

Components of Stator

Fig. 3 – Components of Stator

The winding circuit of the stator is called Stator Winding. It is supplied 3 phase AC power.


Rotor is the rotating part that rotates exactly at the same speed as the stator magnetic field. It is excited by a DC source.

The rotor consists of a number of poles, which depends on the speed and frequency of the machine. The relation between the pole, speed and frequency is defined as

Relation between Pole, Speed and Frequency of a Motor


N = Speed of Motor in rpm

f = frequency, and

p = No. of poles

Types of Rotor Construction in Synchronous Motor

There are two types of rotor constructions in Synchronous Motors. They are:

  • Salient Pole Rotors
  • Non-Salient Pole Rotors

Salient Pole Rotors

In Salient Pole Rotors, the poles protrude from the rotor surface.

Salient Pole Rotor

Fig. 4 – Salient Pole Rotor

Non-Salient Pole Rotors

In Non-Salient Pole Rotors, winding are placed in slots machined rotors.

Non-Salient Pole Rotor

Fig. 5 – Non-Salient Pole Rotor


It is a small generator placed in the rotor, which provides excitation power for excitation. It consists of a field winding and armature winding. The field winding is placed in stator and the armature winding is placed in the rotor of the machine.


It protects the motor and covers the whole assembly.

Working Principle of Synchronous Motor

The operation of a synchronous motors is that the rotor follows the rotating magnetic field of a stator and rotates at a speed approaching it. The rotor winding is excited by a DC source and the stator winding is excited by AC source.

Synchronous Motor Working Principle

Fig. 6 – Synchronous Motor Working Principle

Salient points regarding the working principle of Synchronous Motor are:

  • Due to 3 phase AC, a 3 phase rotating magnetic field is produced by stator winding.
  • Rotor winding produces a constant magnetic field.
  • At some rotations, the poles of two magnetic fields attract each other while at some instant, they repel each other.
  • The rotor will not start to rotate due to its inertia. So an external source will provide initial rotation.
  • Once the rotor starts moving at the synchronous speed, the external source is shut off.
  • The magnetic field of a rotor is not produced by the magnetic field of the rotor but through induction. Hence, the air gap between rotor and stator is not kept very small.

Types of Synchronous Motor

Synchronous motors can be classified into two types based on how the rotor is magnetized.

  • Non-Excited Synchronous Motors
  • Direct Current (DC) Excited Synchronous Motors

Non Excited Synchronous Motor

The rotor is made up of steel. At synchronous speed, it rotates with the rotating magnetic field of the stator, so it has an almost-constant magnetic field through it. The rotor is made of high-retentively steel such as cobalt steel.

Non-Excited Synchronous Motors are available in three designs:

  • Hysteresis Synchronous Motors
  • Reluctance Synchronous Motors
  • Permanent Magnet Synchronous Motors

Hysteresis Synchronous Motors

Hysteresis motors are single phase motors where the rotor is made of ferromagnetic material. The rotors have high hysteresis loss property. They are made up of Chrome, Cobalt Steel or Alnico.

Hysteresis Synchronous Motor

Fig. 7 – Hysteresis Synchronous Motor

They are self-starting and do not need additional winding. This has a wide hysteresis loop which means once it is magnetized in a given direction; it requires a large reverse magnetic field to reverse the magnetization.

Reluctance Synchronous Motors

Reluctance is always minimum when a piece of iron rotates to complete a magnetic flux path. The reluctance increases with the angle between them when the poles are aligned with the magnetic field of the stator. This will create a torque pulling the rotor into alignment with the pole near to the stator field.

Reluctance Synchronous Motor

Fig. 8 – Reluctance Synchronous Motor

The rotor poles generally have squirrel-cage winding embedded, to provide torque below synchronous speed to start the motor.

Permanent Magnet Synchronous Motors

A permanent Magnet Motor uses permanent magnets in the steel rotor to create a constant magnetic flux. The rotor locks in when the speed is near synchronous speed.

Permanent Magnet Synchronous Motor

Fig. 9 – Permanent Magnet Synchronous Motor

The stator carries winding which are connected to an AC supply to produce a rotating magnetic field. Permanent magnet motors are similar to brushless DC motors.

Direct Current (DC) Excited Synchronous Motor

Direct Current (DC) Excited Synchronous Motor requires DC supply to the rotor to generate a magnetic field. It has both stator winding as well as rotor winding. The direct current can be supplied from a separate DC source or from a DC generator connected to the motor shaft.

Characteristics of Synchronous Motor

Some of the key characteristics of a synchronous motor which differentiates it from other motors are as follows:


Speed of ranges from 150 rpm to 1800 rpm. The speed is synchronous and does not depend on load conditions. Speed always remain constant from no load to full load.

The relation between the pole, speed, and frequency is defined as

Relation between Pole, Speed and Frequency of a Motor


N = Speed of Motor in rpm

f = frequency, and

p = No. of poles

Starting Torque

External force is required to start the synchronous motor as it has no starting torque.


The power rating of synchronous motors ranges between 150kW to 15MW.


The Synchronous Motors are highly efficient machines and their efficiency is much greater than induction motors.


The Synchronous motors use brushless Exciter which decreases the maintenance problem.

Power Factor Correction

These motors have high power factor correction, Hence they are used in areas where power factor correction is needed.

Starting Methods of Synchronous Motor

As we all know that Synchronous motors cannot self-start as it has no starting torque. Therefore different ways are used to start the motor. External force is used at start for bringing up the speed up to synchronous speed. The three main ways are:

  • Reduce frequency of stator to a safe starting level.
  • Use external prime mover.
  • Use of damper winding.

Model Diagram and Phasor Diagram of Synchronous Motor

Field structure is stimulated by direct current in synchronous motor. Due to the rotating magnetic field, the voltage induced in the stator winding and this voltage is called counter emf (E).

Model Diagram of Synchronous Motor

Fig. 10 – Model Diagram of Synchronous Motor

The effect of armature reaction is substituted by Fictitious Reactance (Xa). When Xa is combined with the leakage reactance of the armature it gives Synchronous Reactance (Xs). When Xs is combined with the Armature Effective Resistance (Re), it gives the Synchronous Impedance (Zs).

Phasor Diagram of Synchronous Motor

Fig. 11 – Phasor Diagram of Synchronous Motor

In order to draw the phasor diagram, Vt is taken as the reference phasor and below points are to be followed:

  • If a machine works as a asynchronous motor then the direction of armature current will be opposite to that of the excitation emf.
  • Phasor excitation emf always lags phasor terminal voltage.

Application Areas of Synchronous Motor

The application areas of Synchronous motor includes:

  • The basic use of a synchronous motor is “power factor correction” which means to increase the power factor of a system.
  • Synchronous motors are used in voltage regulation
  • Synchronous motors are generally used for low speed, high power loads.
  • Synchronous motors are generally used in air and gas compressors and vacuum pumps.
  • Synchronous motors also find their application in crushers, mills and grinders.
  • They are also used in exhausters, fans, and blowers.

Advantages of Synchronous Motor

The advantages of Synchronous motor includes:

  • The advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous-motor has leading power factor and is operated in parallel to induction motors thereby improving the system power factor.
  • Speed remains constant irrespective of the loads in synchronous motors. This quality helps in industrial machines where constant speed is required irrespective of the load.
  • Synchronous motors are built with wider air gaps than induction motors which make them mechanically more stable.
  • Electro-magnetic power varies linearly with the voltage in synchronous motors.
  • Synchronous motors usually operate with higher efficiencies (more than 90%) especially in low speed compared to induction motors.

Disadvantages of Synchronous Motor

The disadvantages of Synchronous motor includes:

  • Synchronous motors require dc excitation which is supplied from external sources.
  • These motors are not self-starting motors and need some external arrangement for its starting and synchronizing.
  • The cost per kW output is commonly higher than that of induction motors.
  • Unless the incoming supply frequency is adjusted, there is no possible way to adjust the speed.
  • They cannot be started on load because its starting torque is zero.
  • Collector rings and brushes are required which results in high maintenance cost.
  • Synchronous motors cannot be useful for applications requiring frequent starting of machines.
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