Classification of Electric Motors - Part Four

In the previous topic ” Classification of Electric Motors - Part Three “, I explained the Induction motor and its different types.

Today, I will explain the Synchronous motor and its different types as follows.

You can review the following related topics for review and good following.

Second: Synchronous motor 

Synchronous Motor: So called because rotor tries to line up with the rotating magnetic field in the stator. It has the stator of an induction motor, and the rotor of a dc motor.

A synchronous motor is an AC motor, which runs at constant speed fixed by frequency of the system. It requires direct current (DC) for excitation and has low starting torque, and therefore suited for applications that start with a low load, such as air compressors, frequency changes and motor generators. Synchronous motors are able to improve the power factor of a system, which is why they are often used in systems that use a lot of electricity.

Differences between Synchronous and Induction motors:
  1. Synchronous motors are not as widely used as induction machines because their rotors are more complex and they require exciters. 
  2. Synchronous motors are used in large industrial applications in situations where their ability to provide leading power factor helps to support or stabilize voltage and to improve overall power factor. 
  3. In ratings higher than several hundred horsepower, synchronous machines are often more efficient than induction machines and so very large synchronous machines are sometimes chosen over induction motors. 
  4. Unlike an induction motor, the synchronous motor is excited by an external DC source and, therefore, requires slip rings and brushes to provide current to the rotor. 
  5. In the synchronous motor, the rotor locks into step with the rotating magnetic field and rotates at synchronous speed. If the synchronous motor is loaded to the point where the rotor is pulled out of step with the rotating magnetic field, no torque is developed, and the motor will stop. 
  6. A synchronous motor is not a self-starting motor because torque is only developed when running at synchronous speed; therefore, the motor needs some type of device to bring the rotor to Synchronous speed. 


Like the asynchronous (Induction) motor, the synchronous motor consists of a stator and a rotor separated by the air gap. It differs from the asynchronous motor in that the flux in the air gap is not due to a component of the stator current: it is created by magnets or by the field coil current provided by an external DC source energizing a winding placed in the rotor.

The main components of a synchronous motor are as follows:

1- Stator:

The stator consists of a housing and a magnetic circuit generally comprising silicon steel laminations and a 3-phase coil similar to that of an asynchronous motor supplied with 3-phase AC to produce a rotating field.

The stator produces a rotating magnetic field that is proportional to the frequency supplied. This motor rotates at a synchronous speed, which is given by the following equation:

Ns = 120 f / P

f = frequency of the supply frequency

P= number of poles

2- Rotor


Synchronous rotors are designed primarily for applications requiring highly efficient motors. Each pole assembly is made from high strength steel laminations with a DC field winding encircling the pole body. The field winding consists of a rectangular section of insulated copper wire wound directly on an insulated pole body and bonded by a high temperature, high strength insulating epoxy resin which, when cured, results in a coil impervious to dirt, moisture and other contaminants.

The rotor carries field magnets or coils through which a direct current flows and which create interposed North and South poles. Unlike asynchronous (Induction) machines, the rotor rotates with no slip at the speed of the rotating field.

There are two types of rotor structures as follows:

  1. Salient pole rotor. 
  2. Round or cylindrical rotor (Non-salient-pole rotor). 

a- Salient Pole Rotor 

Salient Pole Rotor 

  • Salient pole structure is used for low speed applications, such as hydroelectric generators. 
  • Salient-pole rotor: four and more poles.

b- Round or Cylindrical Rotor (Non-salient-Pole Rotor)

Round or Cylindrical Rotor (Non-salient-Pole Rotor)

  • Round rotor structure is used for high speed synchronous machines, such as steam turbine generators.
  • Non-salient-pole rotor: usually two- and four-pole rotors.

3- Amortisseur (starting winding)

Synchronous motors are provided with an Amortisseur, or starting winding, consisting of copper alloy bars located in the pole face, parallel to the shaft, and brazed at the ends to copper alloy rings. The Amortisseur winding is tailored for the application to provide the required starting performance.

4- Stator Frame

The stator frame contains and supports the other parts and may include bearing housings.

5- Other Parts

Large machines include additional parts for cooling the machine, supporting the rotor, lubricating and cooling the bearings, and various protection and measurement devices. 

Operation of a Synchronous Motor 

The operation of a synchronous motor is simple to imagine. The 'Stator' winding, when excited by a poly-phase (usually 3-phase) supply, creates a rotating magnetic field inside the motor. The rotor winding, which acts as a permanent magnet, supplied with a DC current and creating a field which simply locks in with the rotating magnetic field and rotates along with it? During operation, as the rotor field locks in with the rotating magnetic field, the motor is said to be in synchronization and a torque is developed.

Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the breakdown load, the motor falls out of synchronization i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization. 


  1. Synchronous motors find applications in all industrial applications where constant speed is necessary. 
  2. Improving the power factor as synchronous condensers. 
  3. Low power applications include positioning machines, where high precision is required, and robot actuators. 
  4. Mains synchronous motors are used for electric clocks. 
  5. Record player turntables. 
  6. Large plant compressors. 
  7. Fans, pumps, and large industrial grinders. 
  8. Mills in the steel industry. 
  9. Larger high-speed motors are popular in the natural-gas pipeline system. 


Synchronous motors have the following advantages over non-synchronous motors:

  1. Speed is independent of the load, provided an adequate field current is applied. 
  2. Accurate control in speed and position using open loop controls, e.g. stepper motors. 
  3. They will hold their position when a DC current is applied to both the stator and the rotor windings. 
  4. Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation. 
  5. Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus). 
  6. They run either at the synchronous speed or they do not run at all. 


There are two major types of synchronous motors as follows:

  1. Non-excited motors. 
  2. DC-excited motors. 

1- Non-excited motors

These motors employ a self-starting circuit and require no external excitation supply.

In non-excited motors, the rotor is made of solid steel. At synchronous speed it rotates in step with the rotating magnetic field of the stator, so it has an almost-constant magnetic field through it. The external stator field magnetizes the rotor, inducing the magnetic poles needed to turn it. The rotor is made of a high-retentively steel such as cobalt steel. These are manufactured in three types as follows:

  • Reluctance motors. 
  • Hysteresis motors. 
  • Permanent magnet motors. 

A- Reluctance motors 

Reluctance motor is A synchronous‐induction motor. The rotor has salient poles and a cage so that it starts like an induction motor, and runs like a synchronous motor.

Principle of operation:

Reluctance Rotor

  • A classic squirrel cage rotor with notches (or flats) in the rotor periphery. The number of notches will correspond to the number of poles in the stator winding. The sections of the rotor periphery between the high reluctance areas are known as salient poles. Since these poles create a low reluctance path for the stator flux, they are attracted to the poles of the stator field. 
  • The reluctance synchronous rotor starts and accelerates like a regular squirrel cage rotor, but as it approaches the rotational speed of the field, a critical point is reached where there is an increased acceleration and the rotor “snaps” into synchronism with the stator field. 
  • If the load (particularly inertial) is too great, the motor will not attain synchronous speed. Motor “pull-in” torque is defined as the maximum load that the motor can accelerate and pull into synchronism at rated voltage and frequency. 
  • An applied load greater than the rated “pull-in” torque will prevent the motor from pulling the load into synchronism and will result in rough, non-uniform operation. 

Reluctance synchronous motors may be designed for poly-phase operation, as well as single-phase versions in split-phase, CS and PSC configurations.

Reluctance synchronous motors ratings range from sub-fractional to about 30 hp. Sub-fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral horsepower motors use squirrel cage construction with toothed rotors.

Switched Reluctance Motors 

Switched Reluctance Motors

  • The switched reluctance motor (SRM) is an electric motor in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized. 
  • SRM is a type of synchronous machine. It has wound field coils of a DC motor for its stator windings and has no coils or magnets on its rotor. 
  • It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient, singly excited machine. 
  • Stator windings on diametrically opposite poles are connected in series or parallel to form one phase of the motor. 
  • Several combinations of stator and rotor poles are possible, such as 6/4 (6 stator poles and 4 rotor poles), 8/4, 10/6 etc. 
  • The configurations with higher number of stator/rotor pole combinations have less torque ripple. 

  1. Flameproof drive systems for potentially explosive atmospheres. 
  2. Washing machine. 
  3. Environmentally friendly air conditioning system for passenger trains. 
  4. Servo systems for advanced technology weaving machine. 

B- Hysteresis motors: 

Hysteresis motors

  • Although the stator in a hysteresis synchronous design is wound much like that of the conventional squirrel cage motor, its rotor is made of a heat-treated cast permanent magnet alloy cylinder (with a nonmagnetic support) securely mounted to the shaft like "hard" cobalt steel. This material has a wide hysteresis loop (high retentively), meaning once it is magnetized in a given direction, it requires a large reverse magnetic field to reverse the magnetization. 

Stator of a Hysteresis Motor

  • The motor’s special performance characteristics are associated with its rotor design. The rotor starts on the hysteresis principle and accelerates at a fairly constant rate until it reaches the synchronous speed of the rotating field. 

Cobalt Hysteresis Ring Rotor
  • Instead of the permanently fixed poles found in the rotor of the reluctance synchronous design, hysteresis rotor poles are “induced” by the rotating magnetic field. During the acceleration period, the stator field will rotate at a speed faster than the rotor, and the poles which it induces in the rotor will shift around its periphery. When the rotor speed reaches that of the rotating stator field, the rotor poles will take up a fixed position. 
  • if the load is increased beyond the capacity of the motor, the poles on the periphery of the rotor core will shift. 
  • If the load is then reduced to the “pullin” capacity of the motor, the poles will take up fixed positions until the motor is again overloaded or stopped and restarted. 
  • The hysteresis rotor will “lock-in” at any position, in contrast to the reluctance rotor which has only the “lock-in” points corresponding to the salient poles on the rotor. 

Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.

C- Permanent-Magnet Synchronous Motors 

Permanent-Magnet Synchronous Motors

  • The stator portion of Permanent-Magnet Synchronous Motors has an uneven distribution of magnetic Poles and the solid steel rotor has permanent magnets embedded in it, the purpose of this is to give the rotor a preferred starting point while providing an apparent shift in field during starting due to the uneven reluctance of the stator. 
Permanent-Magnet clock Motor and Rotor
  • They are not self-starting. Because of the constant magnetic field in the rotor these cannot use induction windings for starting, and must have electronically controlled variable frequency stator drive. 
  • Some of these motors have a spring return mechanism to reverse the rotation just in case it starts turning the wrong way. 


Industrial drives, e.g., pumps, fans, blowers, mills, hoists, handling systems, elevators and escalators, people movers, light railways and streetcars (trams), electric road vehicles, aircraft flight control surface actuation.


The use of permanent magnets (PMs) in construction of electrical machines brings the following benefits:

  1. No electrical energy is absorbed by the field excitation system and thus there are no excitation losses which mean substantial increase in the efficiency. 
  2. Higher torque and/or output power per volume than when using electromagnetic excitation.
  3. Better dynamic performance than motors with electromagnetic excitation (higher magnetic flux density in the air gap). 
  4. Simplification of construction and maintenance. 
  5. Reduction of prices for some types of machines. 


  1. High cost of permanent magnets. 
  2. Magnet corrosion and possible demagnetization. 
  3. Large air gap in surface mount PM machines. 

In the next Topic, I will continue explaining other types of Synchronous Motor. So, please keep following.

Note: these topics about Motors in this course EE-1: Beginner's electrical design course is an introduction only for beginners to know general basic information about Motors and Pumps as a type of Power loads. But in other levels of our electrical design courses, we will show and explain in detail the Motor and Pumps Loads calculations.  

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