Synchronous Motor - Working, Applications & Advantages


     An electric motor is a device that converts electrical energy into mechanical energy. The motors can operate on dc or ac ( single-phase as well as three-phase ) supply. In fact, as ac supply is very commonly available, ac motors are very popularly used in practice. AC motors are mostly 3-phase motors ( except small motors used in many types of equipment in homes, offices, shops, etc. ).

     AC motors can be classified into two major categories. They are synchronous motors and asynchronous motors ( induction motors ). In this article let us see about the synchronous motor.


Definition :

     A device that converts electrical energy into mechanical energy running at synchronous speed ( or constant speed ) is called 'Synchronous Motor'. The speed of the synchronous motor is independent of load ( speed doesn't change if load changes ). A reverse operation of the alternator ( input as electrical and mechanical as output ) will makes the machine a synchronous motor as it rotates at synchronous speed.



Construction and Working Principle :


     The construction of the synchronous motor is similar to an alternator or synchronous generator. The 3-phase armature winding is placed on the stator slots, while field winding is placed on the rotor. Due to the number of advantages, most of the synchronous machines ( motor and generator ) are constructed with a stationary armature and rotating field. The field winding is excited by an external dc source supplied through slip-rings.


     When a 3-phase supply is fed to the armature winding it setups a rotating magnetic field rotating at synchronous speed. The field winding is fed through dc supply and produces its stationary magnetic poles. When these field poles are get aligned with rotating stator poles ( by force of attraction between two unlike poles ), they establish a strong magnetic locking between rotating stator poles and stationary rotor poles. This makes the rotor to rotate with the stator rotating magnetic field as shown above.

     Since the stator is supplied through alternating current, there will be a quick reversal of stator poles ( for every half cycle ), before the rotor starts rotating. This is because the inertia of the rotor makes it delay to rotate and before it happens the stator poles reverse. Thus motor fails to rotate, hence the synchronous motor is not a self-starting motor. In order to start, an initial starting has to given by means of an external device.

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Some characteristics features of synchronous motors are,
  1. Practically, the speed of the synchronous motor remains constant i.e., synchronous speed and it doesn't change other than synchronous speed.
  2. It is not inherently self-starting.
  3. Synchronous motors can be operated at both lagging and leading power factors.
  4. The back emf of a synchronous motor depends on the excitation and not on the speed.
  5. When there is a sudden change in load on the synchronous motor, then a condition known as 'Hunting' is produced.


Advantages of Synchronous Motor :

  • The power factor can be controlled easily. The motor can work at lagging, unity, and leading power factors.
  • An over-excited synchronous motor works as a synchronous condenser when connected in parallel with a load having a lagging power factor, which improves the power factor of the combined load.
  • The speed is constant and irrespective of the load.


Disadvantages of Synchronous Motor :

  • It is not self-starting and needs some arrangement for starting and synchronizing the motor. It requires separate dc excitation.
  • The initial cost is very high.
  • It can not be used for variable speed jobs.
  • If the motor is overload, it may fall out of synchronism and stop.
  • It has a tendency to hunt.



Applications of Synchronous Motor :


Power Factor Correction :

     Overexcited synchronous motors work as a synchronous condenser, are used for the improvement of power factor of industrial loads having lagging power factors.


Constant Speed Applications :

     Synchronous motor providing constant speed at high efficiency, so they can be used for constant speed applications such as centrifugal pumps, blowers, line shafts, motor-generator sets, synchronous clocks, air-compressors, textile mills, paper mills, cement mills, etc.


Improving Voltage Regulation of Long Transmission Lines :

     A synchronous motor with a field regulator can be used to control the voltage at the end of long transmission lines by varying its excitation.


Frequency Changer :

     A synchronous motor can be used to drive another alternator to generate supply at different frequencies due to its constant speed nature. In this case, it is called a 'Frequency Changer'.


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