Synchronous Motor,Types of Synchronous Motor,Dc current Excited Motor,Characteristic of Synchronous Motor,Application of Synchronous Motors,Comparision of Synchronous & Induction Motor

Synchronous Motor
            A Synchronous motor is electrically identical with an alternator or A.C. generator .It can define as an A.C. Motor in which at steady state rotation of the shaft is in Syn with the frequency of the applied current.In Synchronous motor the total number of rotations made by the shaft is equal to the applied current.In Synchronous motors unlike induction motor multiphase A.C electromagnets are present on the stator which produces a rotating magnetic field.Here rotor is of a permanent magnet which gets synced with the rotating magnetic field and rotates in synchronous to the frequency of current applied to it.Most synchronous motors are rated between 150KW and 15MW and run at speed ranging from 150 to 1800 rpm.
Types of Synchronous Motor
           According to the method of magnetization of the rotor we have two types of synchronous motors.
  • Non-excited.
  • Direct current excited.
         In these type of synchronous motor the rotor is magnetized by the external stator field.The rotor contains a constant magnetic field.These are further classified as.
  • Permanent magnet.
  • Reluctance.
  • Hysteresis.
Dc Current Excited Motor
          In D.C current exited motors the rotor is excited using the D.C current supplied directly through slip rings.A.C induction and rectifiers are also used.D.C current excited motors are usually large in size normally more than 1H.P.
Characteristic of Synchronous Motor
  • It runs either at synchronous speed or not at all .It means that while running it maintains a constant speed. Ns=120f/p   It shows that we can change only its speed by changing its supply frequency.
  • Synchronous motor is not self starting.It has to be run up to synchronous speed by some external means until it synchronized to the supply.
  • Synchronous motor have the capability of being operated under a wide range of power factor both lagging and leading.
Applications of Synchronous Motors 
  • Synchronous motors are used for more precise and constant speed.
  • Synchronous motors are used in generating stations and in sub-stations connected to the bus bars to improve the power factors.
  • Synchronous motors are used to regulate the voltage at the end of transmission lines.
  • In textile and paper industries synchronous motors are employed to attain wide range of speeds with variable frequency drive system.
Comparison b/w Synchronous and Induction Motor 
  • When frequency is fix the speed of synchronous motor does not change what ever is the load but the speed of induction motor falls with the increase of load.
  • The Synchronous motor is not self starting when as induction motor is self starting (3 Phase).
  • Synchronous motor can be operated over a wide range of power factors lagging and leading.Where as induction motor always run with a lagging p.f. and it is very low on light loads.
  • A D.C excitation is required by synchronous motor bu not for induction motor. 
  • Torque of the Synchronous motor does not change .When we change the applied voltage where as torque effects in the case of induction motor.
  • Synchronous motor is more costly and complicated than induction motor.
  • For low speed drives below 300rpm synchronous motor is attractive because its power factor can be adjusted from 1-0 and their efficiency is high ,but induction motor is good above 600rpm.
  • Synchronous motor can be run at ultra low speeds by using high power electronic converters which generate very low frequencies.Such type of motor of 10MW range are used for driving crushers,rotary kilns and variable speed ball mills.

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