Best Tutorial on Synchronous Motor

In this tutorial, we are going to learn about Best Tutorial on Synchronous Motor

Synchronous Motor

  • Typically, the majority of three phase motors which can be found in use with commercial ventilation systems, Elevators, conveyors, etc. are classified as induction motors. They are designed to run at a speed slightly below.

 Synchronous speed

  • The speed at which a given machine’s field fluxes rotates – so that there is always relative motion between this field flux and the machine‘s rotor conductors. If relative motion were to cease, zero E.M.F would be inducted into the rotor conductors and the machine would be inducted into the rotor conductors and the machine would be inducted into the rotor conductors and the machine would fail to operate.

 Synchronous Motors Applications –

  • As the name implies – are designed to run at synchronous speed. They typically find application in equipment where a precise, constant speed of operation is desired – for example : timing equipment, certain types of fan, paper mill rollers, run with a leading power factor, making them a suitable alternative to capacitor banks where  power factor  correction techniques are to be employed.
  • In order to run at synchronous speed, synchronous motors are in fact fed with two separate electrical supplies. The first is a three phase A.C. supply connected directly to a machine’s stator windings, the second being a D.C. supply connected to that machine’s rotor conductors. As with any three phase induction motor, a synchronous motor’s stator windings will produce a rotating magnetic field. Placed inside that field is of course the rotor, which will produce its own static magnetic field in response to the application of a D.C. supply.
  •  Synchronous speed running will be achieved when a machine’s D.C. excited rotor.
  • Magnetically locks with the stator’s rotating field flux, just as in the following diagram:  Essentially, the rotor’s North and south poles will try to align themselves with the North and south poles of the stator, following them in rotation at an equal velocity. It is easy to see therefore why this type of machine is ideal for constant speed applications. Now, there are a couple of different methods of the supplying direct current to a synchronous motor’s rotor conductors and these will be covered under the construction section of this report. For the moment though, it is important to consider the fact that synchronous motors cannot actually reach synchronous speed without some external means of starting At rest, a synchronous machine’s rotor may at one instant be faced with an attractive force toward the field flux- why rotor and stator poles align with opposing polarities- and the next instant, be faced with a repulsive force –when poles align with like polarities. The net result is that the motor will be unable to rotate in any one direction thanks to the opposing forces acting upon it. Read on for the solution.
  • There are in essence two widely used methods of starting three phase synchronous motors – induction motor starting and damper winding starting. The former method makes use of a separate three phase induction motor which is mechanically coupled to the driveshaft of the synchronous motor. Initially. The induction motor will be used to bring the synchronous motor’s rotor up to near zero. It is at this point that direct current is fed to the synchronous motor’s rotor conductors, allowing it to become magnetically locked with the stator field and begin running at synchronous speed.
  • Obvious disadvantages of this method include the fact that it can only really be used with unloaded synchronous motors; hence it is most commonly found where these machines are employed as a means of power factor correction.
  •  Damper winding starting incorporates additional windings into the rotor of a synchronous motor which are not fed by a D.C. supply, but instead give the effect of a standard three phase induction motor below synchronous speed.
  • Once the synchronous motor’s stator windings are energized initially by a three phase supply, its damper windings will produce their own magnetic field in response to the E.F.M.S. induced within them. The machine will accelerate up to a given speed below synchronous speed in a similar manner to any standard induction motor, at which point direct current is fed to the rotor and magnetic locking takes place, finally bringing the motor up to full synchronous speed.

 Constructional Features-

  • Three Phase synchronous motors do not differ a whole lot from their induction motor counterparts in terms of construction. There are however a few crucial differences- mainly centered around rotor components – which will be identified in the breakdown following this ator may be mounted directly on the driveshaft as a means of producing direct current for the rotor. Observe the following photo of such a setup.

  Commutator & Brushes-

  • Typically used where an external source of direct current is supplied to the rotor, perhaps from an A.C. – D.C. motor/ generator set or a simple rectified mains power source.

Cooling Fan-

  • Attached to the non – drive end of the driveshaft, the cooling fan pulls in air which passes over the motor’s vital components, helping to prevent overheating.

End Shields-

  • Fixed to each side of the motor’s frame, the end shields provide additional structural rigidity and help further prevent mechanical damage.


  • Mounted inside each end shield and encompassing opposite ends of the driveshaft, bearings reduce mechanical friction and provide a means of smoother running for the motor.  So as stated, not massively dissimilar to standard induction motors besides a few additional components and a modified rotor assembly. Moving on – let’s now take a look at some advantages and limitations.

Advantages and Limitations of synchronous Motors:

  • There are of course some pros and cons to consider when selecting a type of three phase motor for a particular task. While induction motors may, in my experience at least, be more widely used than synchronous motors for light to medium duty applications, the latter most certainly have their place and in some cases, might be the only option.

  The following table will serve to outline the advantages and limitations of each of these types of motor:

            Three phase Induction Motor                                                           Three phase synchronous Motor
Generally is financially less expensive than the                              synchronous motor.                                                                          Can be set to run with neutral of leading power Factor so is useful as a means of P.F. correction.          
Is simpler in overall construction than the                                       synchronous motor and may feature fewer                                          individual components.                                                                     Operates at synchronous speed regardless of load, making the synchronous motor perfect for constant Speed drives and timing equipment.  
Inherently self- starting whereas the                                                        Synchronous motor is not.                                                                Generally more efficient than the induction motor, especially in heavier duty setups.  

 So, it’s easy to see that synchronous motors have a wider breadth of capabilities capital cost, more complex make up and need for additional starting equipment mean that it is in fact the latter which finds greater overall use.           

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