Now, the speed of the motor depends on the frequency of the supplied current. Speed of the synchronous motor is controlled by the frequency of the applied current. The speed of a synchronous motor can be calculated as. If the load greater than breakdown load is applied, the motor gets desynchronized. The 3 phase stator winding gives the advantage of determining the direction of rotation. In case of single-phase winding, it is not possible to derive the direction of rotation and the motor can start in either of the direction.
To control the direction of rotation in these synchronous motors, starting arrangements are needed. The moment of inertia of rotor stops the large-sized synchronous motors from self-starting. So some additional mechanism is required to help the rotor get synchronized. Induction winding is included in the large motors which generate sufficient torque required for acceleration. For very large motors, to accelerate the unloaded machine, pony motor is used.
Changing stator current frequency, electronically operated motors can accelerate even from the zero speed. For very small motors, when the moment of Inertia of the rotor and the mechanical load are desirably small, they can start without any starting methods. These motors must be controlled with a variable frequency drive VFD , as the only way to alter their speed and torque is by changing the stator AC frequency.
More information can be found in our article on permanent magnet motors. The only major current-excited synchronous motor available is the direct current-excited synchronous motor, which requires a DC input as well as an AC input. The DC supply goes to the rotor, which contains windings similar to the stator, and these windings will produce a constant magnetic field induced by the DC power supply. This will excite the motor and cause its poles to align with the stator RMF, causing synchronicity.
The different synchronous motors discussed are simply different means to create synchronous speed, and they can generally be used for applications where precise speed is desired. They are not inherently self-starting, and should not be chosen if self-starting is a requirement. Synchronous motors are the preferred choice for low speed, high power loads, and excel as power sources for crushers, mills, and grinders. Their speed remains constant regardless of the load, and their speeds can only be changed via a VFD as the input current directly correlates to output speed.
If adjustable speeds are desired, consider reading about wound rotor motors. Induction motors of the same output and voltage rating are generally less expensive than a synchronous motor of the same specifications.
This means that induction motors are the preferred choice for driving machines most of the time. Synchronous motors have the ability to correct power grid distribution losses and are highly useful in regulating voltage. Synchronous motors are most often found in large generators, or in parallel with induction motors meant to correct power losses.
The magnetic field develops on the rotor because of the DC supply. The polarity of the DC supply becomes fixed, and thus the stationary magnetic field develops on the rotor.
The term stationary means their north and south pole remains fixed. The speed at which the rotating magnetic field rotates is known as the synchronous speed.
The synchronous speed of the motor depends on the frequency of the supply and the number of poles of the motor. When the opposite pole of the stator and rotor face each other, the force of attraction occurs between them. The attraction force develops the torque in the anti-clockwise direction. The torque is the kind of force that moves the object in rotation. Thus, the poles of the rotor dragged towards the poles of the stator. The 3 phase AC supply produces rotating magnetic field in stator.
The rotor winding is fed with DC supply which magnetizes the rotor. Consider a two pole synchronous machine as shown in figure below.
Now, the stator poles are revolving with synchronous speed lets say clockwise. If the rotor position is such that, N pole of the rotor is near the N pole of the stator as shown in first schematic of above figure , then the poles of the stator and rotor will repel each other, and the torque produced will be anticlockwise.
The stator poles are rotating with synchronous speed, and they rotate around very fast and interchange their position. But at this very soon, rotor can not rotate with the same angle due to inertia , and the next position will be likely the second schematic in above figure.
In this case, poles of the stator will attract the poles of rotor, and the torque produced will be clockwise.
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