Single-Phase Synchronous Motors

The Single-phase is a noticeable-point with induction motor with it and is typically known as reluctance motors. In the rotor of any equally dispersed in a single-phase, the introduction of the motor is changed so that the plastic covering do not interrupt the production of noticeable poles of the rotors which are represented in the Fig. 24. Here the disinclination of the air-gap flux way will gradually be larger upto that extent where there is absence of conductors implanted in holes. Therefore a motor like this which is about to enhance the speed of an induction motor is usually dragged into synchronism with the energetic A.C. which is a single-phase field and is developed by the reluctance torque. Thus the noticeable iron shafts consist of lower reluctance air openings.

Working with a reluctance motor:

To gain a grip over the working of a motor like this is the only requirement. However, it has to be noted that when a magnet is located within a magnetic field, then a force starts acting on the material whose mechanism is to pull it to the thickest portion of the field.

Then this force gets lined up the sample materials in a manner that the declination of the magnetic course that goes over the things will make it the least value.

When the source is given to the stator coiling, the rotating field of magnet will utilize the torque on the basis of the asymmetrical rotor that seems to line up with the prominent pole alignment with the rotor. Provided the axis of the magnetic field is revolving.

On the other hand if the reluctance torque is enough to flinch the engine and the load it has.The rotor will attract it into step along with the rotating field and finally linger to course at the speediness of itself.

The Reluctance motors are one-third as shown in the Fig. 24. The Reluctance-motor has a horsepower rating that consists of induction motors which have tube-shaped rotors. Even though the ratio of height may be improved to one-half by improving the design of the field, the power and effectiveness become inferior for equal motor. These Reluctance motors are amatter of ‘cogging,’meanwhile, the locked-rotor torque differs from the rotor position, but the consequence can be lessened by twisting the rotor and without entertaining the number of rotor slots with an equal or multiple numbers of poles.


In spite of its inadequacies, the reluctance motor is used extendedly in numerous applications like recording instruments, time expedients, regulator gadget, regulators, and disc spinner gramophones.

Reversing the motor can be gained as in any single phase induction motor.

Speed-torque characteristics:

The Fig. 25 represents the speed-torque which has the features of a typical single-phase reluctance motor.

  • The motor if kick started at anywhere a point within 300 to 400 percent of its torque when fully loaded then, the duo-phase motor as a consequence of the rotating magnetic field is thus created by the activities of the winding which is actually displaced at 900 in both space and time.
  • The extreme speed that head starts the winding with the initiation of a centrifugal switch, and with this,the motor improves from a single-phase torque formed by its consecutively winding.

Now with the gaining of the speed, the reluctance torque or one can call it as the synchronous motor, it becomes adequate to attract the rotor into a function with the excited single-phase field.

  • The motor is thus operated at an uniform speed upto 200% or its jam-packed- torque. Now if the motor is loaded out side the value of pull-out torque, then the enduring functioning as a single-stage motor up to 500% of its output.

6.2. Hysteresis motor

Hysteresis motors are Single-stage cylindrical or synchronous-induction motors. A hysteresis motor does not contain a noticeable-pole rotor nor direct excitation. In spite of the fact it revolves at a synchronous speed and is associated with hysteresis torque.

The Hysteresis-type lamination is diagrammatically represented in Fig. 26 and are usually composed of toughened, high retentivity steel and not those commercial low-quality steels with minimum retentivity of the dynamo steel.


The resultant magnetic field obtained by the rotating magnetic field which is thus achieved by splitting up of the phase or a shaded-pole stator, by the eddy currents and are injected in the rotor made of steel which is parallel to the bar tracks of the rotor as represented in Fig. 26.The rotor magnetic field is prepared by eddy current and ultimately results in the rotor to rotate. This gives rise to a high starting torque and is produced as a consequence of the high resistance. Steel that has a high retentivity results in a significant hysteresis damage. Whereas a considerable quantity of energy is used up from the rotating field while changing the direction of the in the rotor The rotor lines up with the synchronous speed, the frequency of the current setback in the cross-bars diminishes abruptly. Thus the rotor becomes magnetized in single direction accompanying with high retentivity of the rotor made off steel. Therefore, the motor as a result of everything keeps rotating at a synchronous speed.

  • This type of motor is extensively used for the rotation of gyroscope rotors which is applied in the inertial navigation and control techniques. One main component of the instrument accuracy is that it contains the gyroscope which is entirely constant. The best achievement of this motor is that it can gain almost an absolute accuracy which is very tough to find. This steadiness necessitates a synchronous motor that is compelled by a controlled constant frequency source.

6.3. Sub-synchronous motor

Asub-synchronous motor is that one which is cylindrical in shape and with a similar outline and toothed at the inner surface analogous to a pole salient pole rotor. A characteristic rotor has 16 teeth or poles, and then in combination with a 16-pole stator, it will generally rotate at a constant speed at 450 r.p.m. when functioned on 60 Hz.but if this motor is temporarily burdened then it will move out of synchronism. This will result in the lessening of the speed toward the ultimate torque point, and this will make lock the motor into synchronism at a sub-multiple speed of 225 r.p.m. these are the reasons it is called a sub-synchronous motor.

This sub-synchronous motor gets started and picks up speed with a particular hysteresis torque similar to the hysteresis synchronous motor. However, the induction motor torque is not equal to the reluctance motors.

Therefore this motors of any size can be developed with a higher initiating torque but a smaller synchronous speed torque than a reluctance motor. All of this is represented in the Fig. 27 aa parts of the sub-synchronous rotor.


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