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Lesson Eight: Synchronous motors

Synchronous motors

All previous chapters have been about Asynchronous motors, now this chapter will explain about how Synchronous motors works.
The big difference between the two kinds of motor types is the difference in the working principle in the rotor as all motors have the same stator functioning principle. In the asynchronous motor the rotor rotated a bit slower than the rotating magnetic field from the stator which is why it’s called an asynchronous motor. In the synchronous motors the rotor will rotate at the exact same speed as the rotating magnetic field from the stator.

1.1Permanent magnet synchronous motor

A synchronous permanent magnet motor have practically same working principle of the stator as in the 3-phase asynchronous induction motor but a different rotor working principle. Instead of using a squirrel cage winding, it uses permanent magnets in the rotor.
As the stator just like in the induction motor creates a magnetic field, the magnets wants to align themselves with the magnetic field. When the magnetic field from the stator rotates, the magnets will follow and make the rotor to spin with the same speed as the rotating magnetic field.
Having the rotor spin at the same speed as the stators magnetic field is called synchronous. Therefore it is called a synchronous motor.
When starting the motor the magnetic field rotates too fast for the permanent magnets to lock themselves with the rotating magnetic field. So in order to start the motor it would need a variable speed drive. With a slower rotating magnetic field, the magnets can easily lock themselves with the stators rotating magnetic field.
To know how the variable speed drive should operate, the motor uses a resolver that tells the position of the rotor angle. Now the variable speed drive can calculate and steer the rotating magnetic field according to the rotors position.
There is also a possibility to start a permanent magnet synchronous motor direct on line but then in additions to the magnets, it would also need to integrate a squirrel cage in the rotor. These types of motors are usually referred to as hybrid permanent magnet motors.
Now the motor will just work like a 3-phase asynchronous motor until it picks up enough speed for the permanent magnets to lock themselves with the magnetic field and the motor will start to spin at the same speed as the rotating magnetic field.

There are many different types of permanent magnet motors with different rotor designs. As seen from the chart here the magnets position could either be on the rotor core surface or inside the rotor core. Each design has its pros and cons and one type could not really be compared to the other. 

Figure 18. Different kinds of magnet positions in a rotor
One can also choose from several kinds of magnets to be used in the rotor when developing the motor. They can either be made from rare-earth magnet materials like Neodymium iron boron or Samarium cobalt or be of ferromagnetic type for example Alnico, an alloy of Aluminum, nickel and cobalt.

1.2  Synchronous reluctance motor

Same as with the permanent magnet motor, the reluctance motor have the same stator design as the induction motor but a different rotor design
The word reluctance means resistance for magnetic flux. An object that has high reluctance won’t conduct a magnetic flux whereas an object that has low reluctance will conduct a magnetic flux.
Now this is used in Synchronous reluctance motors to create magnetic flux barriers and to create magnetic flux paths. As the rotor consists of air gaps with high reluctance and steel arcs with low reluctance. The magnetic flux created by the stator wants to follow the path of least reluctance so it goes through the steel barriers. Now as the magnetic field rotates, the steel barriers wants to keep the magnetic flux path intact and forces the rotor to rotate with the rotating magnetic field from the stator.
There are other kinds of reluctance motors called Switched reluctance motor, variable reluctance motor and variable reluctance stepping motor
But we will only cover the synchronous reluctance motor
When starting the motor the magnetic field rotates too fast for the rotor to lock them with the rotating magnetic field. So in order to start the motor it would need a variable speed drive. With a slower rotating magnetic field, the rotor can easily lock themselves with the stators rotating magnetic field.
There are many different designs of the rotor as can be seen in the picture below but the transverse laminated rotor shape is the most common used. There are also hybrid motors that make use of permanent magnets in the rotor to boost the effect of the motor. Those motors are called Permanent magnets assisted synchronously reluctance motors (PMaSynRM)
There is also a possibility to start a Synchronous reluctance motor direct on line but then it would need to integrate a squirrel cage with the rotor. Now the motor will just work like a 3-phase asynchronous motor until it picks up enough speed for the rotor paths to lock themselves with the magnetic field and the motor will start to spin at the same speed as the rotating magnetic field.


Figure 19. Different rotor designs for reluctance motors. (a) is for high speed design, (b) complicated to manufacture. (c) is electrically superior.
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