Permanent Magnet Motors
In a synchronous machine that is magnetized by permanent magnets composed of neodymium boron iron, or ferrite on the rotor, or samarium cobalt. They are placed on the surface of the rotor core in some motors because across the gap the magnetic field is radially directed. You can find its application in variable speed drives were from a variable frequency the stator is supplied, electronically controlled by source. It is designed for operating at a speed that is synchronous with a stable supply of voltage and frequency. There are no means of controlling the stator power factor over there.
With respect to the supply frequency, the speed is uniquely related and it is one of the rarest features of synchronous motors. There are lots of applications of for several special types of synchronous motors in devices like tape recorders, clocks, and photographs. The most frequently used motor is the hysteresis motor which is composed of a rotor with a ring of semi-permanent magnet material like a high-carbon steel. By pulling the rotor out of synchronism, the speed is reduced by the stator field which leads to the rotor material to be magnetized. Good starting torque is provided by these motors with low ripple and is very silent. It has low efficiency and only small power ratings applications are permitted.
Iron poles are caused to hold a magnetic flux for aligning with each other that forced by the Reluctance motors to work on this principle. There is no electrical windings and has four iron poles. There are six poles in the stator with a current carrying coil in each. Similar to an induction motor, the stator is made and has a controllable supply with three phase. Isolated by non-magnetic spacers, the rotors include longitudinal iron laminations. There are salient poles in the rotor without any field windings. You can also find a damper winding in the rotor surface that helps the machine to start the induction motor.
Single Phase Synchronous Motors
In synchronous motors, there is a chance of producing revolving field from a single phase source with the use of the same method and single-phase induction motors. To the main supply, the main stator winding is directly connected to an auxiliary winding and it can be connected through a capacitor. In combination with hysteresis type rotor, the shaded-pole construction of the stator is widely used for small clock motors. It has a very low efficiency which is less than 2 percent.
Direct Current Commutator Motors
Across the rotor by poles on the stator, a stationary magnetic field is produced. With the help of field coils, these poles may be encircled carrying direct current or may have permanent magnets. There is an iron core with a coil accommodated in slots of the rotor or armature. To the bars of the commutator switch kept on the rotor shaft the coil is also connected. They are highly used in paper mills, steel mills, robots, and machine tools where speed reversal or speed control is required.
Alternating Current Commutator Motors
From a single phase alternating voltage supply, a specially designed series commutator motor can be worked. During the time when the supply current reverses both the armature and the magnetic field are reversed. They have huge application in small domestic appliances like mixers, vacuum cleaners, and portable tools.
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