Three-phase induction motor: This has a statoi with three windings mounted 120o art, each winding connected to one of the three lines of the supply fig. 5.59. Since the three phase reach their maximum current at different times, it can be considered that the magnetic field rotate the stator poles complete one rotation in one full cycle of the current.
Fig. 5.59 : Three Phase
advantage of self-starting. The direction of rotation of the motor can be changed by changing the direction of rotation of magnetic field by interchanging any two of the line connections. The speed of revolving of the magnetic field produced by primary currents is called the synchronous speed of the motor, and is given by N= 120f/p where f is the supply frequency and p is the number of poles. This revolving sweeps across the rotor conductors and thereby induces an emf in these conductors.
Synchronous motors : Synchronous motors has a rotor of permanent magnet or can Demagnetized by supplying d.c supply seperately. The magnetic field of the stator due to a.c supply rotates, and so the magnets of the rotor fig. 5.60. The rotor has two poles and the stator has two poles per phase. The magnetic field rotates through 360O in one cycle of supply and the frequency of rotation will be equal to frequency of supply current. They are used when a precise speed is desired. They are also not of self starting type and a seperate system has to be used for starting them. This gives constant speed from no load to full load. Electromagnetic power varies linearly with the voltage. They operate at higher efficiencies, especially in the low speed but it may fall out of synchronous and stop when over loaded. Advantages of a.c motor over d.c motors
Fig. 5.60 : Three phase two poles synchronous motor
a. Cost is less
b. More rugged
c. Reliable
d. Maintenance free.
Variable speed a.c motor
The disadvantage in a.c motor is the speed control being more complex than d.c motors and hence speed controlled d.c motors are much cheaper than speed controlled a.c motors. Speed of a.c motor depends on the frequency of the a.c supply and one method of controlling the speed is by controlling the frequency of the a.c supply. But the torque developed by a.c motor remains constant when the ratio of stator voltage to frequency is constant. When the frequency is varied for controlling the speed, the torque developed will also vary. To overcome this problem a.c supply is first rectified to d.c using a converter and then using an inverter the d.c again converted back to a.c, this inversion being at selected frequency of a.c. The other method is to convert a.c into a.c at the desired fequency using a cycloconverter without converting a.c to dc. Fig 5.61 shows the basic concept of variable speed a.c motor using a converter and an inverter.