3-phase AC Induction Motor Drives
The AC induction motor is a workhorse with adjustable speed drive systems. The most popular type is the 3-phase, squirrel-cage AC induction motor. It is a maintenance-free, less noisy and efficient motor. The stator is supplied by a balanced 3-phase AC power source.
The synchronous speed ns of the motor is calculated by:
ns120fs×p------------------- rpm[]= (EQ 3-1.)
where fs is the synchronous stator frequency in Hz, and p is the number of stator poles. The load torque is produced by slip frequency. The motor speed is characterized by a slip sr:
srnsnr–()ns--------------------nslns------ -[]== (EQ 3-2.)
where nr is the rotor mechanical speed and nsl is the slip speed, both in rpm. Figure 3-1 illustrates the torque characteristics and corresponding slip. As can be seen from EQ 3-1 and EQ 3-2, the motor speed is controlled by variation of a stator frequency with the influence of the load torque.