Currently AC motors are favorably used in many industrial applications.
Squirrel cage induction motors are particularly popular because of their simple
structure, low cost production and less maintenance. However, the limitation of the
induction motors is the working speed which is lower than the speed of rotating
magnetic field and the changing slip depends on load torque. That is, an increasing in
load torque results in the decreasing in working speed. Hence, the induction motors
are not suitable for applications which require an accurate control of speed and
position such as servo systems. On the other hand, speed of synchronous motors can
be accurately controlled by varying the frequency of the rotating magnetic field which
is called synchronous speed. However, the synchronous motors suffer from high
production and maintenance costs.