Page 107 - Servo Motors and Industrial Control Theory -

Chapter 6
AC Servo Motors

6.1 Principle of Operation

AC motors are the first choice for constant speed applications and where large
starting torque is not required. They are available in three or single phase. The
smaller motors are for household applications and they are made for single phase
operations. For industrial applications, AC motors are available from a fraction to
hundreds of horse power output. The principle of operation is that the rotor is made
of laminated steel and bars of conducting material such as aluminium or copper are
buried in the rotor which are short circuited at both ends. The stator is also made of
laminated steel with properly designed slots. In the slots, a well designed number
of windings is located which is connected to the power supply. The power sup-
ply generates a rotating magnetic field. When the motor is connected to the power
supply, a voltage is induced in the bars located in the rotor which causes a current
flow through them. As a result of the current, an electromotive torque is developed
which accelerates the rotor. As the speed increases, the induced voltage reduces
because the rotor approaches the synchronous speed. At synchronous speed, the
torque becomes zero. Therefore, AC motors always rotate at a speed lower than
the synchronous speed. The synchronous speed is determined by the frequency of
the power supply and number of poles in the stator. In this book, only three phase
squirrel cage type motors will be considered.

6.2 Variable Speed AC Motors

Thyristor/transistor drives have been used extensively in industry. For normal in-
dustrial applications, a DC motor powered by a thyristor converter is now a popular
choice as a variable-speed drive. The performance of thyristor controlled and pulse
width modulated DC servo motors drives were discussed in Chap. 4. This chapter
is concerned with solid-state variable speed drives using AC rather than DC mo-
tors. The static variable frequency AC drive uses a cage-rotor induction motor or
synchronous reluctance motor powered by a static frequency converter. This gives

R. Firoozian, Servo Motors and Industrial Control Theory, Mechanical Engineering Series, 101
DOI 10.1007/978-3-319-07275-3_6, © Springer International Publishing Switzerland 2014