AC Servo Motors:Principle of Operation

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.

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 motors. The static variable frequency AC drive uses a cage-rotor induction motor or synchronous reluctance motor powered by a static frequency converter. This gives a versatile and robust variable speed machine which has an advantage over conventional variable speed drives, of better reliability and reduced maintenance. The main objection to the static AC drive has been on economic grounds and its lower starting torque compared with DC servo motors. However, power semi-conductor prices are steadily decreasing as production volume grows and manufacturing techniques improve. In a static frequency converter, the power supply is first rectified using diodes and then thyristors or transistors are used to convert the DC voltage to a variable frequency output which is then connected to the motor. The most appropriate method of changing the speed of AC motors is to change the frequency of the power supply. At low frequency, the amplitude of the voltage must also be reduced; otherwise, large current will flow in the stator windings which is not desirable.

The speed of rotation of the motor is given by