AC Servo Motors

Abstract

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 supply 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.