新型电子元器件第11章.pptVIP

Chapter 11. The Oscillator and 555 timer 11.1 oscillator 1. introduction Oscillators are devices used to change direct current into alternating current. Their power rating range from a few milliwatts to megawatts their frequency from a few cycles to several billion cycles per second. Oscillators are used in hundreds of applications from providing the small signal pulses for operating computer logic circuits to the heating of metal by magnetic. 2. Constructing the oscillator There are several ways an oscillator can be constructed. The design is determined by several factors: the amount of power it must produce its operating frequency how stable the frequency must be. Industry is making use of more and more oscillators on the production line, so the maintenance electrician should become familiar with their basic principles of operation. 3. Applications of the oscillator AC induction motor is one of the most common applications of oscillators in industry. 1) The speed control of Two things determine the speed of an AC induction motor. the number of stator poles the frequency. By changing the frequency applied to the motor, the speed of the rotating magnetic field (the synchronous speed) can be changed also. For a four-pole induction motor is operated at 60Hz, the synchronous speed is 1800 revolutions per minute (RPM). If the frequency is reduced to 30Hz, the synchronous speed will be 900 rpm. 2) As the frequency is reduced in value, the inductive reactance of the motor winding is reduced also. The problem is solved by reducing the applied voltage as the frequency is reduced to prevent the current from becoming excessive in the stator winding and damaging the motor. 4. Example 1: Most variable frequency drives operate by first changing the AC voltage into DC and then changing it back to AC at the desired frequency. There are several methods used to change the DC voltage back into AC. In the circui