12/4/2010 DC – DC CONVERTER (DC – CHOPPER) ASNIL ELEKTRO FT UNP 1 12/4/2010 BLOCK DIAGRAM OF DC‐DC CONVERTERS 2 12/4/2010 A chopper is a static device which is used to obtain a variable dc voltage from a constant dc voltage source A variable dc voltage from a constant dc voltage source. A chopper is also known as dc‐to‐dc converter. The thyristor converter offers greater efficiency, faster response, lower maintenance, smaller size and smooth control Choppers are widely used in trolley cars control. Choppers are widely used in trolley cars, battery operated vehicles, traction motor control, control of large number of dc motors, etc….. They are also used in regenerative braking of dc motors to return energy back to supply and also as dc voltage regulators energy back to supply and also as dc voltage regulators. 3 12/4/2010 Choppers are of two types: Step‐down choppers Step‐up choppers In step‐down choppers, the output voltage will be less than the input voltage whereas in step‐up choppers output voltage will be more than the input voltage. PRINCIPLE OF STEP‐DOWN PRINCIPLE OF STEP DOWN CHOPPER CHOPPER Chopper i0 V + V0 R − Fig. 2.1: Step‐down Chopper with Resistive Load 4 12/4/2010 Figure 2.1 shows a step‐down chopper with resistive load. The thyristor in the circuit acts as a switch. When thyristor is ON, supply voltage appears across the load and when thyristor is OFF, the voltage across the load will be zero. The output voltage and current waveforms are as shown in figure 2.2. v0 V Vdc t tON tOFF i0 V/R Idc d t T Fig. 2.2: Step-down choppers — output voltage and current waveforms 5 12/4/2010 6 12/4/2010 7 12/4/2010 8 12/4/2010 9 12/4/2010 10 12/4/2010 11 12/4/2010 STEP‐DOWN CHOPPER WITH R‐L LOAD 12 12/4/2010 13 12/4/2010 14 12/4/2010 15 12/4/2010 16 12/4/2010 17 12/4/2010 18 12/4/2010 19 12/4/2010 20 12/4/2010 21 12/4/2010 22 12/4/2010 23 12/4/2010 24 12/4/2010 25 12/4/2010 26 12/4/2010 CLASSIFICATION OF CHOPPERS Choppers are classified as follows ‐ Class A Chopper ‐ Class B Chopper pp ‐ Class C Chopper ‐ Class D Chopper ‐ Class E Chopper CLASS A CHOPPER 27 12/4/2010 28 12/4/2010 29 12/4/2010 CLASS B CHOPPER Fig. 2.16: Class B Chopper 30 12/4/2010 31 12/4/2010 32 12/4/2010 33 12/4/2010 34 12/4/2010 CLASS C CHOPPER 35 12/4/2010 36 12/4/2010 37 12/4/2010 CLASS D CHOPPER 38 12/4/2010 39 12/4/2010 40 12/4/2010 CLASS E CHOPPER 41 12/4/2010 42 12/4/2010 Exercise A Chopper circuit is operating on TRC at a frequency of 2 kHz on a 460 V supply. If the load voltage is 350 volts, calculate the conduction period of the thyristor in each cycle. cycle 43 12/4/2010 44