Single Quadrant Choppers Type A

Single Quadrant Choppers (Type A and Type B)

Q. Classify chopper based on output voltage and quadrants of operation. List any four applications of chopper.

Q. Classify choppers based on quadrants of operation. Sketch only circuit diagram of each.

Q. Draw schematic circuit diagram of Class-B chopper and necessary waveforms.

Q. Write comparisons of type A and type B choppers.

Type A chopper [Motoring chopper]

• Figure (a) shows type A chopper. This chopper operates with periodic opening and closing of SCR S₁ which operates as a power switch.
• The SCR S₁ conducts only in one direction, in the direction shown by the arrow. Similarly the freewheeling diode D₁ conducts in only one direction.

Operation of Type A chopper

Assume the load to be highly inductive. The operation of type A chopper is as follows:

Figure (a)

Figure (b)

• When S₁ is closed, the load voltage v_o = + V with a polarity as shown in Figure (b). The load current is also positive and the load will store the energy.
• When S₁ is open circuited, due to the stored energy in the load, the load voltage will reverse the polarities, which forward biases diode D₁ and freewheeling takes place. Due to the ideal characteristics of D₁ the load voltage during the freewheeling period is zero. Figure (c)
• The load voltage waveform is as shown in Figure (d). This shows that the load voltage will always be positive. Due to the presence of freewheeling diode the load voltage is not allowed to become negative.

  

  Figure (c)
  

  

  Figure (d)

• As the load current also remains positive, the type A chopper operates in the first quadrant of average voltage, average current characteristics as shown in Figure (e)

  

  Figure (e)

The power flow of Type A chopper

• The net power flow in this chopper is from the dc source V to the load.
• If the load is a dc machine, then the machine works as motor, as it always receives the electrical energy and converts it into mechanical energy.
• This type of chopper is therefore known as "Motoring Chopper". It is the same step down chopper that we discussed earlier in this chapter.