Type D chopper, Motoring mode of Type D chopper, regenerative braking mode of Type D chopper

Type D chopper

• Another two quadrant configuration is type D chopper as shown in Figure (a) In this configuration the load is connected differentially between type A and type B chopper.
  

  Figure (a)

  Figure (b)

• This chopper can operate in the first or fourth quadrant as shown in Figure (b).
• It can operate either in the motoring mode (first quadrant) or regenerative braking mode (fourth quadrant).
• The average load current always remains positive but the average load voltage can be either positive or negative.

First quadrant operation (Motoring mode) of Type D chopper

• When type D chopper is to be operated in the first quadrant, Table (i) shows the sequence of operation and load voltage and current waveforms are shown in Figure (c).
  

  Figure (c)

Fourth quadrant operation of type D chopper

• When type D chopper is to be operated in the fourth quadrant, Table (ii) shows the sequence of operation and load voltage and current waveforms are shown in Figure (d).
  

  Figure (d)

• When type D chopper is to be operated in the fourth quadrant the operation is as summarized in Table (ii).
  

  Table (i)

• Note that in order to facilitate the regeneration the polarity of back emf should be reversed. For this the field supply of separately excited motor is reversed.
• As seen from Table (ii), the instantaneous load voltage is always either negative or zero.
• Therefore average load voltage is negative however the load current always remains positive. Therefore chopper operates in the fourth quadrant.
  

  Table (ii)

• Notice the change in direction of E_b i.e. the back emf of the motor. This is achieved by reversing the field winding supply voltage. This is essential to get sustained regeneration (regenerative braking).

Note: In the modes Il and IV freewheeling takes place due to the self induced voltage across the motor inductance.

• It is important to understand that even though the type C and D are two quadrant chopper they cannot reverse the direction of rotation of the dc separately excited motor which is being used as load.