Reversible Drives using Choppers:

Using two quadrant choppers a reversible drive can be achieved, because this combination allows reversal of both current and voltage of the motor termi­nals. We can get a regenerative reversible drive. The motor can operate in all four quadrants. A scheme of a Reversible Drives using Choppers is shown in Fig. 4.85(a). The modes of operation are illustrated in Fig. 4.85(b). The operation in both the directions is achieved by operating the choppers 1 to 4 to provide voltage and current of desired polarity.

Control of choppers S₁ S₂ with their diodes D₁D₂ provides operation in I and IV Quadrants with positive current and reversible by varying T_ON/T_OFF ratio. S₃S₄ with diodes D₃D₄ comprise another chopper providing operation in II and III quadrants with negative current and reversible V_a. Thus the load voltage and current can take up both possible directions. The circuit has similar behaviour of a dual converter made up of two antiparallel connected fully controlled line commutated converters.

The choppers can also be controlled as regenerative choppers (step up choppers). To achieve this S₂ is always ON and S₃ OFF. The choppers S₁ and S₄ are controlled to provide positive V_a and reversible load current. This corresponds to operation in I and II quadrants. The operation in III and IV quadrants can be accomplished by making S₄ always ON, and S₁ OFF with control on S₁ and S₂ to make V_a negative and I_a reversible. The modes of operation are shown in Fig. 4.85(c).

Four quadrant operation can also be obtained with a two quadrant chopper supplying the armature and using a scheme for field current or armature cur­rent reversal. This scheme involves fewer power components. The response is very sluggish, as has already been discussed.

Closed loop control systems are being developed to obtain a precise control of speed with regeneration in both directions for thyristor drives.

A comparison of a reversible drive using a dual converter and a four quad­rant chopper is rather interesting. A four quadrant chopper is costlier than a dual converter. This is because of commutation circuits required by each chopper. A dual converter has a simpler control, if circulating current is al­lowed. A detailed investigation may be required to compare the performance of the drives under dynamic and steady state conditions.