The applet shows a thyristor converter based on the anti-parallel connection of two three-phase half-bridges which allows four-quadrant operation of the converter system. The load voltage is controlled and can be positive or negative, and also the load current can be change direction. The two half-bridge rectifiers have output voltages ud1 and ud2, both with a voltage ripple three times the mains frequency (blue curves). One of the two converters has to operate as a rectifier (triggering angle a < 90 degrees), and the other one has to operate as an inverter (triggering angle a between 90 and 180 degrees). The voltage difference between ud1 and ud2 has no DC offset, and keeps a circular current going (green path animation). This allows a high dynamic and smooth current reversal at small output voltages. A main application for this topology have been DC drives in industry. Today, this circuit has been replaced in many applications by PWM inverters with AC drives. 

We animated the little car to demonstrate four-quadrant operation. The car can go upwards or downwards, introducing positive or negative load torque (proportional to the load current). Furthermore, the car can drive towards the left side or towards the right side, introducing positive or negative speed (proportional to the C voltage). All four quadrants can be reached with this load model.

• Change the triggering angles of the two combined half-bridges. Since the output voltages ud1 and ud2 must be equal on average, triggering angle a2 is defined by a1. Little errors in a2 are encountered by harmless distortions of the circular current ik.
• Change the two symmetric inductances that limit the circular current ik.
• Drag the red point of the car animation to change the torque of the DC motor.
• Drag the time slider from left to right. After passing the triggering point, the trigger pulse is animated. After triggering, the thyristor is in the on-state and the load current keeps flowing until it goes down to zero. Then the thyristor is in the off-state again.