Engineering Notes: Jones chopper

Jones Chopper :

Jones Chopper Circuit diagram

The circuit diagram of Jones chopper is as shown in the figure.
Let us assume that the given polarities in the diagram are positive.
T1 is the main thyristor and T2 is the auxiliary thyristor.
This circuit is an example of voltage commutation. The special feature of this circuit is the tapped auto transformer.
Load current flows through some portion of auto transformer winding. The two portions of the auto transformer winding are closely coupled.
Because of this auto transformer, commutating capacitor gets sufficient energy to turn off the main thyristor.
Load current is assumed constant to simplify the analysis of this chopper.

Jones Chopper waveform

Before turning on the main thyristor T1, load is flowing through free wheeling diode. In this period capacitor voltage is -Vs.
When main thyristor turn on load current shifts from free wheeling diode to the main SCR. In this period an oscillating circuit is formed with Lr and C.
This oscillating current flows through C - T1 - L2 - D1 - C. In this period capacitor gets charged in the reverse direction. By the end of this period Vc = +Vs.
When auxiliary thyristor T2 is turned on, the capacitor will appear across T1 as reverse voltage and T1 will get turn off . Now the load current flows through Vs - C - T2 - L1 - load - Vs.
When capacitor voltage becomes -Vs, free wheeling diode will get turn on and the load current shifted to free wheeling diode.
But the inductor L1 has some amount of stored energy. Inductor stored this energy when load current is flowing through it.
When load current shifts to free wheeling diode, the energy stored in inductor L1 is transferred to capacitor through L2 ( since it is an auto transformer).
Because of this capacitor voltage rises initially and settles down to -Vs.
For the next cycle same sequence of steps are repeated.