Jones Chopper circuit Description

The basic circuit of the Jones chopper is shown in the figure A. The Class D commutation circuit is used in the Jones chopper.

Power circuit diagram of Jones chopper

The SCR T1 is main SCR whereas SCR T2, diode D1, capacitor C and auto – transformer T makes commutating circuit for SCR T1. The inductor L1 and L2 are closely coupled and they provide sufficient energy to capacitor to turn off main SCR T1.

Working of Jones chopper

The capacitor initially charges with upper plate positive and lower plate negative.

SCR T1 turned on

When the SCR T1 is turned on, the load current flows through path (+)V_dc– T1 – L2 – MOTOR ARMATURE – V_dc (–). The capacitor discharges through path (+)C – T1 – L1 – D1 – C(–). Once capacitor discharges, it will again charges with reverse polarity with upper plate negative and lower plate positive. The stored energy of load flows through freewheeling diode when SCR T1 is turned off. The SCR T1 gets reverse voltage and turned off.

SCR T1 turned off

when SCR T2 is turned on. The load current flows through path (+)V_dc– C – T2 – L2 – MOTOR ARMATURE – V_dc (–). As the load current flows, the capacitor again charges with upper plate positive and lower plate negative. When the voltage across capacitor is equal to supply voltage, the charging current of capacitor is less than the holding current therefore the SCR T2 is turned off.

SCR Turned on cycle repeat

When SCR T1 is turned on once again cycle will repeat. The speed of the motor in the range of maximum to minimum can be achieved by switching of SCR T1 frequently or by adjusting its duty cycle.

Waveform of Jones chopper

The waveform of gate pulse IG1, IG2, voltage across SCR T1, voltage across SCR T2, voltage across capacitor and voltage across load is given here. When SCR T1 and SCR T2 turns on, voltage across it closed to 1.0 to 1.5 voltage. The voltage across load is available only when SCR T1 is turned on.