Jones chopper employs class D commutation to turn off the main thyristor. Here, a charged capacitor is switched by the auxiliary thyristor in order to turn off the main thyristor. In Jones’s chopper, the saturable reactor core of Morgan’s chopper is replaced with a tapped autotransformer. The autotransformer ensures reliable commutation and hence eliminates the problem of commutation failure. Let us see the circuit and operation of Jones chopper.

Circuit Diagram of Jones Chopper :

Jones chopper circuit consists of two thyristors T₁ and T_A in which T₁ is the main thyristor and T_A is the auxiliary thyristor. The commutation circuit is formed by T_A, capacitor C, diode D, and autotransformer L₁ and L₂. The coils L₁ and L₂ are tightly coupled, which provides the necessary energy to the capacitor in order to commutate the main thyristor and D_f is the freewheeling diode.

Initially, i.e., at instant t = t₀, the capacitor is assumed to be fully charged to the supply voltage with upper plate positive and lower plate negative as seen in the above circuit diagram.

Operation of Jones Chopper :

The below shows the waveforms of the Jones chopper and the operation of the circuit can be explained in two modes.

Mode – I :

In this mode, the main thyristors T₁ is triggered at an instant t₁ by providing the gate signal. Once T₁ starts conducting, the load current flows through V_dc⁺ → T₁ → L₁ → motor → V_dc^–, and at the same time charged capacitor discharges current I_C through the path C⁺ → T₁ → L₂ → D → C^– as shown below.

Due to the flow of capacitor current, it starts charging in the reverse direction with lower plate positive and upper plate negative. Due to the presence of a diode D, the capacitor retains the charges till the auxiliary thyristor is triggered.

Hence from instants t₂ to t₃, the capacitor voltage remains constant as shown in the waveform and the main SCR continues to conduct. This mode ends at t₃, and the circuit parameters during this mode are,

Mode – II :

This mode starts at instant t₃ when the auxiliary thyristor T_A is triggered. Now the load current flows through the path V_dc⁺ → C → T_A → L₁ → motor → V_dc^–, and the capacitor starts discharging the current through the path C → T_A → T₁ → C as shown below.

Due to the discharge of the capacitor, a reverse voltage appears across T₁ from t₃ to t₄ and hence the T₁ gets turned OFF. Now, the capacitor again charges with upper plate positive and lower plate negative due to which a reverse voltage appears across T_A at t₅, and it gets turned OFF.

Since the load connected to the circuit is of inductive type, it stores the energy. When both the thyristors are in an OFF state, the inductive load releases its stored energy. To carry this released energy, a freewheeling diode D_f is connected across the load. Now the load current flows through the path Motor → D_f → Motor. The cycle repeats when the main thyristor T₁ is again triggered. The various circuit parameters in this mode are given as,

Design of Jones Chopper :

The selection of capacitor and autotransformer is very important while designing a Jones chopper circuit. During the turn-OFF time of thyristor T₁, the energy stored in inductor L₁ releases and gets transferred to capacitor C. Thus,

The capacitor voltage will become zero from V_C during the turn-OFF time of T₁. The turn-OFF time of T₁ is,

Now dividing equation 1 with V_dc, we get,

The voltage across the thyristor T₁ and T₂ is,

V_C = g V_dc

Hence, the voltage rating of the thyristor can be increased by increasing the value of ‘g’.

Advantages of Jones Chopper :

• In the Jones chopper circuit, there is no commutation failure as the commutating capacitor always gets sufficient energy to turn OFF thyristor T₁. This is because the upper and lower inductor of the autotransformer is tightly coupled.
• We can vary thyristor switching ON and OFF time individually due to which the flexibility in control is high.
• It is more efficient as the power dissipation is only due to the winding resistance of the autotransformer, the internal resistance of the diode, and thyristors which will be very less.
• The filtering requirements are reduced because of the presence of autotransformer.
• The motor torque which is proportional to the load current is smooth and ripple-free.

Disadvantages of Jones Chopper :

• The circuit requires thyristors of higher PIV.
• The weight and size of the autotransformer are more.
• The range of operating frequencies is limited due to the autotransformer.