How Jones Chopper Works?

Jones Chopper Working Principle Tutorial:

We have seen already about the basic of chopper circuits, various types of chopper circuits.... In this post let us discuss about the operation of Jones Chopper..
Remember that,

Jones Chopper is the example for Voltage Commutated Chopper.
Morgan Chopper is the example for the Current Commutated Chopper.
Before proceeding further, it is recommended to read about the Principle of Operation of DC-DC Chopper circuits...

Jones Chopper:

  • The Jones chopper circuit is shown below. Here T1 is main thyristor and T2 is auxiliary thyristor.
    JONES_CHOPPER_OPERATION
  • Jones chopper is the example for voltage commutation chopper.
  • The jones chopper circuit is frequently used for the speed control of DC motors.
  • Observe that the potion of the circuit enclosed in the dashed lines.
  • It is the commutating circuit for main SCR T1.
  • A charged capacitor (C) which is switched by an auxiliary SCR T2 and the autotransformer (Tr) provides commutation of T1.
  • In this chopper, for varying the output voltage either tON or tOFF of the main thyristor T1 can be varied.

Jones Chopper Operation:

Consider that T2 is already turned ON first to charge the commutating capacitor C to the polarity as shown in the figure.

  • When T1 has to be turned ON, the charged capacitor discharges around the path C(+) - T1 - L2 - D2 - C(-), and gets charged in opposite direction.
    Ie, plate B is positive and plate A is negative.JONES_CHOPPER_MODE1
  • But the diode D2 prevents further oscillation of the resonating L2C circuit.
  • So the capacitor (C) retains its charge until T2 is triggered.
  • When T2 is tuned ON, the capacitor is in parallel (connected across) with the T1.
  • As the capacitor is reverse-charged (plate B is positive) it will turn OFF the T1.JONES_CHOPPER_MODE2
  • Now the capacitor gets charged with plate A is positive.
  • Consequently T2 turned off because the current flow through it reduces below the holding current value.
  • The cycle repeats itself when T1 is again triggered.

If T1 is turned ON before the capacitor C is charged, then the load current in the autotransformer T1 will induce enough voltage of proper polarity in L2 to charge capacitor C negatively.
So it does not matter which SCR is turned ON first in this circuit.

Role of Freewheeling Diode

  • The diode D1 is the freewheeling diode.
  • This diode is to used carry the inductive current when T1 is turned OFF.
  • Thus the diode D1 prevents the load from the high voltages appearing across it.

Read More:

Power Electronics Interview Questions and Answers: Set-8  
Rectifier Types Comparison Tutorial 
How Buck Converter Works? 
Comparison of MOSFET with BJT 

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