power electronics

POWER ELECTRONICS

Instructor: Eng. Jalal Al Roumy

The Islamic University of GazaFaculty of EngineeringElectrical Engineering Department

الرحمن الله بسمالرحيم

EELE 5450 — Summer 2012

Lecture 16

Chapter six

A Free-Wheeling Diode DC Motor Drive Using the Jones Chopper

This circuit is frequently referred to as a chopper circuit.

Chapter six

Operation of the Jones Commutation circuit

The Jones circuit is an efficient commutating method to turn off the main SCR in the dc chopper circuit.

The circuit characterized by class D commutation (charged capacitor switched by SCR2 and T1) (Ton and Toff can be varied)

Chapter six

Operation of the Jones Commutation circuit

Initially SCR1 and SCR2 are not conducting and the side B of the capacitor C is charged to a negative voltage (from last SCR2”on”).

Commutating current through SCR1,L2, D2 and C charges up the capacitor to opposite polarity, B is positive

At time t1 SCR1 is triggered ,and the current passed through the load.

The capacitor retains its charge until SCR2 is triggered at t3

The discharged of capacitor C reverse biased SCR1 and turn it off.

C again charges up with A positive(energy in L1) and SCR2 turns off because its current falls below the holding value when C is recharged.

The autotransformer action of T1 induced voltage in L2 of correct polarity for charging the commutating capacitor to a voltage > VB

The bottom plate (B) of capacitor C reaches a peak value at t5.

Since Vc (at t5) is charged to a voltage greater than VB, D2 is again forward biased. C discharges to a value lower than VB .

The blocking voltages across SCR1 and SCR2 also changes (t5 to t6).The turn off time of SCR2 must be less than the time from t5 to t6.

The discharging of C stop at t7 when current through L2 falls to zero.

The cycle repeats itself when SCR1 is again triggered.

Chapter six

Design of the Jones commutation circuit

The basic design of the commutating circuit involves the proper choice of ‘C’ and autotransformer T1

Chapter six

Design of the Jones commutation circuit

The magnitude of the commutating capacitor is dependent on the following circuit parameters:

Maximum motor current to be commutated, Im. Turn-off time,t0, of SCR1. The battery voltage, VB.

End of Lecture