Thyristor Commutation in DC Choppers - A Comparative Study

The typical dc chopper motor controller consists of two thyristors, two or three diode rectifiers, a commutating capacitor, and one or more commutating inductances. The major art in circuit design is selecting the most suitable configuration for the given application, particularly the location of the inductances. It is desirable to minimize the inductance in certain loops and to carefully choose the inductance in other loops. A computer-aided analysis and optimization technique has been applied to several attractive chopper circuits. It appears that inductance in series with the auxiliary thyristor favors turn-off of the main thyristor at the expense of the auxiliary thyristor and vice versa. The best arrangement has most of the commutating inductance in a path that is common to both thyristors. Where one of the thyristors has a diode connected directly across it, there is generally an optimum value of inductance which yields maximum turn-off time. If no such diode is used, the maximum turn-off time is obtained when the inductance is zero.