I was wondering why smoothing capacitors are not used in smoothing the output of fully controlled bridge rectifiers, like it is used in the uncontrolled bridge rectifiers.
\$\begingroup\$
\$\endgroup\$
2
-
1\$\begingroup\$ I guess they are mainly used with motors which have lots of inductance and act as their own smoothing inductors? \$\endgroup\$– Stack Exchange Supports IsraelCommented Apr 19, 2022 at 18:00
-
\$\begingroup\$ I don't see a problem with a choke input filter beyond cost. \$\endgroup\$– greybeardCommented Jan 4, 2023 at 11:01
Add a comment
|
1 Answer
\$\begingroup\$
\$\endgroup\$
Capacitive load to SCR rectifier is begging troubles. Thyristors and triacs cannot stand too quickly growing current. See, they have max di/dt spec. Too fast growing current kills them. So, only those applications work which have an inductor between the SCR and the possible smoothing capacitor.
I have seen such 3-phase rectifiers. The triggering angle of the thyristors was controlled by the output DC voltage stabilization circuit.
To be more exact, the fatality of the fast growing inrush peak to the capacitor through a triggered SCR is not the only reason to leave the capacitor off or to use an LC filter instead of plain capacitor. If one calculates how much energy is wasted in losses during the current peak, he finds that the amount is substantial and it can cause intolerable amount of heat in high power systems - in those which pay off to be designed with SCRs. Charging the cap through an inductor like in buck switching regulators is much more efficient.
Finally, as already said by others in a comment, motors can work well with less smoothed voltage. And some high power industrial processes (welding, arc furnance, electroplating) need sustaining DC current. A series inductor can stabilize it well enough.