Step-up and step-down single-phase cycloconverter

A single-phase cycloconverter converts single-phase AC at $f_i$ directly into single-phase AC at a different frequency $f_o$. It uses a positive group (P1, P2) that carries positive load current and a negative group (N1, N2) that carries negative load current. If $f_o < f_i$ it is a step-down cycloconverter; if $f_o > f_i$ it is a step-up cycloconverter.

Circuit (mid-point type)

          a o----+---P1->|---+
            |    +---|<-N1---+
  ~ vs    Tr|                |
  (input) O o----[ Load ]----+  (common point K)
            |                |
          b o----+---P2->|---+
                 +---|<-N2---+

• Positive supply half ($a$ +): P1 gives $+v_o$, N2 gives $-v_o$.
• Negative supply half ($b$ +): P2 gives $+v_o$, N1 gives $-v_o$.

Step-down cycloconverter ($f_o = f_i/2$)

The P group is kept on for two input half cycles (P1 then P2), then the N group for the next two (N2 then N1).

 vo  /\  /\
    /  \/  \
 --/--------\--------/---
             \  /\  /
              \/  \/
    P1   P2   N2   N1

• Current in each SCR falls to zero at the supply zero, so natural (line) commutation is enough.
• $f_o = f_i/n$ when each group conducts $n$ half cycles; rms output $V_o = V_s\sqrt{\frac{1}{\pi}\left(\pi - \alpha + \frac{\sin 2\alpha}{2}\right)}$.

Step-up cycloconverter ($f_o = 2f_i$ example)

Within each input half cycle, P and N groups are switched alternately, several times:

 vs   _____
     /     \
 ---/-------\---------/--
             \_______/
 vo   _       _
     / |     / |
 ---/--+--.-/--+--.------
       |  /    |  /
       |_/     |_/
     P1 N2   P2  N1

• An SCR must be turned off in the middle of a half cycle while the supply is still forward biasing it, so forced commutation circuits are needed.
• The output is made of slices of the supply wave; for $f_o = 4f_i$, each half cycle is split into four slices (P1, N2, P1, N2 ...).

Comparison