Single-phase step-down cycloconverter

A single-phase step-down cycloconverter is a direct AC-to-AC converter that gives an output frequency $f_o = f_i/n$ (n = 2, 3, 4 ...) from a fixed-frequency supply, without an intermediate DC link. It uses naturally (line) commutated thyristors.

Circuit (mid-point / centre-tapped type)

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

• Positive group: P1, P2 — load current from K to O (positive output).
• Negative group: N1, N2 — load current from O to K (negative output).
• Let $v_{aO}$ be positive in the positive half cycle of the supply.

Operation for $f_o = f_i/4$ (e.g. 50 Hz to 12.5 Hz), R load, $\alpha = 0$

Positive output half cycle (4 input half cycles):

• Supply +ve: P1 conducts, load gets $+v_{aO}$.
• Supply −ve: P2 conducts, load gets $+v_{bO}$ (b is now positive).
• This repeats, so 4 positive humps appear across the load.

Negative output half cycle (next 4 input half cycles):

• Supply +ve: N2 conducts (b is negative, current O to K): load voltage negative.
• Supply −ve: N1 conducts: load voltage negative.

Waveforms

 vs  /\  /\  /\  /\  /\  /\  /\  /\
    /  \/  \/  \/  \/  \/  \/  \/  \
 (alternate humps are negative)

 vo  /\/\/\/\
    /        \
 --/----------\----------/--
               \        /
                \/\/\/\/
    |<-- P grp ->|<-- N grp ->|
    P1 P2 P1 P2   N2 N1 N2 N1

All thyristors switch at natural current zeros (R load), so no forced commutation is needed. Output period = 4 × input period.

Conduction table ($f_o = f_i/4$)

Control of output voltage

Delaying each firing by $\alpha$ reduces the rms output (R load):

$$V_o = V_s\sqrt{\frac{1}{\pi}\left(\pi - \alpha + \frac{\sin 2\alpha}{2}\right)}$$

where $V_s$ is the rms voltage of each half-winding. For a closer approximation to a sine wave, $\alpha$ is made large at the ends and small in the middle of each output half cycle.

Features: only step-down frequency with natural commutation; output contains harmonics; used in low-speed large AC motor drives.