Single-phase cycloconverter and applications

A cycloconverter is a direct frequency changer that converts AC power at one frequency to AC power at another (usually lower) frequency in one stage, without an intermediate DC link. A single-phase cycloconverter does this for a single-phase supply and load, using two groups of thyristors: a positive group for positive load current and a negative group for negative load current.

Circuit (mid-point type)

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

• Positive group: P1, P2 (current from K to O through the load).
• Negative group: N1, N2 (current from O to K).

Operation as a step-down cycloconverter ($f_o = f_i/2$, R load)

1. Input half 1 (a positive): P1 is fired; load voltage $= +v_{aO}$.
2. Input half 2 (b positive): P2 is fired; load voltage $= +v_{bO}$, still positive. Two positive humps form the positive output half cycle.
3. Input half 3 (a positive): N2 is fired; current flows O → load → K → N2 → b; load voltage negative.
4. Input half 4 (a negative, b positive): N1 is fired; load voltage negative.

 vs  /\    /\
    /  \  /  \
 --/----\/----\/----\/---
       (alternate humps -ve)
 vo  /\  /\
    /  \/  \
 --/--------\--------/---
             \  /\  /
              \/  \/
    | P1 | P2 | N2 | N1 |
    |<--- To = 2 Ti --->|

So the output has half the input frequency. Using 3 or 4 humps per half cycle gives $f_i/3$ or $f_i/4$. The SCRs turn off at natural current zeros (line commutation).

Voltage control: firing each SCR at a delay $\alpha$ gives

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

Step-up operation ($f_o > f_i$) is also possible, by switching between P and N groups several times within each input half cycle, but then forced commutation is needed.

Inductive load: current lags, so a group continues to conduct after the voltage reverses (inversion mode); long gate pulses are used and a group is switched only after its current reaches zero.

Applications

• Speed control of large, low-speed AC motors: gearless cement and ball mills, rolling mills, mine hoists, ship propulsion.
• Variable-speed constant-frequency (VSCF) aircraft power: variable-frequency alternator output converted to fixed 400 Hz.
• Induction heating supplies (step-up type).
• Static VAR / slip power recovery schemes (Scherbius drives) of wound-rotor induction motors.
• Traction: conversion of 50 Hz to $16\frac{2}{3}$ Hz for railway supply.

Merits and limits