Three-phase to single-phase cycloconverter

A three-phase to single-phase cycloconverter produces a single-phase, variable (lower) frequency output from a three-phase supply, using two three-phase thyristor bridges connected in anti-parallel. One bridge (P) carries the positive load current and the other (N) carries the negative load current.

Circuit (bridge type, 12 thyristors)

      a  b  c                     a  b  c
      |  |  |                     |  |  |
   +----------+               +----------+
   | P-bridge |               | N-bridge |
   |  P1..P6  |               |  N1..N6  |
   +----------+               +----------+
    (+)    (-)                 (-)    (+)
     |      |                   |      |
     A      B                   A      B

        A o------[ Load ]------o B
   P-bridge drives io > 0 (A to B); N-bridge drives io < 0 (B to A)

• Each bridge is a three-phase fully controlled converter, with average output $V_{dc} = \dfrac{3\sqrt{2}V_L}{\pi}\cos\alpha$.
• The two bridges are connected in anti-parallel across the load (P+ and N− to terminal A, P− and N+ to terminal B), so they drive current in opposite directions through the load.

Working

1. Positive half of output: the P-bridge is fired. Its firing angle $\alpha_P$ is varied gradually: large near the start (low voltage), small in the middle (high voltage), and large again at the end. The mean output thus rises and falls like a positive half sine wave.
2. Negative half of output: the P-bridge is blocked and the N-bridge is fired in the same way, giving the negative half.
3. The time for which each bridge works sets the output frequency $f_o$; the depth of the $\alpha$ variation sets the output amplitude.
4. For a sinusoidal output, cosine-wave crossing control is used:

$$\alpha_P = \cos^{-1}(r\sin\omega_o t), \qquad \alpha_N = \pi - \alpha_P$$

where $r$ is the voltage ratio (0 to 1). Then the mean output is $V_o(t) = \dfrac{3\sqrt{2}V_L}{\pi}\,r\sin\omega_o t$.

Waveform

 vo (made of 6-pulse line segments)
      _/\_/\_
    /\       /\
 --/-----------\-------------/--
                 \/\_   _/\/
                     \/\/
   |<- P-bridge ->|<- N-bridge ->|
   alpha_P: 90..0..90  alpha_N: 90..0..90

The output is built from pieces of the six line voltages; its fundamental is at $f_o$.

Modes

• Circulating-current-free: only one bridge is fired at a time; a short dead time at current zero avoids a short circuit between the bridges.
• Circulating-current mode: both bridges are fired with $\alpha_P + \alpha_N = 180°$, and an inter-group reactor limits the circulating current; this gives a smoother output with no dead time.

Points

• The practical output frequency is limited to about one-third of the input frequency for acceptable harmonics.
• With inductive loads, each bridge may operate in rectifying or inverting mode depending on the signs of $v_o$ and $i_o$ (four-quadrant operation).
• Applications: low-speed, high-power drives (ball mills, ship propulsion, rolling mills).