Three-phase cycloconverters

A three-phase cycloconverter converts a fixed-frequency three-phase supply directly into a three-phase output of lower, variable frequency and variable voltage, without a DC link. Each output phase is a dual (positive and negative) phase-controlled converter.

Types

Circuit (3-phase to 3-phase, 3-pulse, one phase shown)

 Supply a b c
  |  |  |      Phase A of output
  +--+--+--> P-group (3 SCR) --+
  |  |  |                      |--> A (load)
  +--+--+--> N-group (3 SCR) --+
 Same for output phases B and C
 (output phases share a neutral)

Operation

1. Each output phase has a positive group (P) that carries positive load current and a negative group (N) that carries negative load current.
2. The firing angle of the conducting group is modulated during the output cycle. With cosine-wave crossing control:

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

so the mean output voltage of that phase is

$$v_{A}(t) = V_{do}\, r\sin\omega_o t, \qquad V_{do} = \frac{3\sqrt{3}V_m}{2\pi}\ \text{(3-pulse)}$$

3. Output phases B and C use the same law with $\omega_o t$ shifted by $-120°$ and $-240°$, giving a balanced three-phase output.
4. $r$ (0 to 1) sets the output amplitude; the rate of modulation sets $f_o$.
5. With R-L or motor loads, the current lags the voltage, so each group works as a rectifier for part of its conduction and as an inverter for the rest; the cycloconverter is naturally four-quadrant and regenerative.

Waveform (one output phase)

 vA (pieces of supply phase voltages)
        _/\/\_
      /\      /\
 ----/----------\-----------/--
                 \/\_  _/\/
                     \/
   |<--P group-->|<--N group-->|

Features

• Output frequency limited to about 1/3 (3-pulse) to 1/2 of input frequency for acceptable distortion.
• Natural (line) commutation, high efficiency, power flow in both directions.
• Poor input power factor and complex harmonic content; many thyristors.
• Applications: large low-speed AC drives (cement and ball mills, mine winders, ship propulsion), variable-speed constant-frequency (VSCF) aircraft supplies.