Page 248 - Power Electronics Handbook

238 Direct a.c. frequency converters
reactance effects are especially important when the cycloconverter is used
in variable-speed constant-frequency (VSCF) systems. These are frequent-
ly met with in aircraft, where the cycloconverter is fed from a local
alternator which has a high-frequency output. Since the alternator speed is
variable so also is its frequency, the cycloconverter now being used to
ensure that the load frequency is maintained at a fixed value. The source
reactance is that of the alternator, and since the cycloconverter represents
its largest load it would create considerable distortion, which would need
to be taken into account in any system design. If p is the overlap angle
caused by instantaneous output current Z through the source reactance X,
(Y is the firing delay angle and V the r.m.s. supply voltage, then equation
(10.3) can be obtained.

cos((Y + p) - cosa = &) 2 7 XI
(10.3)

The input current conditions to the cycloconverter can be defined by
three factors:
(9 The power factor P, which is the ratio of the system watts to the volt
amperes;
The distortion factor D, which is the ratio of the r.m.s. fundamental
input current to r.m.s. total input current;
(iii) The displacement factor L, which is the cosine of the angle between
the fundamental supply voltage and the fundamental component of
the input current.
The relationship between these three factors is given by equation (10.4).
P = DL (10.4)
Therefore for a sinusoidal input current D = 1 and the power factor equals
the displacement factor, as expected.
The performance criterion for a cycloconverter is dependent on many
different conditions. Exact analysis is difficult and is complicated further
by the oscillations of the firing angle throughout the output cycle. The
equations given above are based on the assumption of a relatively high
input-to-output frequency ratio.

10.6 The cycloinverter

The usual form of operation for a cycloconverter is in the step-down mode,
where the output frequency is less than that of the input. Generally, this
frequency is limited to a maximum of one third that of the supply
frequency, since at lower ratios the voltage distortions become appreci-
able.
When the cycloconverter is running in its step-down mode it is naturally
commutated, the leading kVA required to turn off conducting thyristors
being derived from the higher-frequency side, which in this case is the a.c.
supply. There is no reason why a cycloconverter cannot run with an output
frequency greater than that of the input, and such a system is called a