27-12-2012, 03:31 PM
CYCLOCONVERTERS
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INTRODUCTION
In industrial applications, two forms of electrical energy are used: direct current (dc) and
alternating current (ac). Usually constant voltage constant frequency single-phase or three-phase
ac is readily available. However, for different applications, different forms, magnitudes and/or
frequencies are required. There are four different conversions between dc and ac power sources.
These conversions are done by circuits called power converters. The converters are classified as:
1-rectifiers: from single-phase or three-phase ac to variable voltage dc
2-choppers: from dc to variable voltage dc
3-inverters: from dc to variable magnitude and variable frequency, single-phase or threephase
ac
4-cycloconverters: from single-phase or three-phase ac to variable magnitude and
variable frequency, single-phase or three-phase ac
The first three classes are explained in other articles. This article explains what cycloconverters
are, their types, how they operate and their applications.
Traditionally, ac-ac conversion using semiconductor switches is done in two different ways: 1- in
two stages (ac-dc and then dc-ac) as in dc link converters or 2- in one stage (ac-ac)
cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction
and synchronous motors. They are usually phase-controlled and they traditionally use thyristors
due to their ease of phase commutation.
Three-Phase to Single-Phase (3f-1f) Cycloconverter:
There are two kinds of three-phase to single-phase (3f-1f) cycloconverters: 3f-1f half-wave
cycloconverter (Fig. 4) and 3f-1f bridge cycloconverter (Fig. 5). Like the 1f-1f case, the 3f-1f
cycloconverter applies rectified voltage to the load. Both positive and negative converters can
generate voltages at either polarity, but the positive converter can only supply positive current
and the negative converter can only supply negative current. Thus, the cycloconverter can
operate in four quadrants: (+v, +i) and (-v, -i) rectification modes and (+v, -i) and (-v, +i)
inversion modes. The modulation of the output voltage and the fundamental output voltage are
shown in Fig. 6. Note that a is sinusoidally modulated over the cycle to generate a harmonically
optimum output voltage.
Blocked Mode and Circulating Current Mode:
The operation of the cycloconverters is explained above in ideal terms. When the load current is
positive, the positive converter supplies the required voltage and the negative converter is
disabled. On the other hand, when the load current is negative, then the negative converter
supplies the required voltage and the positive converter is blocked. This operation is called the
blocked mode operation, and the cycloconverters using this approach are called blocking mode
cycloconverters.
However, if by any chance both of the converters are enabled, then the supply is short-circuited.
To avoid this short circuit, an intergroup reactor (IGR) can be connected between the converters
as shown in Fig. 9. Instead of blocking the converters during current reversal, if they are both
enabled, then a circulating current is produced. This current is called the circulating current. It is
unidirectional because the thyristors allow the current to flow in only one direction. Some
cycloconverters allow this circulating current at all times. These are called circulating current
cycloconverters.
Blocking Mode Cycloconverters:
The operation of these cycloconverters was explained briefly before. They do not let circulating
current flow, and therefore they do not need a bulky IGR. When the current goes to zero, both
positive and negative converters are blocked. The converters stay off for a short delay time to
assure that the load current ceases. Then, depending on the polarity, one of the converters is
enabled. With each zero crossing of the current, the converter, which was disabled before the
zero crossing, is enabled. A toggle flip-flop, which toggles when the current goes to zero, can be
used for this purpose. The operation waveforms for a three-pulse blocking mode cycloconverter
are given in Fig. 10.
The blocking mode operation has some advantages and disadvantages over the circulating mode
operation. During the delay time, the current stays at zero distorting the voltage and current
waveforms. This distortion means complex harmonics patterns compared to the circulating mode
cycloconverters. In addition to this, the current reversal problem brings more control complexity.
However, no bulky IGRs are used, so the size and cost is less than that of the circulating current
case. Another advantage is that only one converter is in conduction at all times rather than two.
This means less losses and higher efficiency.