AC TO AC (CYCLOCONVERTER)
The Cycloconverter has been
traditionally used only in very high power drives, usually above one megawatt,
where no other type of drive can be used. Examples are cement tube mill drives
above 5 MW, the 13 MW German-Dutch wind tunnel fan drive, reversible rolling
mill drives and ship propulsion drives. The reasons for this are that the
traditional cycloconverter requires a large number of thyristors, at least 36
and usually more for good motor performance, together with a very complex
control circuit, and it has some performance limitations, the worst of which is
an output frequency limited to about one third the input frequency.
The Cycloconverter has four
thyristors divided into a positive and negative bank of two thyristors each.
When positive current flows in the load, the output voltage is controlled by
phase control of the two positive bank thyristors whilst the negative bank
thyristors are kept off and vice versa when negative current flows in the load.
An idealized output waveform for a sinusoidal load current and a 45 degrees
load phase angle is shown in Figure 2. It is important to keep the non
conducting thyristor bank off at all times, otherwise the mains could be
shorted via the two thyristor banks, resulting in waveform distortion and
possible device failure from the shorting current. A major control problem of
the Cycloconverter is how to swap between banks in the shortest possible
time to avoid distortion whilst ensuring the two banks do not conduct at the
same time. A common addition to the power circuit that removes the requirement
to keep one bank off is to place a centre tapped inductor called a circulating
current inductor between the outputs of the two banks. Both banks can now
conduct together without shorting the mains. Also, the circulating current in
the inductor keeps both banks operating all the time, resulting in improved
output waveforms. This technique is not often used, though, because the
circulating current inductor tends to be expensive and bulky and the
circulating current reduces the power factor on the input
In a 1- φ Cycloconverter,
the output frequency is less than the supply frequency. These converters
require natural commutation which is provided by AC supply. During positive
half cycle of supply, thyristors P1 and N2
are forward biased. First triggering pulse is applied to P1 and hence it starts conducting.
As the supply goes negative, P1 gets off and in negative half cycle
of supply, P2
and N1
are forward biased. P2
is triggered and hence it conducts. In the next cycle of supply, N2 in positive half cycle and N1 in negative half cycle are
triggered. Thus, we can observe that here the output frequency is 1/2 times the
supply frequency.
The Cycloconverter are
classified into three types based on the type of input ac supply applied to the
circuit.
- Single Phase to Single phase Cycloconverter.
- Three Phase to Three Phase Cycloconverter.
- Single Phase to Three Phase Cycloconverter
The following sections will describe
the operation principles of the cycloconverter starting from the simplest one,
single-phase to single-phase (1f-1f) cycloconverter.
To understand the operation
principles of cycloconverters, the single-phase to single-phase cycloconverter
(Fig. 2) should be studied first. This converter consists of back-to-back
connection of two full-wave rectifier circuits. Fig 3 shows the operating
waveforms for this converter with a resistive load.
The input voltage, vs is an ac
voltage at a frequency, fi as shown in Fig. 3a. For easy understanding assume
that all the thyristors are fired at α=0° firing angle, i.e. thyristors act like diodes. Note that the
firing angles are named as αP for the positive converter and αN for the
negative converter.
Consider the operation of the cycloconverter
to get one-fourth of the input frequency at the output. For the first two
cycles of vs, the positive converter operates supplying current to the load. It
rectifies the input voltage; therefore, the load sees 4 positive half cycles as
seen in Fig. 3b. In the next two cycles, the negative converter operates
supplying current to the load in the reverse direction. The current waveforms
are not shown in the figures because the resistive load current will have the
same waveform as the voltage but only scaled by the resistance. Note that when
one of the converters operates the other one is disabled, so that there is no
current circulating between the two rectifiers.
Fig:2
Single Phase to Single Phase Cyclo Converter
Fig:3 Single
Phase to Single Phase Cyclo Converter
- Input Voltage
- Output voltage for Zero Firing angle
- Output voltage with firing angle π/3rad
- Output voltage with varying firing angle
Thus by varying α, the fundamental
output voltage can be controlled.
Constant α operation gives a crude output waveform with rich harmonic
content. The dotted lines in Fig. 3b and c show a square wave. If the square
wave can be modified to look more like a sine wave, the harmonics would be
reduced. For this reason α is
modulated as shown in Fig.3d. Now, the six-stepped dotted line is more like a
sinewave with fewer harmonics. The more pulses there are with different α's,
the less are the harmonics.
Three-Phase
to Single-Phase (3Φ-1Φ) Cycloconverter
There are two kinds of three-phase
to single-phase (3Φ-1Φ) cycloconverters: 3φ-1φ half-wave cycloconverter (Fig. 4) and 3Φ-1Φ bridge cycloconverter (Fig. 5). Like the 1Φ-1Φ case, the 3Φ-1Φ 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 α is sinusoidally modulated over the cycle to generate a harmonically
optimum output voltage.
- + converter output voltage
- Cosine timing waves
- – converter output voltage
The polarity of the current
determines if the positive or negative converter should be supplying power to
the load. Conventionally, the firing angle for the positive converter is named
α P, and that of the negative converter is named α N. When the polarity of the
current changes, the converter previously supplying the current is disabled and
the other one is enabled. The load always requires the fundamental voltage to
be continuous. Therefore, during the current polarity reversal, the average
voltage supplied by both of the converters should be equal. Otherwise,
switching from one converter to the other one would cause an undesirable
voltage jump. To prevent this problem, the converters are forced to produce the
same average voltage at all times.
Three-Phase to Three-Phase (3Φ-3Φ) Cycloconverter:
The three-phase cycloconverters
are mainly used in ac machine drive systems running three phase synchronous and
induction machines. They are more advantageous when used with a synchronous
machine due to their output power factor characteristics. A cycloconverter
can supply lagging, leading, or unity power factor loads while its input is
always lagging. A synchronous machine can draw any power factor current from
the converter. This characteristic operation matches the cycloconverter
to the synchronous machine. On the other hand, induction machines can only draw
lagging current, so the cycloconverter does not have an edge compared to
the other converters in this aspect for running an induction machine. However,
cycloconverters are used in Scherbius drives for speed control purposes driving
wound rotor induction motors.
Cycloconverters produce harmonic rich output voltages, which will be
discussed in the following sections. When cycloconverters are used to
run an ac machine, the leakage inductance of the machine filters most of the
higher frequency harmonics and reduces the magnitudes of the lower order
harmonics.
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