Cycloconverter And Synchro Converter
Norine Wiltshire
Whereas phase-controlled semiconductor controlled rectifier devices (SCR) can be used throughout the range of CCVs, low cost, low-power TRIAC-based CCVs are inherently reserved for resistive load applications. The amplitude and frequency of converters' output voltage are both variable. The output to input frequency ratio of a three-phase CCV must be less than about one-third for circulating current mode CCVs or one-half for blocking mode CCVs.(Lander 1993, p. 188)[3] Output waveform quality improves as the pulse number of switching-device bridges in phase-shifted configuration increases in CCV's input. In general, CCVs can be with 1-phase/1-phase, 3-phase/1-phase and 3-phase/3-phase input/output configurations, most applications however being 3-phase/3-phase.[1]
The competitive power rating span of standardized CCVs ranges from few megawatts up to many tens of megawatts. CCVs are used for driving mine hoists, rolling mill main motors,[4] ball mills for ore processing, cement kilns, ship propulsion systems,[5] slip power recovery wound-rotor induction motors (i.e., Scherbius drives) and aircraft 400 Hz power generation.[6] The variable-frequency output of a cycloconverter can be reduced essentially to zero. This means that very large motors can be started on full load at very slow revolutions, and brought gradually up to full speed. This is invaluable with, for example, ball mills, allowing starting with a full load rather than the alternative of having to start the mill with an empty barrel then progressively load it to full capacity. A fully loaded "hard start" for such equipment would essentially be applying full power to a stalled motor. Variable speed and reversing are essential to processes such as hot-rolling steel mills. Previously, SCR-controlled DC motors were used, needing regular brush/commutator servicing and delivering lower efficiency. Cycloconverter-driven synchronous motors need less maintenance and give greater reliability and efficiency. Single-phase bridge CCVs have also been used extensively in electric traction applications to for example produce 25 Hz power in the U.S. and 16 2/3 Hz power in Europe.[7][8]
Whereas phase-controlled converters including CCVs are gradually being replaced by faster PWM self-controlled converters based on IGBT, GTO, IGCT and other switching devices, these older classical converters are still used at the higher end of the power rating range of these applications.[3]
This example presents a synchronous motor fed by a 12-pulse cyclo-converter. The circuit includes a long transmission line as well as the harmonics filters bank. It is possible to analyze the line-filter interaction. The 12-pulse cyclo-converter is commutated by the AC network. Each DC-link supplies a phase of the synchronous motor. the field current rectifier is also taken into account. The control scheme is based on a dynamic flux model of the synchronous machine. It allows a very high dynamic, even if that kind of drive has a very low supply frequency.
The simulation results present the behavior of the system in steady-state at rated operating point as well as a load change from 50% to 100% at rated speed.
SIMSEN is able to simulate such a complex topology, including more than 210 differential equations. Values related to each semiconductor can be displayed.
Basically, cyclo-converters are AC to AC converters and are used to vary the frequency of a supply to a desired load frequency. These are naturally commutated, direct frequency converters that use naturally commutated thyristors. These are mainly used in high power applications up to tens of megawatts for frequency reduction.
Some of the applications of cyclo-converter include high power AC drives, propulsion systems, high frequency induction heating, synchronous motors in sea and undersea vehicles, electromagnetic launchers, etc. Let us discuss this concept in detail.
It converts the frequency without help of any intermediate DC link. The output voltage and frequency of a cyclo-converter can be varied continuously and independently using a control circuit. Therefore, unlike other converters, it is a single stage frequency converter.
Cyclo-converters are constructed using naturally commutated thyristors with inherent capability of bidirectional power flow. These can be single phase to single phase, single phase to three- phase and three-phase to three phase converters.
So the control circuit implementation is not simple because large number of SCRs, typically 4 or 8 SCRs for single phase and 36 for three- phase supply. For such controller, a microcontroller or microprocessor or DSP is used to trigger SCRs.
In case of step-down cyclo-converter, the output frequency is limited to a fraction of input frequency, typically it is below 20Hz in case 50Hz supply frequency. In this case, no separate commutation circuits are needed as SCRs are line commutated devices.
But in case of step-up cyclo-converter, forced commutation circuits are needed to turn OFF SCRs at desired frequency. Such circuits are relatively very complex. Therefore, majority of cyclo-converters are of step-down type that lowers the frequency than input frequency.
Besides the frequency control, cyclo-converter output voltage can be varied by applying phase control technique. These can be used to provide either fixed frequency output from variable frequency input value or variable frequency output from fixed frequency input.
The equivalent circuit of a cyclo-converter is shown in figure below. Here each two quadrant phase controlled converter is represented by a voltage source of desired frequency and consider that the output power is generated by the alternating current and voltage at desired frequency.
The diodes connected in series with each voltage source represent the unidirectional conduction of each two quadrant converter. If the output voltage ripples of each converter are neglected, then it becomes ideal and represents the desired output voltage.
So the voltages produced by these two converters have same phase, voltage and frequency. The average power produced by the cyclo-converter can flow either to or from the output terminals as the load current can flow freely to and from the load through the positive and negative converters.
Due to the unidirectional property of load current for each converter, it is obvious that positive converter carries positive half-cycle of load current with negative converter remaining in idle during this period.
The figure below shows ideal output current and voltage waveforms of a cyclo-converter for lagging and leading power factor loads. The conduction periods of positive and negative converters are also illustrated in the figure.
The positive converter operates whenever the load current is positive with negative converter remaining in idle. In the same manner negative converter operates for negative half cycle of load current.
It consists of two full-wave, fully controlled bridge thyristors, where each bridge has 4 thyristors, and each bridge is connected in opposite direction (back to back) such that both positive and negative voltages can be obtained as shown in figure below. Both these bridges are excited by single phase, 50 Hz AC supply.
During positive half cycle of the input voltage, positive converter (bridge-1) is turned ON and it supplies the load current. During negative half cycle of the input, negative bridge is turned ON and it supplies load current. Both converters should not conduct together that cause short circuit at the input.
To avoid this, triggering to thyristors of bridge-2 is inhibited during positive half cycle of load current, while triggering is applied to the thyristors of bridge-1 at their gates. During negative half cycle of load current, triggering to positive bridge is inhibited while applying triggering to negative bridge.
By controlling the switching period of thyristors, time periods of both positive and negative half cycles are changed and hence the frequency. This frequency of fundamental output voltage can be easily reduced in steps, i.e., 1/2, 1/3, 1/4 and so on.
The above figure shows output waveforms of a cyclo-converter that produces one-fourth of the input frequency. Here, for the first two cycles, the positive converter operates and supplies current to the load.
It rectifies the input voltage and produce unidirectional output voltage as we can observe four positive half cycles in the figure. And during next two cycles, the negative converter operates and supplies load current.
Here one converter is disabled if another one operates, so there is no circulating current between two converters. Since the discontinuous mode of control scheme is complicated, most cyclo-converters are operates on circulating current mode where continuous current is allowed to flow between the converters with a reactor.
These cyclo-converters can be half-wave or full bridge converters as shown in figure. Like single phase cyclo-converters, these also produce a rectified voltage at the load terminals by each group of thyristors.
During positive half cycle of the input, conduction of the positive group thyristors is controlled and during negative half-cycle, conduction of negative group of thyristors is controlled in order to produce an output voltage at desired frequency.
In a bridge type of cyclo-converter, both positive and negative converters can generate voltages at either polarity, but negative converter only supplies negative current while positive converter supply positive current.
The above figure shows the conversion of three phase supply at one frequency to single phase supply of lower frequency. In this, the firing angle to a positive group of thyristors is varied progressively to produce single phase output voltage.
Three-phase to Three-phase Cyclo-converters
These are obtained by connecting 3 three-phase to single-phase cyclo-converters to the load. These converters can be connected in star or delta. Three phase cyclo-converter of both half-wave and bridge types are shown in figure below.