Basic Principle of Operation of cyclo-converters
Basic operating principle of cycloconverters:
Fig: Equivalent circuit of a cycloconverter & Waveforms
- Each two-quadrant converter (phase-controlled) is represented as an alternating voltage source, which corresponds to the fundamental voltage component obtained at its output terminals.
- The diodes connected in series with each voltage source, show the unidirectional conduction of each converter, whose output voltage can be either positive or negative, being a two-quadrant one, but the direction of current is in the direction as shown in the circuit, as only thyristors − unidirectional switching devices, are used in the two converters.
- Normally, the ripple content in the output voltage is neglected. The control principle used in an ideal cyclo-converter is to continuously modulate the firing angles of the individual converters, so that each produces the same sinusoidal (ac) voltage at its output terminals.
- Thus, the voltages of the two generators have the same amplitude, frequency and phase, and the voltage of the cyclo-converter is equal to the voltage of either of these generators.
- It is possible for the mean power to flow either ‘to’ or ‘from’ the output terminals, and the cyclo-converter is inherently capable of operation with loads of any phase angle − inductive or capacitive.
- Because of the uni-directional current carrying property of the individual converters, it is inherent that the positive half-cycle of load current must always be carried by the positive converter and the negative half-cycle by the negative converter, regardless of the phase of the current with respect to the voltage.
- This means that each two-quadrant converter operates both in its rectifying (converting) and in its inverting region during the period of its associated half-cycle of current.
- The displacement angle of the load (current) is 00 In this case, each converter carries the load current only, when it operates in its rectifying region, and it remains idle throughout the whole period in which its terminal voltage is in the inverting region of operation. When Φ= 600, the displacement angle of the load is lagging.
- During the first period of each half-cycle of load current, the associated converter operates in its rectifying region, and delivers power to the load.
- During the latter period in the half-cycle, the associated converter operates in its inverting region, and under this condition, the load is regenerating power back into the cyclo-converter output terminals, and hence, into the ac system at the input side.
- Any other case, say capacitive load, with the displacement angle as leading, the operation changes with inverting region in the first period of the half-cycle as per displacement angle, and the latter period operating in rectifying region.