Line-Commutated And Load Commutated Cycloconverter
The load commutated cycloconverter differs from line commutated cycloconverter in that the thyristors can be commutated by the reversal of the load voltage. In this case, the load must possess a generated or back emf that is independent of the source voltage. The most common example of such a load is a wound-field or permanent magnet synchronous machine. In such cases, the load frequency can be equal to or greater than the source frequency and still allow natural thyristor commutation. The thyristor gating is based on two control signals: first, with respect to the source voltage, to control the load voltage and second with respect to the synchronous machine-generated emf, to ensure that current will flow in the correct phase of the load machine at the correct time. For optimum torque per ampere in a synchronous machine, commutation should take place with 180° degree of tiring delay with respect to the machine voltage. This, however, leaves no margin for commutation overlap and the thyristor tums off. As in the other types of thyristor-based synchronous motor drive, the commutation must be advanced from the optimum position. It is the function of the shaft position sensor and the control electronics to ensure that the computation is sufficient in advance of the Line-to-line voltage crossing. This condition results in a phase current that leads the phase voltage. In aircraft applications, load-commutated cycloconverters are used to start and accelerate a gas turbine through its high-frequency synchronous a.c. generator operating as a motor. After the turbine has started and reached its operating speed, the generator cycloconverter delivers 400 Hz power to the aircraft.