Electrical transmission or interconnection systems – Nonlinear reactor systems – Parametrons
Patent
1982-04-05
1985-03-12
Zazworsky, John
Electrical transmission or interconnection systems
Nonlinear reactor systems
Parametrons
307252B, H03K 1772
Patent
active
045047421
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an electrical switching device of the kind comprising a semiconductor thyristor having first and second terminals the state of conduction between which terminals, when a periodically varying first signal is applied thereacross, is controllable to change from a first to a second state at a particular time during a cycle of the first signal, in response to a change, at a third control terminal of the thyristor, from a first to a second state of a control signal, and a control signal generating circuit for generating said control signal, the control signal generating circuit including a first switchable element switchable from a first to a second state of conduction to effect said change.
Devices of the above kind are in common use, the thyristor normally comprising a triac or silicon controlled rectifier. Usually, the control signal generating circuit includes a diac coupled via a suitable network across the first and second terminals of the thyristor and so arranged as to cause the diac to be switched from a state of nonconduction to a state of conduction at a predetermined time during each cycle or half cycle of said first signal whereby to correspondingly switch the thyristor from a state of nonconduction to a state of conduction at such time. By choosing the network, the particular time at which conduction of the thyristor is initiated during each cycle or half cycle of applied said first signal is selectable so that the effective power delivered to a load connected in series with the switching device is correspondingly selectable. Normally, the said network is arranged to enable variation of the period of thyristor conduction during each cycle of applied first signal to allow selective control of power dissipation in the load.
Particular difficulty arises with devices of the above kind in that it is found that, apparently unexplained surges or pulses occur, more particularly on inductive loads so that, frequently, damage to the thyristor will occur in use even through the nominal rating of the thyristor is well in excess of the expected power dissipation requirements. It has been supposed that the consequent unreliability of thyristors under these circumstances is due to heavy transients induced in the switching device due, for example, to interference in the first signal. Thus, it is customary to include sometimes elaborate means for eliminating such transients with a view to improving reliability of the switching device.
The present invention is based on a premise that unreliability of devices of the kind described is not due simply to transients occurring in the first signal, but rather occurs through faulty operation of the first switchable element.
Accordingly, a switching device as described above is characterized, in accordance with this invention, in that the control signal generating circuit further includes a second switching element connected whereby in use to be responsive to said change in state of conduction of said first switching element to revert said control signal to said first state after said change therein. More particularly, the second switching element may be switched to a state of conduction upon detection of the change of state of the first switching element to effect said reversion. The first switching element may, as is known, comprise a diac coupled to the third terminal of the thyristor via a network forming part of said control signal generating circuit, said network being coupled across said first and second terminals and said diac being responsive to variations in electric signal across part of said network occurring during application of said first signal to the thyristor to switch said diac on pursuant to occurrence across said part of said network of a particular voltage level, this switching on causing said change in said control signal. The said second switching element may be arranged across said part of said network to at least substantially short-circuit said part pursuant to detection of said change in said control signal.
The second sw
REFERENCES:
patent: 3763395 (1973-10-01), Shilling et al.
patent: 4052624 (1977-09-01), Hamstra
patent: 4220910 (1980-09-01), Chiba et al.
Middendorp Electric Co. Pty. Ltd.
Zazworsky John
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