The present invention relates to apparatus for controlling the application of power to a load and, more specifically, to a novel assisted-commutation power relay, in which the current conducted through, and energy dissipated in, a contact-shunting device is minimized.
It is now well-known that relay contact damage, caused by arcing and the like phenomena normally occurring during making and breaking of a current-carrying contact between a source and a load, can be substantially reduced or eliminated by providing an element in parallel with the relay contacts, and controlling the shunting element to conduct while the relay contacts are actually being opened and closed. Apparatus using a controllably conducting semiconductor device across the relay contacts, is found in such prior art descriptions as U.S. Pat. Nos. 3,474,293 to Siwko et al.; 3,555,353 to Casson; and 3,868,549 to Schaefer et al., amongst others. These early apparatus may be considered as "brute force" approaches to the relay contact damage problem, and typically require shunting semiconductor devices capable of withstanding relatively high current and power dissipation. In U.S. Pat. No. 4,074,333 to Murakami, et al., and the like, conduction in the shunting semiconductor device is enabled for relatively fixed time intervals, in an attempt to reduce the energy dissipation of the shunting device. However, the current handling capability of such shunting solid-state switches, in parallel with the relay contacts, must be sufficiently high to conduct the full load current of the power relay apparatus for a relatively large number of cycles of the power line frequency. Because of the resulting current-time rating, the solid-state switch is relatively large and expensive, and generally requires a bulky heat sink to adequately protect the solid-state switch from over-temperature conditions, particularly when the apparatus is utilized with certain loads, such as motors which may have the rotor thereof locked at the time that motor starting may be commanded. Relatively long conduction time periods also occur due to the relatively long pull-in and drop-out times found in power relays utilized for controlling relatively large motors and the like. The pull-in time has a relatively large spread due to the A.C. coil having different characteristics dependent upon when, in a source waveform half-cycle, that coil is energized; if the coil is energized while the line voltage is at a peak, the relay contacts pull in much more rapidly than if the power relay is energized at a line voltage minimum, e.g. close to a line voltage waveform zero crossing. Friction and damping effects of structures utilized for modern power relays also introduce a variability in the pull-in time of the power relay. Similar large time spreads are found in the drop-out characteristics of power relays. Therefore, the shunting solid-state switch must continue to carry load current, when the relay is commanded to its open condition, on the possibility that a particular relay may be slow enough to require a longer period to drop out thn another relay of the same type. Accordingly, power relay apparatus which will minimize the current rating, conduction time and energy dissipation of the shunting solid-state switching device, is highly desirable.