Abstract: A control arrangement for a multilevel convertor employs an AC reference signal which is locked to the AC system frequency and is compared with a number of DC threshold levels. Intersections of the reference signal and threshold levels triggers the firing and turning-off of GTOs within the convertor to produce the desired multilevel voltage output. Normal changes in system voltage, for example, are compensated by variation of the phase relationship of the reference signal with respect to the system voltage, and/or of the voltage values of the threshold levels. Sudden and drastic changes, especially reductions, in system voltage level are sensed and used to alter the amplitude of the reference signal so as to reduce the difference between the convertor multilevel voltage and the system voltage, thereby reducing the stress on the GTOs during faults.

Abstract: A piston for an internal combustion engine includes an insert defining an air chamber opening into the combustion chamber. The insert is preformed to a desired configuration and the piston cast around the insert thus securing the insert within the structure of the piston. The insert may be located wholly within the piston or may have a flange part which extends over an outer surface of the piston.

Abstract: A case is provided for protecting a second stage breathing apparatus such as those used in underwater scuba diving equipment. The case is in the nature of a device for keeping the second stage breathing apparatus clean and for protecting the same and for completely covering the second stage of a diver's two-stage diving apparatus. The case includes a member made of a yieldable or resilient material such as a suitable plastic, rubber-like compound or the like, and the case includes a hinge construction for permitting opening and closing of the case, and wherein there is also provided a fastener for maintaining the case in closed position until it is to be opened, and there is also provided a ring-like member for permitting the diver to attach the case to a belt or strap worn while diving. The internal configuration of the case is such that the second stage breathing apparatus is snugly and properly received therein.