switch which is employed to protect the resistor from dangerous overheating due to some unusual conditions, such as fan stoppage or blocking of the air-intake door. The fan motor is fed from the main contact tips on the control undervoltage relay and through the control fuses which should give fan operation as long as the control is energized.The resistor is very difficult to protect, as it is essential to trip out the control for a small amount of power in the stack or from a relatively large amount of power in the same length of time. The reason for this is that the resistor is made up of six sections, used in various circuit locations, and each section has a different power rating and time on requirements. A number of different types of thermal switches was tested, and it was found that the switch had to have a best heat response to protect the edgewise resistor units which do not have a great amount of thermal capacity. The next problem was to determine the exact spot where it should be placed in order to protect as many of the sections as possible on the various operating parts. The location of the thermal switch was above the re sistor stack, so that the heat representing large kilowatt dissipation low in the stack would trip the switch. The unit also would function with a relatively low kilo watt dissipation on the top units. The action in the latter case was mainly from radiation from the top units. This switch is entirely waterproof, as it is located where rain could damage it. It is not necessary to remove the thermal switch to take out resistor boxes, as it is mounted on the top of the air baffle. The thermal switch selected protected the resistor sections on most operating points as well as protecting the resistor on a normal operating cycle.
I N the past four years, the many con structional and operational advan tages of sealed ignitron rectifiers have greatly aided the war effort by saving critical materials and manufacturing fa cilities. Two-wire sealed ignitron rectifi ers 1 have met industry's urgent need for efficient d-c power and have found wide" application in the range of 75-to 1,000-kw capacity, at voltages ranging between 250 and 600 volts direct current for general in dustrial, railway, mining, and-electro chemical service.The field of application of the ignitron rectifier is considerably enlarged by the introduction of equipments for threewire service, and it is the purpose of this paper to describe the design, character istics, and performance of these units.
The Field for the Three-Wire RectifierOne field of usefulness of the three-wire rectifier is for supplying d-c power to in dustrial three-wire systems. There is an other application to be found in metro politan districts where steps are being taken to retire the Edison three-wire d-c system and existing building generating plants. In many cases it will not be eco nomical to rewire buildings for a 208-volt three-phase 60-cycle supply or to replace d-c motors and control for a-c operation. The problem of supplying d-c power for three-wire systems without the installation of rotating apparatus has long delayed the conversion in many districts.Three-wire sealed ignitron rectifiers offer several distinct advantages when compared with other forms of converting equipment. The rectifier is largely static, thus eliminating special foundation and space requirements. Rectifiers are quiet in operation and easy to operate. The noload losses are small and the efficiency is fairly constant and reasonably high over the entire load range. Rectifier equip ments are factory assembled as unit d-c substations thereby minimizing installa tion time and expense.
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