Timmie McElwain Gas/Propane Basics... President, Gas Appliance Service Thermal Electricity gastc@cox.net & Electromagnetism Part 2 Riverside, RI the copper winding to the housing of the power unit. This wire lead acts as the ground for the windings on the electromagnetic assembly. The wire lead attached to the other copper winding extends down below the “U” shaped core. This wire lead is attached to a recessed, or female, tin-plated contact that is manufactured into the threaded base of the housing. Current that is used to energize the electromagnet enters the power unit by way of the tinplated contact. Figure 7: Electromagnetic Safety Assembly Above the electromagnet assembly, shown in Figure 6, is a flat steel keeper plate. The keeper plate has a shaft attached to it that extends through to the top of the housing of the power unit. A rubberized shutoff disc is attached to the end of the shaft. When the electromagnet assembly is energized, the keeper plate is attracted to the “U” shaped core poles, pulling the shutoff disc with it. When the electromagnetic assembly is de-energized, the keeper plate is released and the shutoff disc can return to its original position (normally to close a gas valve). In order to ensure that the keeper plate and shutoff disc return to their original position, a drop-out tension spring is located between the top of the housing of the power unit and the shutoff disc. The added force from the tension of the drop-out spring is used to snap the shutoff disc back to its original “closed” position. Operational Description As stated earlier, current used to energize the electromagnet assembly enters the power unit through the female contact in the threaded base. Current then flows through the wire lead that interconnects the copper windings on the "U" shaped core poles and the tin-plated contact. Once current passes through the copper winding, the “U” shaped core becomes energized. A magnetic field (magnetic force of attraction) is then generated at the poles of the core. The magnetic field is strong enough to hold the keeper plate in position when the keeper plate is brought in contact with the “U” shaped core. When the OOn Sept. 1, 2012, standing pilots on new equipment were done away with. This only pertains, however, to new equipment, not existing equipment. There are still many systems, both heating and hot water, that use a thermocouple as a pilot safety system. This series of articles addresses these systems— how they operate, their many uses and how to diagnose and troubleshoot thermocouple systems. These systems appear on both propane and natural gas equipment. The fact that so many standing pilot systems are still operating makes this article very important. A large number of heating and water heating systems are still operating with a thermocouple pilot safety system. Being able to understand and service these systems is vital to keeping the older systems, which still have some life in them, operating. In the proceeding articles, I am going to present some indepth information about thermocouples and the electromagnetic principle that works in conjunction with this very reliable and long standing system. The first chapter in my manual on Circuitry and Troubleshooting Volume II is on thermocouples. Electromagnetic Power Unit Definition and Purpose An electromagnetic power unit is a safety device in a gas appliance control system that stops gas from flowing through the system when unsafe conditions exist. This is accomplished through the use of an electromagnetic/valve assembly. Physical Description The most important component of the electromagnetic power unit is the electromagnet assembly, shown in Figure 6. The electromagnet assembly consists of a “U” shaped core that is normally manufactured from soft-iron bar stock copper wire and is wound around each of the legs, or poles, of the “U” shaped core. When current is passed through these windings, a magnetic force is generated in the soft-iron bar. This magnetic force is the basic principle of operation of the electromagnetic power unit. Each of the copper windings has a wire lead attached to it. One wire lead extends from Figure 6: Electromagnetic Power Unit 14 ICM/September/October 2016
ICM September-October 2016
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