keeper plate is held in this position by the magnetic field, the shutoff disc is pulled away from a gas valve seat. When current ceases to flow into the copper windings of the magnet assembly, the electromagnet assembly will deenergize. The “U” shaped core will then lose its magnetic attraction (magnetic field). Consequently, the keeper plate will be released. The tension of the drop-out spring will then pull the keeper plate away from the magnet core. Since the keeper plate and the shutoff disc are attached to the same shaft, the spring tension will also force the shutoff disc back onto a gas valve seat. Figure 7 shows an electromagnetic power unit installed in a gas valve. The current needed to energize the electromagnet assembly is supplied by a thermocouple. When the thermocouple is properly heated by the pilot burner flame, a sufficient amount of voltage is generated to energize the electromagnetic assembly. When the electromagnet assembly is energized by the thermocouple, it is not strong enough to overcome the drop-out spring tension and pull the shutoff disc off of the gas valve seat. Therefore, a reset button is normally incorporated into most gas valves using an electromagnetic power unit. The reset button has a shaft attached to it that rests on the top of the shutoff disc. When the reset button is manually depressed, the shaft forces the shutoff disc off the valve seat. In turn, the keeper plate is forced up against the core poles. When the thermocouple is generating enough millivoltage to energize the magnet assembly, the keeper plate will be held against the core poles by the magnetic field. The reset button can then be released. The purpose of most electromagnetic power units is to cut off the supply of gas to all burners in the event of a pilot outage. When the gas valve is de-energized, the shutoff disc is seated on the gas valve seat; therefore, the gas supply is cut off. In order to energize the gas valve, the reset button must be depressed so that gas can flow to the burners for combustion. In most valves, depressing the reset button allows gas to flow only to the pilot burner. The pilot burner can then be ignited. Next, the pilot burner heats the thermocouple until the required voltage output that is necessary to energize the power unit is achieved. At that time, the reset button can be released, and gas will continue to flow to the pilot burner. If the pilot burner is extinguished for any reason, the thermocouple will cool, and the voltage output will drop. As the voltage applied to the power unit drops, the strength of the magnetic field weakens. When the magnetic field is no longer strong Figure 8: The Pilotstat power unit is the heart of the safety shutoff mechanism. enough to hold the keeper plate to the core poles, the dropout spring will snap the shutoff disc back against the valve seat, cutting off the supply of gas to all burners. The point at which the shutoff disc returns to the valve seat is called the drop-out point. The voltage level at which the magnetic field weakens sufficiently, to allow the shutoff disc to return to the valve seat, is called the drop-out voltage. Possible Malfunctions The electromagnet power unit is part of an electromagnet safety shutoff system used in many gas appliance control systems. The thermocouple or thermopile, pilot burner, electromagnetic power unit, reset button and, in some cases, a lever assembly are all components of the safety shutoff system. When the safety shutoff system malfunctions, any one of the components can be at fault. For this reason, it is important to understand malfunctions associated with each component, as well as the entire system when it is in operation. Malfunctions of the complete safety shutoff system will be covered in depth in the Electromagnetic Safety System Summary in an upcoming issue. Malfunctions associated with only the Electromagnetic power unit and with their respective causes and corrective actions, are contained in Table 1. Table 1. SYMPTOM CAUSE CORRECTIVE ACTION 1. Power unit will not energize. Keeper plate will not remain against core poles. 1. Female contact corroded or oxidized. 1. Clean female contact. 2. Core winding broken. 2. Replace power unit. 3. Hot wire lead or ground wire lead for core windings 3. Replace power unit. is broken. 4. Core of magnet assembly oxidized or corroded. 4. Replace power unit. 2. Power unit will not de-energize. Shutoff disc does not cut off the supply of gas to burners. 1. Dropout return spring broken. 1. Replace power unit. 2. Trash lodged between shutoff disc and valve seat. 2. Remove power unit. Clean shutoff disc and valve seat. 3. Shutoff disc damaged. It does not rest properly on valve seat. 3. Replace power unit. 4. Power unit not completely seated in gas valve. 4. Tighten power unit with adjustable wrench until it is firmly seated in gas valve. Note: Specific millivolt test procedures for drop-out voltage will be covered in the Electromagnetic System Summary. Summary The electromagnetic power unit is a device normally used to cut off the flow of gas to all burners in the event of a pilot outage. The magnet assembly is the heart of the power unit. It is normally energized by voltage generated from a thermocouple that is being heated by a pilot burner. When the pilot burner is extinguished, the thermocouple output will decrease until the magnet “drops out,” closing off the supply of gas to all burners. There are really two parts to the magnet assembly. One is the coil, which is wrapped around the “horseshoe” shaped metal piece. The other is the “horseshoe” shaped metal piece. When current passes through the coil it creates a magnetic field. This, in turn, will magnetize the metal piece, which is engaged by pushing down on the pilot-on-off button. This piece is shown below in Figure 8. In Part 3, we will discuss thermocouple and electromagnet safety troubleshooting procedures. ICM ICM/September/October 2016 15
ICM September-October 2016
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