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NORA Oilheat Technicians Manual

Figure 8-15: Electromagnet 8-14 Basic Electricity Figure 8-16: Solenoid valve Chapter 8 Basic Electricity one closer to the heater expands faster. Since the two pieces of metal are fused together and they are secured on one end, the quickly expanding piece of metal bends away from the heat and makes the strip warp. This is why it is called a warp switch. As you can see in the illustration, as the bimetal warps, it releases the movable contact and the spring on the movable contact opens the circuit. This puts the oilburner on ‘safe’ and stops the flow of electricity to the heater, see Figure 8-15. The only way to close this switch again is to wait for the bimetal to cool and warp back and then someone must push the ‘reset’ button to reset the moveable contact under the lip of the bimetal. Electricity and magnetism Electromagnetism describes the relationship between electricity and magnetism. If you move a wire in a magnetic field, you generate voltage if that wire is part of a complete electrical circuit. The generated voltage will cause electric current to flow through the wire. As electric current flows through the wire, it creates a magnetic field around the wire. A coil of wire creates a stronger magnetic field than a straight wire. The coil is called a solenoid. If an iron core were placed inside the coils of a solenoid and the solonoid is energized, the magnetic field created magnetizes the iron core while the current flows. This is an electromagnet. See Figure 8-15. Two factors affect the strength of an electromagnet: 1. The intensity of the current in the coil. 2. The number of turns of wire in the coil. Electromagnets are rated in amp-turns. Solenoid valves To make a solenoid valve, an iron bar is placed into the center of a coil of wire and electricity is sent through the wire. The electromagnetic force will pull and hold the bar in the center of the coil. If a spring is attached to the iron bar (or valve stem), it returns to its original position when the electricity is shut off. If a valve seat is put on the end of the valve stem you have an electrically operated valve. Figure 8-16. Relay switches Relays are electromagnetic switches found in primary controls, operating controls, and switching relays that use electromagnets to open and close. To create a magnetic field, wrap a coil of wire around a metal core. The magnetic field magnetizes the core and this electric


NORA Oilheat Technicians Manual
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