Chapter 8 Basic Electricity Figure 8-18: Basic design used in an iron core transformer and forth in the iron core causing the north and south poles to switch back and forth. This pulsating magnet field creates a pulsing electric field in the secondary coil. Step-down transformers have more primary coils than secondary and reduce voltage. Step up transformers have more coils in the secondary coil and increase voltage. There is no electrical connection between the primary and secondary coils. The only connection between the two is the magnetic field. When voltage is applied to the primary coil, a magnetic field is generated which creates voltage in the secondary coil. The amount of voltage generated in the secondary coil is determined by the ratio of coils between the primary and secondary coils. Transformers are very efficient and can provide as much as 90% of the energy put into the primary side to the secondary side. The small energy loss is due to the heating of the wires in the coils. Why are transformers and solenoids loads? The greater the current, the greater the magnetic field around the conductor. The Primary Secondary 8-16 Basic Electricity magnetic field limits current flow by pushing back against the source voltage. Remember, current is not only caused by voltage from the power source, it is caused by any electromotive force that acts upon the conductor. As the electricity goes through the coil of wire in the transformer or solenoid, it creates a magnetic field that resists the flow of electricity. It becomes a load. This is called this “back emf” because it counteracts the voltage (electromotive force) from the power source. Motors Motors turn electrical energy into mechanical energy. If a permanent magnet is mounted on an axle and placed between the opposite poles of two fixed magnets, the magnet on the axle will spin until its north pole faces the south pole of the fixed magnet, and its south faces north. (Like poles repel each other, opposites attract.) If you could reverse the poles of the fixed magnets just as the rotating magnet was coming to a stop, it would force it to turn another half circle until the opposite poles lined up again. Figure 8-19. If you make the fixed magnets electromagnets every time the alternating current in the electromagnet changes direction the poles in the magnet flip (north becomes south and south becomes north) driving the rotating magnet around again. It will keep turning as long as the current in the fixed magnets keep flipping. The fixed (stationary) electromagnets are called the stator, and the rotating magnet the rotor. Measuring electricity Since electricity can not be seen (except for sparks), testing and measuring devices are needed in order to properly troubleshoot heating and air conditioning systems.
NORA Oilheat Technicians Manual
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