#### AC Alternator Types Alernating Current Review

Electric Generators Suppliers
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The characteristics of three- wire, three-phase (or delta) alternators are: - The amount of current through the alternator terminals is the algebraic sum of current through the alternator coils.
- The currents are not equal in magnitude or time.
- Connection between coils can be made either inside or outside the generator.
- In a 60-Hertz machine, each coil experiences maximum instantaneous voltage, first positive and then negative, 120 times each second. Disregarding voltage direction, the maximum instantaneous voltages occur on successive coils 0.003 seconds apart. Due to time differences between the voltages and resulting currents, the amount of current through the alternator terminals and the amount through the alternator coils are not equal in magnitude or time. The current through the alternator is 73 percent greater than through the coils. Coil and terminal voltages are the same magnitude. Three voltmeters and three ammeters (or equivalent) are required to measure the load on the alternator. The average value of the three currents times the average value of the three voltages plus 73 percent gives a close approximation of the alternator load in kilovolt-amperes. Two single-phase or one two element polyphase kilowatt-hour meter is required to measure the alternator output in kilowatt-hours.
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Four-wire three-phase, dual voltage and frequency alternators are also used. These are supplied in sizes from 15 to 1500 kW, 127-220 volts, three-phase, 60 Hertz, or 230-400 volts, three phase, 50 Hertz. Dual stator coils are used on each phase. Coil ends are brought out to a terminal board for making connections. Voltage and frequency combinations are shown in below (
Most parts of the world have standardized on either 50 or 60 Hertz alternating current power. Sixty Hertz power is commonly used in the United States. Fifty Hertz power is used in many countries outside the United States. The ratio be-teen the 60-50 Hertz frequencies is 6:5. Electrical energy received at one frequency can be converted to a different frequency by using a frequency changer. If a large power requirement exists, it may be more economical to use a special alternator to produce power at the desired frequency. The applicable equation is: Where: V = generated voltage The generated voltage is proportional to the strength of the magnetic field, phase, and number of turns in series between terminals and the speed. References: “Joint Departments of the Army and the Navy, Operation Maintenance and Repair of Auxiliary Generators, 26 August 1996”
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