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Alternator

An alternator produces AC current that passes through a diode bridge rectifier to become nearly DC current. It has two sets of electromagnets built up from wire windings and nested together. The stationary outer windings are called the stator. The rotating inner windings, turned by the engine, are the rotor. The voltage regulator applies about 2 amps of DC field current to the rotor windings, activating the electromagnets, which then induce 50 amps or more of AC current in the stator as the rotor magnetic field rotates through the stator windings.

Rectified alternator output is stable enough to charge a battery but not nearly stable enough to operate most electronic equipment. Never run an alternator with the battery disconnected, for this reason, because without the buffering effect of the battery, transient voltage spikes can damage other equipment, including perhaps the voltage regulator. You can run the engine for a while without the alternator, on the other hand, as might be necessary if the drive belt fails, keeping in mind that the voltage regulator will continue to draw about 2 amps for field current.

Amperage
rating

The alternator that produces battery charging current must be adequate for the size of the battery bank being charged. If it is not, the alternator will overheat and can fail. Most alternators are designed to recharge one starting battery that rarely if ever falls below 90% of capacity and quickly comes up to full charge. They cannot reliably recharge a bank of two or more deep cycle house batteries that have fallen to 50% of capacity and will accept a bulk charge for some time.

The amperage rating of the alternator should be at least one-fourth of the amp-hour capacity of the battery bank being charged. Higher capacity is generally better, since the alternator will not work as hard, and over-sizing the alternator will not harm the batteries. Most alternators cannot produce their rated current contin­u­ously without overheating. Some, called continuous output hot rated, or kkk-rated, can do so. Ideally, the charging system should sense the alternator temperature and reduce its load if it overheats. It might also provide a switch for disconnecting the charging system when full engine power is needed.

Mounting

Alternators come with four kinds of mount for attaching to an engine: Single foot, which has one bolt hole an inch deep; double foot with one bolt hole two inches deep; and dual foot or saddle mount with two bolt holes that are either 3.15 in apart or 4 in apart. A single foot mount can be fitted with a spacer to make it double foot. Alternators also come designed to turn clockwise when viewed from the front, or counter-clockwise, or both. This difference refers only to the angle of the cooling blades.

Grounding

The alternator casing can be grounded to the engine, or the alternator can have an isolated ground that attaches to a bus bar. Isolated ground is useful on steel boats, for example, where ground current can cause electrolysis. If the negative battery terminal connects to the engine, a case grounded alternator is adequate.

Belts

The belt that drives an alternator is usually a 3/8 or 1/2 inch v-belt but can also be a multi-groove serpentine belt. A 3/8-in belt can produce up to about 80 amps. A half-inch belt can produce up to about 110 amps. High capacity alternators over about 80 amps usually require two belts, which should always be replaced in pairs. V-belts must be properly tensioned and must ride snugly along the sides of the pulley groove but must not protrude above the edge of the groove.

Engine
load

When producing its full rated output, an alternator consumes one HP for every 25 amps. A high capacity, dual belt alternator can place an unacceptable sideways load on the engine crankshaft, which can lead to premature bearing failure. The best way to avoid this is by adding a second belt-driven device, such as another alternator or a refrig­er­ation compressor, on the opposite side of the crankshaft to balance the load. Many engine makers specify a maximum unbalanced side load.
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