Red Mist said: (quoted from post at 14:33:23 07/11/09) Owen:
In addition to what you've explained, don't alternators make AC current (hence the need for the diode on the one wire versions) and generators make DC current? Inquiring minds want to know.....
mike
The external diode is not needed on the one wire hookup. It is used with the three wire hookup which needs an external source of power to excite the regulator to get the alternator to start to charge. Once the alternator is charging current is available internally to keep the regulator working. This current also back feeds the wire used to excite the regulator. Since most farm equipment does not use an ignition switch with an accessory position that is separate from the ignition circuit, this back feed provides power to the coil on gas engines so that shutting of the switch will not shut off the power to the coil. This is not a problem with diesel engines unless you have an electrical shut off on the injection pump.
Electrically the generator and alternator work the same. In the generator the magnets are stationary and wire coils move. In the alternator the wire coils are stationary and the magnets move.
The current is generated by passing a wire coil through magnetic lines of force from the magnets. As the coil approaches the am pole end of the magnet the current will be generated in one direction. As the coil moves away from the pole the current is generated in the opposite direction. In a generator the armature is made up of many individual wire coils. The ends of each coil are attached to commutator bars 180 degrees apart. The number of coils in the armature is half the number of commutator bars it has.
The current produced in each coil of the armature is alternating current. The brushes are arranged so that the current can flow through them as the coil approached the magnetic pole. As the individual coil passes the pole of the magnet the current reverses in the coil but the brushes are no longer in contact with the commutator bars for that coil so the current produced in the opposite direction is not used until the commutator bars contact the brushes again after rotating nearly 180 degrees. The current in the coil is now flowing in the opposite direction but the brushes now contacting the commutator bars are now opposite of what they were before so that current flow through the brushes is in the same direction as it was when the armature was 180 degrees different. It is the way the brushes contact the commutator bars at the end of the wire coils that converts to AC current in the armature to DC current at the A terminal in the case.
Alternators use the same principles to produce current. Mounted inside the case of the alternator are three field coils called the stator. (They can have one end tied together in a common connection and three individual wire ends so that they form a "Y" shape or they may have their ends tied together to form a triangle or Delta winding. Delta windings will have three pairs of wire ends rather than three individual wire ends.) Inside the stator is the magnet mounted on the shaft called the rotor assembly. The rotor is made up of a wire coil used as an electro magnet and the iron ends that look like interlaced fingers. When current is applied to the inner coil one end of the assembly becomes a magnetic North pole and the other ends becomes a magnetic South pole. Because the fingers interlace they become alternating North and South poles all around the outside of the rotor. As the rotor spins these magnetic poles and their associated line of force are passed by the stator windings producing current in one direction as the pole approaches and current in the other direction as the pole moves away.
Diodes are necessary in the alternator to convert the alternating current produced in the coils to direct current usable with a battery system. Each diode is an electrical switch that allows current to flow in one direction only. A positive diode will have its base attached to the positive output stud. A negative diode will have its base attached to the alternator case. Each end of the stator coil is attached to a pair of diodes, one N and one P. One allows current to flow in one direction, the other allows current to flow only in the opposite direction. There are three wire coil ends (or pairs of ends in a Delta wound stator) so a total of six diodes are used. As current flows through the coil the P diode on end and the N diode on the other end will have the current flow through them. As the current is reversed in the coil, the other two diodes now carry the current. Because of the way the diode bases are mounted, the output flow form the alternator is always in one direction only.
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