Generator vs alternator

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Originally submitted by T: Back to Electrical Index

Note the toothed Fan Belt for increased purchase on this 12 Volt Generator. Large Alternators can benefit from the use of one of these. Click to Enlargen. Photo by Sherro.

It can be argued that there is an ideal environment for each.

Alternator Advantages:

It's no secret that an alternator is a very efficient and high output device compared to a generator. An alternator, having the current generated in the outer windings (called the stator) provides an alternator with a distinct advantage. The stator windings can be thick high capacity cable, they can be long and it's easier to induce the higher voltage and current in them by exploiting the dv/dt factor of a generating device. The dv/dt factor means that the faster a magnet goes past a wire, the more power the magnet induces in the wire. The stator windings of an alternator can be made thick and plentiful because there is increased space available to accommodate them. The voltage in an Alternator is produced by having an electrically controlled electromagnet rotated by the alternator's centre shaft.
85 amp Bosch Alternator on a 202 powered UC Torana. Note the Alternator uses a Fuel Injector type Connector. Click to Enlargen. Photo by T.
 The Regulator measures the output of the alternator and increases the current feed to the rotor when the voltage is low, and decreases the current to the rotor when the voltage is high. The Regulator supplies pulsed current to the rotor, too much at one instant followed by too little at the next instant. The average of too much/too little is the correct output voltage. Alternators can automatically current limit once the loading on the magnetic flux is exceeded that's why they don't need a current limiting relay. Because it uses diodes to rectify the alternating current generated in the stator windings, alternators cannot self excite. They are not able to generate sufficient voltage from the residual magnetic flux stored in the rotor's winding to overcome the forward bias voltage of the diodes. The diodes need in excess of 0.5 volts applied to them before they will conduct. Alternators are 3 phase devices. They are effectively three alternators in one, connected in parallel but 120 degrees apart so they deliver power in sequential intervals. Because an alternator has 6 power diodes it allows the output of the 3 phases to be connected together further boosting the device's efficiency. It also has 3 exciter diodes which are used by the regulator to excite the rotor once the alternator starts producing power.

Alternator Disadvantages:

Generally an Alternator cannot self excite at as low a speed as a generator, but some of the late model high capacity can self exciteif the RPM becomes high enough.

Generator Disadvantages

A generator, having the current generated in the armature, or centre rotating part of itself, has limited ability to produce power (for the same physical dimensions as an alternator). Because the diameter of the rotating output windings (called the armature) is about half that of the stator windings in an alternator, a generator does not gain from dv/dt switching. That is the windings pass the generator's magnets at a slower speed, even if the armature is rotating at the same speed as the alternator's rotor so less voltage and power can be induced in them. If the armature windings are made large to permit more current to be produced, the voltage output will diminish due to the reduction in the number of windings held in the restricted space of the armature. A generator uses stationary windings connected to the outside of the case to provide the magnetic flux needed to generate the output voltage and current. So a generator's windings are in the opposite position from an alternator. A generator's electromagnet is formed in the outer windings called the "field". Current flow into the field determines the amount of voltage that is generated by the device. When the battery voltage is low, the regulator feeds a high current into the field. When the battery voltage is high, the regulator feeds a low current into the field. Just as in the alternator's case the regulator provides a field current that is at one instant too high, the next instant too low. The average of the high/low output is the desired output voltage. Because there is minimal voltage drop across brushes that have been properly bedded, generators can self excite at a significantly lower speed than alternators. Be advised that a self excited generator can reverse its polarity if it is dropped, hit or bumped in any way. A self excited generator should have its output checked to ensure it has not reversed its polarity. A battery excited generator, one that is in the car, will automatically come up with the correct polarity because the generator light will "flash" the generator's "field" into the correct magnetic sense. Generators are capable of producing far more current than they are rated for, and doing so will exceed the current rating of the armature windings and they will burn out. Therefore, they must have the protection of a current regulator. The current regulator adjusts the generator output by varying the field current in exactly the same way as a voltage regulator does. Between them, they protect the whole system from overvoltage and overcurrent.

Generator Advantages:

A generator can self excite.



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