Re: Generator vs. Alternator


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Posted by Gordon Maney on Sunday, November 17, 2002 at 12:27PM :

In Reply to: Generator vs. Alternator posted by Mark Toper on Saturday, November 16, 2002 at 6:56PM :

There are several important differences between alternators and generators.

The alternator will charge the battery better at lower speeds, since the alternator does not use a cutout relay like the DC generator regulator. As long as alternator output voltage is higher than battery terminal voltage, current will flow from the alternator to the battery. The alternator uses a rectifier bridge to change the AC to DC, and also prevent current flow from the battery to the alternator.

The cutout relay was one of the three units used in the DC generator regulator. HIstorically, it was an electromechanical device, meaning magnetically controlled mechanical switches. This was true of the voltage limiter and the current limiter, the other two units on the three unit regulator. In some later years there were solid state regulators for DC generators, but you PW people will not have those, probably only the M-series people will see them.

A regulator comparison.... the alternator regulator has no current limiter. It does not need one because an alternator of a given physical description will be self limiting in current output. There may be one additional unit in an alternator regulator, that being a field relay to turn on field current during operation.

In the DC regulator current limiter, you have all of the output current flowing through the windings of the unit, with temperature rise becoming an issue. Anytime we have full output current running through something, we raise the possibility of increased failure rate.

Sorry, back to the cutout relay... The cutout relay disconnects the battery from the generator at a voltage that is higher than the highest terminal voltage that the charged battery may attain. Specifications may vary, but the point is that it disconnects the two units at a range of voltage in which the alternator and the battery would still be connected. Consequently, we are not availing ourselves of some important charging opportunities, those being at low speeds.

Another difference is that the sliding brush connection in the alternator is a low current with the brush running on smooth slip rings. The DC generator brush is running on an interrupted surface and carrying high current, meaning the output current. If you have a 60 amp DC generator, at maximum output all of that current is being handled by that brush/commutator connection. Hence, brush life in an alternator is enormously greater than that of a DC generator. Enormously.

One advantage, realistically minor, that a DC generator has over an alternator, is that a DC generator has residual magnetism in the field winding cores, so it can generate some field current with a dead battery and begin the charging process. An alternator can not "self excite" its field, and so, with no battery voltage to excite the field, it has no output. I will prudently choose to omit any humorous reference to self-excitement in this discussion.

An interesting side note will require a definition of a term. Armature is a term that has several definitions. The DC generator has a rotating part called the armature. Also, the term armature is used to refer to the conductor in a generating device in which current is generated. So, an alternator includes wires in which current is generated. Those conductors are part of a component called the stator. The rotating part of an alternator is called the rotor, and it provides the magnetic field.

Now, getting to the point, since I know you are desperately hoping I will.... both devices have "armatures" of the second definition, and both of them produce AC in those armatures. Both devices utilize rectification, which is a change from AC to DC. The alternator does it with diodes (electrical check valves, so to speak), and the DC generator uses the cooperative efforts of the segmented commutator on the armature (both definitions now!) and the brushes.

So..... if AC is prouduced in both, and DC is the output of both, then why do we call one of them the DC generator and the other an alternator? Danged if I know.

In summary, then, for those of you still awake.... The alternator charges better at low speeds, and lasts longer due to longer brush life, and has a simpler and longer lasting regulator. Once the alternator came into production, little more development work was done with DC generators. Consequently, you won't generally find high output DC generators of a physical size suitable to use in a pickup truck. There were much higher output units used on buses, big trucks, and construction equipment, but you almost have to have a shop crane to move the things.

You can buy some really nice alternators with high output today. They are a good thing.





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