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NathanAllan
May 15th, 2004, 01:14 AM
I have never really understood the diffeence between an ALTernator and a GENerator. A generator is pretty stright forward, but then noone will explain to me the diffeence between the two. I am interested because I am kinda tired of having a generally non-servicable alternator on my car when I could use a generator and an extra battery to save the juice from it. So could somebody please explain the difference to me?

I hope this is off-topic enough for eeryone :lol:

Nathan

CP/M User
May 15th, 2004, 02:25 AM
"NathanAllan" wrote:

> I have never really understood the diffeence
> between an ALTernator and a GENerator. A
> generator is pretty stright forward, but then
> noone will explain to me the diffeence
> between the two. I am interested because I
> am kinda tired of having a generally
> non-servicable alternator on my car when I
> could use a generator and an extra battery
> to save the juice from it. So could somebody
> please explain the difference to me?

I know that for a Generator to function, it needs
fuel all the time. Without fuel, the Generator
can't run (I thinking in terms of a power
generator).

Doesn't an Alternator only get power when it needs
it (hence save the power source & make that last
longer). Mate, I think that unless you're going to
hookup one of those Wind Generators which stores
power from when you're on the move, that maybe
the only way to effectively have a Generator (well
that's my thought).

> I hope this is off-topic enough for eeryone LOL

I could be wrong about my theory, but then this is
the off-topic section, which means anything is open
to discussion! ;-)

Cheers,
CP/M User.

Terry Yager
May 15th, 2004, 08:06 AM
Technically speaking, there is no difference, an alternator is considered a type of generator, but let's not be picky about that minor detail.

The main difference is that generators are DC and alternators are AC output devices.

define:alternator

A device which produces alternating current (AC) by converting the engine's turning (mechanical) energy into alternating electrical current at all engine speeds. The AC must be rectified (converted from AC to DC) before reaching the vehicle's electrical system. The alternator is driven by a belt at the front of the engine.
www.peiauto.com/dictionary/a.shtml

This unit converts the 12-volt DC (Direct Current) from the battery into AC (Alternating Current) at the rate of 13.8 to 14.2 volts, which is necessary to operate your car's performance and electrical systems. By nature, alternating current is generated at a higher rate and is more stable. This is critical to automotive application.
www.carshowtomt.com/glossary/Welcome.html

As per the definitions above, AC output is greater, at the same RPM, and the AC current is more stable, therefore easier to regulate and otherwise manage. (Another big difference is that the alternator's output is at a constant rate, regardless of rotational speed, whereas the generator's output is variable, depending on RPM).

I see no reason why you could not convert your car to use a generator if you wanted to. Hot-rodders have been doing just the opposite for many years. There are even kits available to convert your vintage automobile over from generator to alternator, so why not go in the other direction?

--T

Rick Ethridge
May 18th, 2004, 08:28 AM
I have never really understood the diffeence between an ALTernator and a GENerator. A generator is pretty stright forward, but then noone will explain to me the diffeence between the two. I am interested because I am kinda tired of having a generally non-servicable alternator on my car when I could use a generator and an extra battery to save the juice from it. So could somebody please explain the difference to me?

I hope this is off-topic enough for eeryone :lol:

Nathan

Generally generators use permanent magnets to create direct current power. Alternators use inputted direct current to provide an electromagnet and create alternating current. As long as there's a source of dc running to an alternator, and the unit is driven, there will be power created. Alternators will sometimes have internal rectifiers to convert the AC to DC on the unit. RLE

dongfeng
May 19th, 2004, 11:33 AM
I believe they are opposites to each other. the generator (or dynamo as they're called over here) has the coils in the commutator, whereas the altenator has them in the case.

Altenators are much more effective than dynamos at charging, hence most cars changed over in the late 1960's.

Dyanmos are a breeze to rebuild though! The one in my Volkswagen has an external regulator (the car is 6v electrics). Altenators usually had an internal one.

CP/M User
May 22nd, 2004, 01:46 AM
"dongfeng" wrote:

> Altenators are much more effective than dynamos at
> charging, hence most cars changed over in the late
> 1960's.

So, in theory, would it be better to buy a car from that
period, or get your existing car & modify it?

Cheers,
CP/M User.

booboobear
October 23rd, 2006, 07:49 PM
"Alternator" is jargon for "alternating current generator."
All electromechanical generators generate alternating current internally.
Since 1993 the SAE (Society of Automotive Engineers) has discouraged the misleading term "alternator," calling on all to return to the proper term "generator."
A commutated generator is made like a "universal" motor. It has windings on a rotating armature (frame the wires are wound around), the "rotor," that spins within an outer set of coils, called the "stator."
The commutator is a switch that changes the connections between the rotating coils and the output terminals in time with the changing polarity of the current coming from the rotor coils, resulting in pulsating direct current.
The disadvantage of the old-fasioned commutated generator is that the commutator has a lot of friction, the current capacity of the brushes rubbing against the commutator is low for their size, and the whole thing generates a lot of waste heat in comparison to the power it generates.

The "alternator" (sorry, SAE) has a small amount of direct current running through brushes on slip rings, which are like the commutator in a way but are continuous rings of copper rather than a single ring cut into segments with each segment connected to a loop of the wire wound upon the rotor armature.

Each slip ring is connected to one end of the rotor winding.

In the old-fashioned commutated generator both the rotating coils and the stationary coils carry heavy current. In the modern rotating field generator the slip rings and brushes carry only the relatively small field current, called the "excitation" current. This is a great advantage along with having no gaps in the copper rings.

A large automotive generator of the commutated type would generate perhaps 30 amps at full speed. A modern rotating field generator will be rated 85 amps, and that's a little one on a small car. Cars with A/C, electric windows, electronic fuel injection, and lots of other power accessories will have at least a 100 amp. generator, and many have 125 or 150 amp generators.
These generators use solid-state diode rectifiers to change the AC generated in the stator (stationary) coils into the DC used in the car's electrics. DC is used because of the needs of the chemical starting battery, which can be charged only with DC and can put out DC only. A (usually for the last 30 years or so) solid state voltage regulator meters the excitation current sent through the brushes and slip rings to the rotor coil to keep the output voltage within narrow limits, around 13.8-14.6 volts direct current (14.6 V is the limit for the "12 volt" battery). This regulator is now usually mounted inside the generator case, but some applications still mount it in a box elsewhere in the motor compartment, like they all were 40 years ago.

Better generators used on stationary units use the brushless design. A small generator on one end of the rotor is used to generate current to excite the main coils of the rotor. A diode bridge rectifier is mounted within the rotor to turn the AC coming from the exciter coils into DC to excite the main field coil. This costs more to make, especially in the small sizes of auto generators, so the brush-type still rules. you will find it it higher-grade generator sets used on motor homes, as emergency power generator sets for homes, hospitals, etc.

DimensionDude
October 24th, 2006, 02:08 PM
Good info, folks!

In my experience, the generator in the air-cooled VWs is easy to maintain and repair. The commutator doesn't wear appreciably and the brushes are easily replaced. The externally mounted voltage regulator is easy to replace, too. The disadvantage is the low output (about 30 amps as noted by BooBooBear). This is adequate if you don't have a bunch of electrical goodies in your car.

I repaired an alternator in a 1970-something Ford Maverick by replacing the diode trio (alternators are 3-phase) and the brushes. This design also had an external regulator.

The advantages of the alternator are high output and compact design. Also, you get a significant percentage of the rated output at relatively low engine speed.

Note that most alternators have internal regulators now. I believe that they can be replaced by the average Joe, but it's not always an easy job.

I haven't rebuilt an alternator in quite a while, I bought the "lifetime warranty" alternator at Auto Zone for my Isuzu Trooper and had that exchanged for free when the bearings failed.

Kent

Chris2005
October 24th, 2006, 04:00 PM
"I repaired an alternator in a 1970-something Ford Maverick by replacing the diode trio (alternators are 3-phase) and the brushes. This design also had an external regulator."

Alternators produce 3-phase? I forget how many diodes it takes to make a rectifier (something tells me 2 or 4, but 2 might just be a half wave rectifier).

Terry Yager
October 24th, 2006, 06:48 PM
Speaking as a mechanic, not an electrical engineer, yes, the diode sets in an automotive 'alternator' come as a trio, usually sold as a complete component, FWIW.

OTOH, I've replaced/rebuilt diode bridges in everything from a Tilt-A-Whirl carnival ride to a pinball machine, and the diodes usually come in two pairs, oriented opposite one another. I dunno why there's three of 'em inna automobile alternator.

--T

DimensionDude
October 24th, 2006, 07:16 PM
"I repaired an alternator in a 1970-something Ford Maverick by replacing the diode trio (alternators are 3-phase) and the brushes. This design also had an external regulator."

Alternators produce 3-phase? I forget how many diodes it takes to make a rectifier (something tells me 2 or 4, but 2 might just be a half wave rectifier).


Yeah, 3-phase, half-wave rectified with one diode per phase. I suppose that full-wave rectification (4 diodes per phase) isn't cost effective. Two diodes can be used for full-wave rectification but it requires a center tapped coil. Using a full-wave bridge (4 diodes) gives you a DC output approximately half that of the AC input.

As an aside, the car's battery serves as a filter, but not a very good one. You can still get alternator "buzz" in audio components with sub-standard input power filters.

Kent

Terry Yager
October 25th, 2006, 09:13 AM
Is that why my radio always buzzes? I thought it was just picking up some stray RFI from the (old, unshielded) plug wires.

--T

Brendan
October 25th, 2006, 11:13 AM
The generally accepted source of car stereo buzz (which varies with engine RPM) is the alternator, and I recall following this line of thinking and actually placed 2 filters between my stereo power input and the battery, one after the other when the first didn't help much. Eventually I did replace the old stereo (Realistic tape deck) with an Alpine tape deck/CD changer controller, and that change completely resolved the buzz issue. I'm guessing, then, that the Alpine tape deck is better shielded...

Incidentally, I got tired of replacing the external regulator for the ~40 amp alternator on the same 1970 Toyota Land Cruiser (as it seemed to go bad about once a year and only provide charge at the low/maintenance rate), so more than 10 years ago Dad and I replaced the alternator with a ~70 amp internally regulated rebuilt tractor alternator. Haven't had a problem since! *knocks on wood*

Brendan
October 25th, 2006, 11:27 AM
Sorry, a slight clarification on the above - I made the assumption that the Alpine deck was better shielded against RFI which was being put out by my plug wires since the filters helped so little.

Dwight Elvey
July 6th, 2013, 07:20 AM
I can help but comment on this old post.
Some of the explanations of the two were a little off.
A generator is considered to have a fixed field and a rotating
armature.
An alternator has a rotating field and fixed stator winding.
Both generator and alternator in a car used an variable field
to control voltage.
Both generators and alternators in a car have brushes. Alternators use them on slip rings
rather than a commutater. The brushes are about the most common failure in either type.
Alternators produce AC that must be rectified with diodes.
Alternators are a subset of generators when the word generator is used to indicate
a class of electric current producing devices but in a car they refer to the
two types.
Dwight