.707 of peak? Less tiny losses from the rectifier components?
It would be the same peak voltage.
So does a generator.
The real difference is strictly technical. A generator creates a current flow in only one direction.
An alternator creates a current flow that goes back and forth in both directions. Hence, it alternates. Hence, the name “alternator”.
A rectifier simply takes the “negative” wave, the downward swing, and flips it over using a “bridge circuit” which is an arrangement of diodes arranged to flip the downward swing. It then sends it through a “filter”, which is a circuit of capacitors and resisters arranged such that the capacitors constantly charge and discharge in a controlled manner, “chopping off” the peaks by absorbing the electrical energy and then “filling in” the valleys with the electrical energy that the capacitor has stored. If the components suffered no losses in their jobs, the mathematical result would be an average DC voltage out of .707 of the peak sinewave. Some energy is lost, however, in all electrical devices as heat energy, so the number is never perfect. I don’t know what the acceptable loss is in an automotive alternator or generator & rectifier circuit.
An automotive “generator” is strictly any device that converts rotational energy to electrical energy. An “alternator” is a device that converts rotational energy to electrical energy but in a sinusoidal wave, with current going first in one direction and then in the other direction.
Alternators are used in current cars because they can convert the rotating energy to electrical energy more efficiently. The output then needs to be converted to Direct Current, because it needs to be stored in the battery and Alternating Current cannot be stored.
All alternators are generators, but not all generators are alternators, just as all squares fit the definition of a rectangle but not all rectangles fit the definition of a square.
I sincerely hope this helps explain the question. Or, I could be crazy. Some have suggested the latter.
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Generators of old were big, heavy, expensive to manufacture and had brushes that carried high currant leading to a short service life. Their output was limited at low speed…
Harleys still used generators up through the 70s and into the 80s on some models although they were the 2-brush variation; an improvement over the old 3-brush units.
It might be noted that some permanent magnet alternators generate electricity on their own and the majority need help from a battery source.
I do know alternators generate electricity which makes the headlights work, so if you have a bad alternator, you cannot drive at night. I learned that the hard way about 20 years late at night on a country road in Wisconsin.
Thanks for the historical perspective, the energy/electrical details and of course, a math problem!
“.707 of peak? Less tiny losses from the rectifier components?”
"It would be the same peak voltage."
Actually, in an idealized, three-phase full-rectification alternator, the output voltage varies between √3 Vpeak and 3/2 Vpeak. That is, if Vpeak were the peak votage generated by each of the three stator windings, the alternator output voltage would vary between 1.7321 Vpeak and 1.5000 Vpeak. See sketch below.
Why this is so can be seen by examining a typical alternator schematic:
For sake of discussion. let’s label the three stator windings in the “Y” configuration as A, B, and C. (It doesn’t matter which is which.) At some point in time, A will have the highest, positive voltage, B will have the lowest, negative voltage, and C will be somewhere in between. Then a complete electrical circuit can be sketched through winding A, through some diode into the battery, out of the battery through its grounding strap (not shown), into the alternator again through its ground, then through another diode and into winding B. Note that winding C does not play a role in any of this. A voltage is generated in winding C, but any current it tried to produce would be blocked by diodes from passing tnto the battery.
At a later point in time, B may have the highest positive voltage, C the lowest negative voltage, with A lying somewhere in between. The electric circuit then goes through windings B and C, leaving out A. And so it goes …
The point to be taken from all this is that at any moment in time, only two of the three windings are contributing to the output voltage of the alternator and generating a current through the battery. (it’s not exactly true that only two stator windings contribute to the alternator output voltage at any one time; there is some overlap with the third winding. But that would muddy the waters for purposes of this discussion. Let’s just say it’s “almost” true.)
Now let’s apply this observation to the sketch of full-wave rectification above. Consider the time 0.50T when the output voltage is a maximum. This occurs when the green sine curve is at 60* (sin 60º = 0.8660 = √3/2) and the red sine curve is at 300º (sin 300º = -0.8660 = -√3/2). Thus the output voltage at this maximum is (√3/2) - (-√3/2) or √3 Vpeak. The value of the blue sine curve is immaterial at this point — it does not contribute to the output voltage, as its values lie between the green and red curves.
Just to the right of this point at 0.5833T, the blue curve becomes the most negative at 210º (sin 210º = -0.5), the red curve becomes the intermediate curve and drops out of consideration, while the green cutve reaches its maximum at 90º (sin 90º = 1.0.). This yields a minimum of the alternator output voltage of (1.0) - (-0.5) = 1.5 Vpeak.
A little mathematics would show that the dc voltage is given by Vdc = (3√3/π) Vpeak,
or Vdc = 1.6540 Vpeak.
A similar expression for Vrms in terms of Vpeak could also be derived, from which one could determine the ratio of Vrms / Vdc, independent of Vpeak. This would provide a guide to the acceptable ripple voltage when testing for diode leakage. Actual ripple voltage will be less than the value theoretically calculated, as the battery acts as a large capicitor in a low-pass filter. A ripple voltage of 100 mV or less is considered fine; a ripple voltage of 500 mV or more could indicate leaky diodes.
@Mechaniker I did just fine in my one electrical engineering course until we got to three phase power. Thank you for making my head hurt again.
The nomenclature isn’t just a translation thing. The alternator in my Jeep Compass is called a generator in the shop manual. I’ve seen generator mentioned on other cars too.
I think it’s safe to say that all modern cars (certainly after 1980) have what we would call an alternator.
Well done, mechaniker. A tip of the hat to a truly superior clarification… and, for me, a correction. Ah, to be young again…
TSM
Re: the terminology, while the terms originated as descriptives, they’ve become just nomenclatures, interchangeable to all but the technical community… and apparently almost interchangeable there too. I can live with that.
I think the Society of Automotive Engineers changed the term for any device that creates current to “Generator” from Alternator sometime in the 1990’s. This changed the common definition of an Alternator being an A/C power generation creator and a Generator being a DC power creator. Manuals followed suit.
Alternator is a marketing term for a three phase generator with a diode bridge used in automobiles. BTW, the big wind generators are also three phase generators with a rectifier bridge, and they are not called alternators.
Mechaniker, sorry but you got it wrong. The 1.732 value is the difference you get in a three phase system when you measure phase to phase vs phase to ground. In a typical three phase distribution system, you will see 13470Y7200VAC which translates to 13470 volts phase to phase, 7200 volts phase to ground. The Y indicates a 4 wire transmission line.
In a modern automotive generator, aka alternator, Vavg ≈ .91Vpeak. Vavg cannot be higher than Vpeak. I do like your drawings though.
For all those who got the .707 figure, that for a single phase. In a full wave rectified, but unfiltered single phase, the transition from one half wave to the next occurs at 0V. In a half wave rectified but unfiltered three phase system, the crossover from one wave to the next occurs at 50% of Vpeak. In a full wave rectified three phase unfiltered system, the crossover is at .866 Vpeak
The cosine of 60 degrees is .5. In a three phase system, the crossover occurs +/- 60 degrees for a total of 120 degrees of waveform. This is the reason that all electrical power is generated in three phases, not 2, not 4 or 5 etc.
“In a full wave rectified but unfiltered three phase system, the crossover from one wave to the next occurs at 50% of Vpeak.”
I get the crossover at 0.866 of Vpeak:
I’m sure Juanita is now Dazed and Confused (credit to Led Zeppelin) after the charts and explanations…
Regarding wind turbines, the farm here has been in operation less than 2 years and the crane crews are out there now about to replace generator/transmission for the eighth time out of 140 units; and that doesn’t include the small potato stuff.
Those things require more maintenance than an old air-cooled VW Beetle…
That 25 year service life projection is not looking too good.
@ok4450
I have talked to people who service wind turbines, they are known to be maintenance intensive and problematic. They are green source of energy but they produce “dirty” power.
Wind turbines generate a sine wave of variable frequency in order to be able to take advantage of the full range of wind speeds. However, the grid only operates at 60Hz, so the variable frequency is converted to DC and then an inverter is used to convert the DC signal to 60 Hz AC. This is the signal that is put on the power line. Most inverters generate an extremely “dirty” signal, which is a 60Hz waveform polluted with a lot of high frequency transients.
Dirty 'ol wind turbines.
The major advantage of an ac alternator(used in gas vehicles vs the dc generator (used in old vehicles and now in treadmills, and varying sizes in many other apps) Is getting the power “out” of the windings. The basic design of the DC “brush” generator requires all output power go thru a “brush-commutator” connection and is practically limited to about 30 amps and requires frequent
brush replacement and commutator cleaning.
AC generator (ALTERNATOR) design eliminates this 30 amp limit by only requiring a very small excitation current to create a magnetic “field” in the rotor(rotating) windings. Power is provided (by auto motor, wind turbine, water turbine, etc)to rotate this “field” inside inside the much heavier “stator”(stationary) windings. This is just the opposite of a DC generator, which must have heavy windings in the spinning portion and heaving brushes to carry the current out. The alternator can provide a heavy current on the “stationary” wound stator, but at the disadvantage of being AC current instead of DC. That’s why you have the diodes, they convert the AC to DC.
The alternator ALWAYS requires some “feeder” current, even after it gets up to speed it can only provide this self-sustaining current thru a diode/regulator system. This current MUST be DC, so unless you want to keep providing this DC power from a battery (and it WILL add up) you will need a properly functioning regulator (either internal or external) on your alternator.
Unfortunately efficiency is not that great in a small alternator like autos use, about 50%. I don’t know, but my guess is the DC brush generator is even less efficient.
The huge AC generators at a coal-fired power plant exceed 95% efficiency. I don’t know, but my guess is the old DC generator is even less efficient. I imagine small alternator designs will emerge that are much more efficient, but the REAL advances are taking place on the DC motor/generator front. It won’t be long before they are mass-producing some very efficient backyard generation units based on BRUSH-LESS DC generation.
insightful, I caught my mistake in proofreading (after posting of course). I guess I was correcting it while you were posting.
Where’s Rick, you are correct about wind turbines, but after the inverter, the voltage is stepped by two transformers, the first one at the turbine steps it up from 690 volts phase to phase to 34,500 volts phase to phase (690Y400 to 34500Y19920). From there it goes to a substation to be stepped up to 115kV. Transformers filter out the high frequency components so by the time it leaves the farm, it is pretty clean.
Also the inverters are a little more sophisticated than the one you have plugged into your dash to charge your computer.