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Automotive and some marine alternators typically have an efficiency curve with a peak of 60%.

alternator efficiency curve http://www.intechopen.com/source/html/38166/media/image3_w.png
Image from http://www.intechopen.com

Whereas alternators to other types of applications reach 96% of efficiency (excited rotor type).

What are the factors that makes then so inefficient compared to other applications? Is pole-number, air-gap? Could the efficiency be improved with some modifications for stationary use or the unique way is maintain the speed at the peak of efficiency?

UPDATE

@olinlathrop suggested somethings, among that the environment the alternator works (temperature, vibrations, dust) means their robustness can be a trade off to the efficiency.

I agree in some points, although there's no objective answer, good sealed bearings would not decrease the efficiency that much, so I think temperature can be one of the factors as they are small and works near a combustion motor, even trough have their own forced ventilation.

Anyway where is an alternator with stated 80% efficiency at 450A 24A! That's ~10Kw. http://www.emp-corp.com/media/MarketingMaterial/Power450/SpecificationSheets/Power450.pdf

The difference is that this alternator uses magnetics instead of electro-magnets in the rotor. Anyway, the ~95% efficiency alternator product sheet I read is a excited rotor one.

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I don't design automobile alternators, so I can't say exactly what goes into the engineering decisions. However, here are some reasonable speculations.

Alternator efficiency is simply not a big deal in a car. The power the engine has to put out to move the car dwarfs what the alternator requires. If this tiny fraction of overall motor power were 1/3 less it wouldn't make much of a difference. Therefore efficiency can be traded away to get other more important parameters. Some of those probably include high reliability in a harsh environment, operation over a wide temperature range, and keep going while being splashed with water containing dirt and road salt and other crud. The volumes are very high, so keeping cost down must also be a major desire.

First, look at the cost of a 90% effecient generator of the same power compared to a car alternator. I expect it will be several times more. Then try operating the high-efficiency generator in a harsh environment like under the hood of a car and see how long it survives. Car alternators routinely survive this for 10-15 years. The high end efficient generator that cost several times more probably won't last a month in bad conditions.

It's all about what's really important and making the appropriate engineering tradeoffs in the design.

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    \$\begingroup\$ Another factor probably relates to what one must do to maintain a constant output voltage at differing speeds. Purpose-built engine-and-generator assemblies can be designed to run the engine at whatever speed works best for generation. Automotive alternators have to run at whatever engine speed makes the car travel with the desired velocity. \$\endgroup\$ – supercat Jan 10 '14 at 21:51
  • \$\begingroup\$ Thanks, I know that the internal combustion engine itself is normally 25% efficient. My question is really at the design level, not why the industry go this way. Something we can suppose is over dimensioned bearings, because of dust, vibrations, and temperature. Anyway that will not take efficiency to this point. Also they are air cooled, like the other, anyway I don't know the temperatures it experience in the hood, this will alter windings parameters, and so can be a more considerable factor. I'm not in this industry too, but with magnetics and some mechanical knowledge we can speculate. \$\endgroup\$ – Diego C Nascimento Jan 10 '14 at 21:54
  • \$\begingroup\$ @supercat the voltage is regulated by changing the excitation voltage. The effiency varies greatly with speed if you see the graph, less than 40% at near 8000rpm (well I think no one will put your motor at this RPM). \$\endgroup\$ – Diego C Nascimento Jan 10 '14 at 21:57
  • \$\begingroup\$ Correction, it is belt driven, so it can have a different ratio than the motor shaft RPM. \$\endgroup\$ – Diego C Nascimento Jan 10 '14 at 22:04
  • \$\begingroup\$ @Diego: Car engines aren't anywhere near 25% efficient. \$\endgroup\$ – Olin Lathrop Jan 10 '14 at 22:05
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Voltage: at 14v it is very hard to be efficient.

  • diodes loss: car alternator operate around 14v, with 0.6v for each diode pass you have 1.2v loss: almost 10% loss only this one.

  • windings: you loose a lot of power over resistive loss in the winding because of the high current for the winding size (you could compute this loss with the winding gauge at a given current)

  • Connections: it's very easy to loose a few percent of efficiency at this current and voltage: a 0.1 ohm connection at 65A would be 6.5v across, loosing almost 50% of the power!

  • core loss: at higher speed core loss are probably increasingly important (big guess trying to explain the graph)

I think we could get a far better efficiency just by operating the same alternator at a higher voltage.

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  • \$\begingroup\$ Welcome to EE.SE! Unfortunately, your answer doesn't seem very realistic. To address the middle two points: the windings are designed for the currents and the connections are much lower resistance than 0.1ohm, when bolted down correctly. \$\endgroup\$ – user2943160 Jul 18 '16 at 3:26
  • \$\begingroup\$ I think it's more a tradeoff (of the I2R losses) between cost and efficiency. But, what you say to be the voltage (electric potential), is not directly the question, it's the current in the case of I2R losses. And big generators work at even bigger currents and are far better efficient. Anyway today alternators tend to be more efficient as the automotive market uses more electrical devices. \$\endgroup\$ – Diego C Nascimento Jul 18 '16 at 16:14
  • \$\begingroup\$ I forgot a point: that graph is at maximum power for a given RPM. I think that's why the efficiency is so low and the resistive loss is high. In case of lower current the efficiency is going to be better as the resistive loss are RI2. \$\endgroup\$ – alfor Oct 21 '17 at 21:46
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I believe that a good part of the low efficiency can be attributed to the fact that the individual coils don't charge the battery until their voltage exceeds the battery voltage + 2 diode drops. If the peak coil voltage is 16V, and the battery voltage is 12.6V, then no current flows from a coil until the AC waveform of that coil exceeds about 14.6V (12.6V + 1V + 1V). So each winding produces no current at all until its voltage is above 14.6. While it is true there are 3 phases, which greatly reduces the battery ripple current, that doesn't change the fact that each individual coil doesn't produce any current at all for the majority of each cycle. Such a low utilization of the coil would have to have a negative impact on the overall efficiency.

That is unless an alternator is a constant-current source and not a constant-voltage source...

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  • \$\begingroup\$ alternators are not designed for efficiency - they are normally bolted to an engine of some 150+ bhp and so who cares if they take 10 or 15.... Oh and they are not constant current either... \$\endgroup\$ – Solar Mike Nov 17 '17 at 20:11
  • \$\begingroup\$ But I would think that when cars are shedding all the weight they can, every HP saved would make a measurable difference in mileage. This looks like an area ripe for the application of electronics. \$\endgroup\$ – Charles Linquist Nov 18 '17 at 20:44
  • \$\begingroup\$ And that is why some alternator charging control systems do exactly that ie match output precisely with the needs of the car and battery , not only that some also cycle the air conditioning load as well, but it depends a lot on the price and quality of the car : the charging system on my car does match itself to the load and battery needs very precisely, in fact it also charges at up to 15.2 v at some points - before you ask yes I have an accurate meter... \$\endgroup\$ – Solar Mike Nov 18 '17 at 22:10
  • \$\begingroup\$ @solarmike Yes some ECU controls the alternator, I believe that by controlling excitation current. But 10 HP is much power, some air intake systems not cheap, are designed to add less than this. Also today with more electronic devices, the demand for electric current is growing, as well as a better efficiency. \$\endgroup\$ – Diego C Nascimento Nov 19 '17 at 21:21
  • \$\begingroup\$ I performed a LTSPICE simulation and found that I could greatly increase efficiency by using FETs to act as boost converters,using the coils inductance as the converter's inductors. I ran the switcher at about 22 Khz and used synchronous rectification. \$\endgroup\$ – Charles Linquist Nov 19 '17 at 23:04
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Car alternators are way better than DC generators because they are more effective and produce enough electricity to power everyrhing you need at all rpm ranges. The trouble is that they as you find out of name produce AC while everything in your car needs DC. So on entry you loose some efficiency. The current must be also stabilized - next loos. Inspite of all loses they are still better than DC generators that barelly can power headlights of a car. And when something works well why to fix it?

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  • \$\begingroup\$ Im not promoting dc denerators but saying dc generators can barely turn on headlights is wrong. Proper sizing can make DC generators that produce more than enough power. Rather, dc generators have been wrong because voltage regulation required expensive and inefficent dc to dc conversion and commutation causes more brush wear than slip rings on an alternator. Granted dc to dc conversion efficency and cost may have come down. But you still have more ware and the expence of perminate magnets that need to work in elevate temperatures under the hood. \$\endgroup\$ – Keith Reynolds Oct 4 '18 at 12:43

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