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Old 01-02-2019, 05:08   #1
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Alternator Output

When I was having trouble fitting my large 24V 60A alternator to my Perkins I told my mechanic mate I was tempted to fit a 45A alternator that was half the size.

He told me to stick with the large alternator as it was a bit like comparing a 400HP engine in a street car with a 400HP truck motor. He said one will output the rated HP for a short time and the truck motor will output 400HP continually. Is that a good analogy?

NEXT QUESTION My large 24V alternator outputs 60A but when I research the internet the identical sized alternator on a later model Toyota outputs 85A. Without getting too technical what determines the output of an alternator and can I tweak mine up?

Clive
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Old 01-02-2019, 05:19   #2
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Alternator Output

Itís either been de-rated by the internal regulator, or wound differently to reach a max output of 60 amps.

If wound de-rated, you can feel very confident (not absolutely) that it will be able to output 60 amps continuously. This is better than an 85 amp alternator that becomes hot quickly and the output drops to 50 amps within a few minutes.

Of course all of the above may be wrong.......
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Old 01-02-2019, 13:33   #3
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Re: Alternator Output

Go to these links and do some learnin'


https://marinehowto.com/


Electrical Systems 101 Electrical Systems 101
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Old 01-02-2019, 15:13   #4
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Re: Alternator Output

Quote:
Originally Posted by Stu Jackson View Post
Go to these links and do some learnin'
https://marinehowto.com/
Electrical Systems 101 Electrical Systems 101
WOW! That looks like an excellent site

Now I have the alternator mounted I was going to ask if it is OK to start the motor up without wiring up the alternator. I presume it would be OK as there would be no field current.

(I've got a lot of learning to do but I really do not want to become an marine/auto-electrician)

Thanks

Clive
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Old 01-02-2019, 21:08   #5
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Re: Alternator Output

Quote:
Originally Posted by Stu Jackson View Post
Go to these links and do some learnin'
https://marinehowto.com/
Electrical Systems 101 Electrical Systems 101



That's a good site but these two sources are first class.




MikeML AAC Fanatic!

Actually field current controls output current, not output voltage.

In a typical automotive alternator, the field resistance (measured at DC) is about 5 to 8Ω, so the max field current when connected to 12V is ~2A.

At a given RPM, the alternator acts as a current-controlled current-source; the output current is proportional to field current, so if you connect 12Vdc to the field, the field current will be about 2A, the output current will be about 50A, so the current gain is 50/2 = 25. If the output side of the alternator is open circuited with full-on field excitation, the output voltage will shoot up to hundreds of V (hence current source, not voltage source).
The typical automotive voltage regulator senses battery voltage, and controls the field current using PWM, which determines the output current such that it just exactly matches total load current, keeping the battery at a nominal 14.2V. A simulation of this behavior is shown here.



So just by changing the field current that will change the alternator output amperage. Maybe that explains how my alternator is only 60A while others exactly the same size can be 85A.


Alternator DEMO Wiring, connection to Battery, Capacitors, Inverter, Modification

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