How powerful of a motor do you need to run an alternator?

From questions I have read and step by step’s I have read a question came to my mind.
How powerful of a motor do you need to run an alternator?
I checked the Delco Remy alternator series from 10SI in 37 Amps, to the 40SI durable brushless model, in 240, 275, and 300 Amps.
All their performance charts show rpm’s at the alternator not the motor.
The pulley on the crankshaft is considerably larger than the pulley on the alternator, giving you more rpm’s at the alternator than at the motor.
All their performance charts show rpm’s from 1200 to 8000 rpm’s.
All their performance charts show at 3000 rpm’s you get 13/16ths of max power.
The last 5000 rpm’s only gaining 3/16ths of power.
The thing is none of the performance charts say the horsepower to drive the alternator.

I decided to approach the problem from a different direction
Typical alternator efficiencies are in the 54%-60% range.
Brushless alternator efficiencies are in the 60% 70% range.
A typical 12v 60 Amp alternator produces 720 watts at 55% efficiency.
It needs just 1310 watts to drive it or a little under two horsepower at 1492 watts.
This does not account for mechanical efficiency of connecting the alternator to the motor.
With this in mind a three horsepower motor should drive a 12v 60 Amp alternator.
This is just an educated guess but I would like a horsepower chart.

720 watts would do well at charging a battery bank while you sleep; however I could not run my microwave and charge batteries at the same time.

iceng3 years ago
Here is my mechanical 60 Hz  to 50 Hz sine wave  frequency changer.  
A 50 cycle MG set ( Motor Generator ),  The alternator has been modified
( no diodes ) to output 120 VAC at 50 Hz by virtue of the belt pulley diameters.
The drive is a 115 VAC induction motor drawing 9.6 Amps 60 Hz at 1725 RPM,
that's over a Kilowatt about 1½ HP of input Power. 
The alternator was rewound deliver over 6 Amps at 110 VAC 50 Hz. about 700 Watts.
There is a step-up 600 VA transformer is used to provide 220 VAC at 50 cycles.

The dual duplex outlet on the left provides 220 VAC and 110 VAC at 50 Hz.
The single duplex and motor power switch on the right is the 115 VAC 60 Hz side.
Efficiency of the whole system is a whopping 60%  but with it I can qualify designs
for the rest of the world including the UK :-)
There is a 12 sec video of it running.  The last pic is where it helps hold down a
Chinese metal lathe.

In my opinion a Transformer Rectifier would be a much better battery charger
then an MG setup...........................   A
Josehf Murchison (author)  iceng3 years ago
Now that is neat, years ago I worked for a company that rebuilt electric motors.
We would get brand new motors from manual lath’s imported from Europe that ran on a variety of voltages and cycles, our job was to convert them to 600v 60cy. (Canadian Industrial Power) large electric motors are cheaper to rebuild then they are to buy.
That looks easier to build than a variable power inverter or rebuilding the motor without the specialised equipment. Especially if you cant buy just the right motor.
akumar1999 months ago
well i also want to checkout that a low power DC motor can run heavy/semi-heavy alternator? to develop high power. if yes, on what parameters? what parameters should match in between them? thanks in advance.
Josehf Murchison (author)  akumar1999 months ago
In the case of what icing did he attached an AC generator to an AC motor to convert 115 volts 60 cycle to 120 volts 50 cycle. The same thing can be achieved with an AC PWM or a AC to AC converter but since at the time he did not have the skills to make one he found a different way, and it worked even though it worked at 50% efficiency it worked.
akumar1999 months ago
well i also want to checkout that a low power DC motor can run heavy/semi-heavy alternator? to develop high power. if yes, on what parameters? what parameters should match in between them? thanks in advance
Wroger-Wroger11 months ago
Yeah going through the demoic incantations to work out what is the BEST RPM to run an alternator at under full load.

Unfortunately the Delco Remy site has more or less only the current models and their specs on site - but the "Spec Sheets" say nothing more than "Clean the battery terminals with baking soda, water and a wire brush..."

(Doh!!!! - grinds teeth a little - why????)

No maximum / Minimum RPM and output curve.....

Since I figure that using the alternator to charge up a battery bank - for the odd occassions I need to do light gauge welding, and usually not a whole heap of it, in conjunction with a small solar array - may in fact be better value than actually paying to keep the power connected at my place.

That is currently !$90 for 90 days..... THAT is an awful lot of fuel for a generator.- like about 65 litres - or 30 hours of continuous welding.... actual running an arc time..

2 or 3 minutes running a weld... 8 to 10 minutes charging up the batteries....

Might be worthwhile.....
lemonie3 years ago

I can't think of a good reason to connect two such electrical devices together.
There will be a better way to convert electricity-in to electricity-out, why do it this way?

iceng lemonie2 years ago
MG sets run many an older elevator and before inverter technology got
Reliable that was the best conversion technique..
lemonie iceng2 years ago
That's a frequency or phasing conversion then?

iceng lemonie2 years ago
Yes, mine does frequency 60 cps to 50 cps.
And a Ward Leonard control used to do elevators with a much wider
speed change ie zero RPM...

Ragnarr2 years ago
You can use the start with watts and convert that to horsepower and provide for the alternators efficiency, which averages about 55%. So rounding things off, a 100 amp GM 12si alternator at full load is approximately 3.5 hp just generating your electricity. The Denso square wire alts. are the most efficient design available now.
One thing a lot of people miss is how much hp it takes to turn the fan.
In working with alternators in racing applications, the 3.5hp is a small load. When you rotor is in the 5 to 6k range the fan can demand 15+ hp. When testing alternators in excess of 7500rpm, I have seen a 17 lb GM CS144 alternator with a 20 lb+ cage around it, rise off a test bench because the fan is moving so much air. Alternators with internal fans like Densos, the GM AD244, and Ford 3,4, and 6Gs use a little less power.
Josehf Murchison (author) 3 years ago
By the way this is a gas or diesel motor connected to an alternator.
Horsepower can be expressed in 550 foot-pounds per second, 746 watts, 746 Joules or 7.46E+09 Ergs.
From this you should be able to see why the international standard is watts.
I use 746 watts per HP a lot.
I know 550 ft_LB/sec, and 746 Joules.
But never had the 7.46+9 ergs to a HP,  thanks for that one.

I always referred to an erg as the energy of blowing one nostril
into a paper tissue without tearing the tissue, LoL

Josehf Murchison (author)  iceng3 years ago
Ha Ha Ha Ha Ha
Josehf Murchison (author) 3 years ago
I already figured a governor on the motor’s accelerator would be necessary to compensate for load breaking. And my calculations are based on loaded output as near as I could figure. I just haven’t received an answer from the manufactures and it is not in their information brochures.
That is where I got, typical alternator efficiencies are in the 54%-60% range and brushless alternator efficiencies are in the 60% 70% range.
An ideal alternator and motor combo would be one horsepower in 746 watts out.
However real life does not always match theory.
In real life 1 horsepower in, .54 to .60 horsepower out, or 447 watts out, it should be an easy calculation.
Just cant get a conformation.
Vyger3 years ago
I know there have been people who have experimented with using alternators for wind power electric generation. (belt driven pulley systems). I would imagine that they have posted the info they have gathered somewhere. Try Google and see what you can find.
seandogue3 years ago
A rough calculation of the required minimum input power can be reverse-calculated from the output power.

1) Determine the maximum power you want out of the alternator.

2) Multiply the value ascertained from the above by a value of ~1.5 to conservatively account for mechanical and electrical efficiency losses presented by the alternator

3) Convert the result to units appropriate for the motor and you should have your tidy little answer.

To be more accurate, you'd need to know the specific losses involved.
orksecurity3 years ago
I think this comes down to how much torque it takes to keep the alternator running at that speed under full load (and, yes, load affects how much resistance there will be; that's the principle behind variable electric braking). The specs, and/or the manufacturer's help desk, should be able to give you that information.

Then, according to a quick websearch:
(Torque x Engine speed) / 5,252 = Horsepower
where torque is in foot-pounds.

Actually, if you contact the alternator's manufacturer, they can probably give you a horsepower number without your having to calculate it.

Caveat: This is a semi-informed guess.