How much power does it take to turn an alternator

[oil pan 4]

I am trying to find a second cheap DR44G alternator.
I had the compressor go out on one of my air compressors so I have an electric motor with mounting plate that will be free for a while.

[teoman]

Any updates or data for this project?

[oil pan 4]

I just picked up my second DR44G alternator those past Saturday and I have not messed with my air compressor with the bad compressor head unit.

[oil pan 4]

I finely got to building my test stand and putting an alternator on it.
The alternator is a modern 150 amp output ACdelco DR44G, also the AD244 is almost the same. So it's not a small alternator.
Turning the alternator at an unrealistically slow speed of about 3,600rpm only took between 100 and 120 watts for the alternator plus belt..
A little less than what I expected.
Next I had a 4 inch pulley to put on the motor. The larger pulley should spin it up to 5,760rpm.
A bit more plausible for a usable speed but no where near cruise RPM. It only used between 120 and 140 watts to turn the alternator plus belt.
A minimum cruise speed is going to be at least 7,500rpm.
Edit: no field current. Just free spinning.

[serialk11r]

That's with field current right? That's higher than I would've hoped It would be worse if that was just the bearings and belt, although if I understood correctly, your belt is less efficient than the common 6 rib belts on modern cars.

One thing to keep in mind is that the field current is adjusted downwards as the rpms go up, and your gasoline engine's efficiency is going up with increased load. Still, it's useful to see ballpark numbers.

Seems like the 0.5mm laminations on car alternators rob a fairly good amount of power. A cheapo Turnigy Rotomax 120mm only takes 200W to spin at 10000rpm despite its neodymium permanent magnets, and still not so great 0.2mm laminations, which reduce the eddy current losses by 83%.

[teoman]

I just performed a test on my passat (2011 1.8T), I was unable to attach to the cable directly connected to the alternator, but I was able to get the cable exiting the battery (so the power consumption of the car).

I must admit the test most probably did not work properly beacuse of the clamp type voltmeter that I used. But the car was drawing about 2A, when revved up 2.5A when all of th edevices inside (stereo, seat heaters, heater blower, rear window defrosting) the current went to 6A. That does not seem right as that makes about 85W, the seat heaters alone should be consuming that much.

[oil pan 4]

I know from my alternator welder, energizing the field with no output puts a pretty good load on my 5hp OHV briggs and Stratton engine.
The steel core laminates on this alternator are more like 1mm thick.
With my welding alternator making around 2.2kw it takes everything the 5hp motor can muster

[oil pan 4]

The DR44G alternator is rated to peak at 170amps. I bought an after market 250 amp stator.
When welding it makes around 24 to 29 volts at up to 90 amps. When not welding it makes up to 80v when there is no current flowing.

[serialk11r]

Originally Posted by oil pan 4

The DR44G alternator is rated to peak at 170amps. I bought an after market 250 amp stator.
When welding it makes around 24 to 29 volts at up to 90 amps. When not welding it makes up to 80v when there is no current flowing.

That means your efficiency is extremely poor, as most of the energy is lost to resistance in the stator. If you can, you should rewire it to produce less voltage but have lower resistance. Any idea if it's delta or wye wound?

[oil pan 4]

The AD244 and DR44G alternator stator are delta wound.
This is normal for constant current welding machines.
Almost all 3 phase to DC welding alternators are delta.
I need it to produce up to 30 welding volts and at least 50 volts open current to help start the welding arc. The way it's making up to 80 volts is perfect.
The resistance is already very low in the stator. The reason for the voltage drop is welding creates a short circuit.
If I was tig welding the arc voltage would be even lower, between 11 and 16 volts.