Generator idea.

[ibdennyak]

Originally Posted by duivendyk

I think that is incorrect.It takes very little power to turn an alternator under no load conditions (just the friction of the slipring brushes and the bearings.The only power required is for supplying the rotor magnetization,which is in the order of 20-50 W (my guess).Alternators can typically supply around 1kW at 1100 rpm at 14.4 V and a lot more at higher rpm ( output voltage).For instance at full rotor excitation,at 4400 rpm, it could supply up to 4kW (70 A at 57V).The output is ultimately limited by the PIV of the diodes( usually about 150V ). So 5kW at 70V would be about the top limit.

You are right, but you underestimate whatthe little suckers will do. I use alternators as the basis for welders, and have gotten over 130 volts and 120 amps out of them. You do have to upgrade to diodes with higher PIV voltage on the newer alternators though. They run avalanche diodes that let the smoke out at about 36 volts as a precaution against toasted the vehicles computer. At this kind of output you will need something on the order of 10-12 hp to run them though.

I am looking for a small (as in physically small and light) alternator to retry my electric pusher hybrid project someday. In reading some of your threads, you seem very knowledgeable, and can expect a few PM's from me if you don't mind.

Denny

[jared8783]

ok so from what i understand we can spin a dcv motor and it will put out power

so the big question is

What motor do I get to get 12v 100watt output and what rpm's will it need?

[jared8783]

http://www.econvergence.net/electroacc.htm

I just ran across this it may be useful.

And you can buy just the gererator part.



Here is another VERY useful site.

http://www.greeleynet.com/~cmorrison...-building.html

I think it has even answered my wuestion from my last post.

So I think that I am looking to get a 12 volt 10-12 amp dc motor and spinning it at the rpm the manufactuar says is is capable of to get the 100 watts I am looking for.

Correct me if I am wrong.

Does anyone know where I can get a Motor like this?

And does anyone know an electrition who can join in on the conversation?

[duivendyk]

If you replace the diodes with higher PIV units you can get lots of power,something like at least 5 kW at 100V.The older alternators usually have better diodes.With a variable frequency 3 phase ac source you could make a dandy synchronous motor out of one.Most car units have at least a 50A capacity and are good for 700W plus at 14 V or around 1200W at 24V and 1,8kW at 36V.So something of an overkill.
As far as using dc motors as generators it should be noted that, depending on the efficiency of the unit the output voltage (for the same current and speed) is only 60-70% of the original motor rating.To get the original voltage& current the speed has to be increased to 1/0,6= 1.6 or 1/0.7= 1.4 the original speed.This is based on a efficiency of 80% as a motor. For instance a permanent magnet dc motor running at 24V,20A,3000rpm with 80% eff. will as a generator put out 19.2V at 3000rpm no load ,and when supplying 20A ,the output would drop to 14.4 V.To supply 24V ,the speed would have to be increased to (24/14.4)x 3000= 5000 rpm and the nl voltage becomes 29V
So either the speed has to be increased and lower output ,or a combination of the two has to be acepted, when using a motor as a generator and a lot obviously depends on it's efficiency or rather the lack of it.

[duivendyk]

Guidelines for using dc perm.magnet motor as a 12 V, 100W generator
Power output rating,For 100 W output at 12V:
If motor voltage 12-15 V ,power rating should be 130W output (about 0.2 hp) or higher.If voltage from 15-25 V ,power rating reqd: (V/15)x130 (V is motor voltage)
Motor speed:
If nominal motor speed at voltage 'V' and 130 W output is called Snom then the required speed can be computed from Sgen= (15/V)xSnom
These numbers are based on a motor efficiency of 75-80 %.Note that the motor speed is not mentioned explicitly,the speed affects motor size and to some extent the efficiency.Note also that these are 'rules of the thumb',thus must be considered guidelines.If the motor power rating is lower/higher the generator output needs to be proportionally reduced/raised.

[jared8783]

Helpful info duivendyk.

I must say you have confused me a bit as well.
I understand that you are saying that I would need a 12-15 volt 130 watt rating or higher. Or I could even use a 24v 210 watt motor as well, I think that is what you are saying.

But your equation on how to figure the rpm I need to spin it at has left me a bit confused. If you could help me out with that a little more that would be really cool.
thanks

[duivendyk]

OK,you are right,it is a bit confusing I should have said required MINIMUM speed,sorry about that .Take your 24V motor running at say 3000 rpm.you need around: 15/24x 3000=1875 rpm or more.Remember these are reasonable, but conservative estimates.Be advised that the motor power numbers are based on the OUTPUT power,not the input power!.It is sometimes unclear just what is being referred to.If it is clear that the input power is stated (rather meaningless really) ,then these figures need to be upped by 25%.

[NiteSpy]

duivendyk is there a particular or specific motor that you'd suggest for his type of application? I'm liking the idea of a small motor being used to maybe charge an on-board SLA battery?

[duivendyk]

You could charge a 6V battery with a 12V car dc motor,such as a radiator fan motor or an ac blower motor.Probably too much to hope for charging 12V batt with.Would take at least twice the normal rpm.These motors are good for a couple of amps at least.They are not likely to be efficient though and the bearings are questionable (not designed to take sideways loading).Probably need series resistor or 317 type regulator to limit output if driven from variable speed source (output proportional the rpm).Could be found in junk yards.

[jared8783]

ok so are you saying that if i get a motor with an output rating of 24volt 210 watt and spin it at 1875 rpm that should be capable of putting out 12-15 volts 100-150 watts?
Or am I way off.