Alternator output current limiting

[(unknown)]

I have a 12 V/70 A alternator driven by  a 5.5 hp lawn motor. The alternator runs at 1500 RPM. The motor is started with no current to rotor. When the rotor current is applied  to charge an empy or low  battery the motor stall due to the high current to start charging.  I guess remedy to this migth be  A) limiting the rotor current with series resistors  or B) limiting the rotor voltage to less than 12 V. Would this work?

What is the typical current drawn by thr rotor and is there a treshold  voltahe to start charging

Artsi

[D]

[maker of toys]

mmmm . . . alt at 1500 rpm.  

the ones with a built-in regulator are going to drive the field pretty hard to try and 'recover the start,' as it assumes it's still in a car.

I'm assuming that the engine is reasonably fresh, it's got good spark and a clean carb, airfilter, oil, yada, yada.  Any tune-up or wear issue that the engine has will severely limit its ability to generate torque.

if you turn the alt faster, the torque requirement should go down. if you've ever had a loose belt on a car alternator, you know that it slips worse at low engine speeds, indicating that the belt is the torque limit, not the alternator output.

  you don't say how fast the engine is turning.   The torque curve of a typical small engine is actually pretty peaky.  Most small engine manufacturers will have the torque curve on their website; my advice is to look at that information carefully, then govern the engine so it runs a bit faster than peak torque under a light load, so as you load it, it falls 'up' the torque curve, rather than down, off the peak.

otherwise, yes, limiting the current into the field will reduce your torque requirement.  it will also reduce your output current and raise your 'cutin' speed some, so you'll have to run harder and longer to fill your battery bank.

the real durable answer is either a bigger engine or a smaller alt, though.

[hiker]

sounds like you need a bigger pulley on the alt.........

most likly not as big  as this........


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[John II]

Hello: 12 volts on the field winding is way too much voltage ! At normal car rpms, 12v in at the fields will normally give you 120vdc output !

I'm not sure of the exact voltage... but it's something like 3 or 4 volts on the field that is required. You could put your test instrument leads on your car while it is running to find out.

Someone else on this forum might also know. I do know that 12v input is too much.

John II

[finnsawyer]

Yes.  Measure the rotor resistance.  Then get a wire wound potentiometer of about 8 - 10 times that value.  Put it in series with the rotor and the battery.  Starting with the resistance low run a series of trys to start charging as you slowly increase the potentiometer resistance.  When you find where charging will start without killing the engine with the minimal value of resistance replace the pot by a fixed resistor of that value.  If this does not give a reasonable rate of charging you will have to get a regulator for the alternator.  

[maker of toys]

so then I got to thinking, which usually leades to calculation.

figureing 75% alternator efficiency, then tacking on another 30% for belt, fan, bearing, vibration and brush losses:  I get 2 hp and 7 ftlbs required for 70 amps at 1500 Alternator rpm.

then I went and looked at the honda gx160 specs, which had a much flatter torque curve than I remembered, and it shows that a reasonably modern 5.5hp engine should make about 7 ft lbs at that same rpm. . . .you could direct-drive the alternator from the engine, and save a few watts and a lot of decibels. <g>  the engine would last longer, too.

so my pulley comments were a red herring.  go with the wire-wound potentiometer idea lower in this thread.

-Dan