Jerry Avins <jya@ieee.org> wrote: (snip) (also, I probably should have changed the subject along the way)> An alternator with fixed field excitation has an equivalent circuit that > consists of an inductor in series with an AC source whose voltage and > frequency are both proportional to rpm. (The magnetic field due to the > armature current directly opposes the excitation field. the equivalent > circuit follows from that.) The result is that the current is constant > when the alternator looks into a short circuit, and nearly so at high > loads, such as your headlights or a storage battery. The diodes, of > course, prevent reverse current. I used to isolate the alternator and > fiddle the voltage regulator to run 110-volt power tools. With no > battery and no voltage regulator, the headlights would blow. As long as > a battery was in place, it set the system voltage.I remember ads for boxes that would let you run 110VDC power tools off the alternator. (I don't remember if the ad specified DC, though.) At some point, I figured out that it wasn't quite as useful as could be, and wanted a real power inverter. (A favorite project for Popular Science and Popular Electronics over the years.) First year college, my dad bought a boat. With the excuse of needing one to shave (electric, non-battery razor) I bought the Heathkit power inverter. I still have that one. It won't run most power tools, though, but still enough things to make it useful. -- glen
complex notation
Started by ●October 12, 2012
Reply by ●October 15, 20122012-10-15
Reply by ●October 16, 20122012-10-16
On 10/15/2012 3:33 PM, glen herrmannsfeldt wrote:> Jerry Avins <jya@ieee.org> wrote: > > (snip) (also, I probably should have changed the subject along the way) > >> An alternator with fixed field excitation has an equivalent circuit that >> consists of an inductor in series with an AC source whose voltage and >> frequency are both proportional to rpm. (The magnetic field due to the >> armature current directly opposes the excitation field. the equivalent >> circuit follows from that.) The result is that the current is constant >> when the alternator looks into a short circuit, and nearly so at high >> loads, such as your headlights or a storage battery. The diodes, of >> course, prevent reverse current. I used to isolate the alternator and >> fiddle the voltage regulator to run 110-volt power tools. With no >> battery and no voltage regulator, the headlights would blow. As long as >> a battery was in place, it set the system voltage. > > I remember ads for boxes that would let you run 110VDC power > tools off the alternator. (I don't remember if the ad specified DC, > though.)DC it was. Power drills and such have AC/DC motors. Without the rectifiers, the frequency would be too high to get significant current through the tool inductances.> At some point, I figured out that it wasn't quite as useful > as could be, and wanted a real power inverter. (A favorite > project for Popular Science and Popular Electronics over > the years.)More useful and more costly. What else is new? :-)> First year college, my dad bought a boat. With the excuse of > needing one to shave (electric, non-battery razor) I bought > the Heathkit power inverter. I still have that one. > > It won't run most power tools, though, but still enough things > to make it useful.I have a few 75W units that work off the cigarette-lighter outlet, and a 600W unit with clip leads to the battery. Nice! Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●October 16, 20122012-10-16
On 10/15/2012 3:24 PM, glen herrmannsfeldt wrote:> Jerry Avins <jya@ieee.org> wrote: > > (snip) >> The disconnect relay that you cite is called a reverse-current cutout. I >> don't know why it could be expected to fuse, but as I wrote, it can make >> a big loud spark. It has both a voltage coil in shunt with the generator >> and a current coil in series with it. The way it works, the voltage on >> the disconnected generator builds up enough to pull the relay in on the >> voltage coil alone. When it pulls in, current flows to the battery >> through the current coil, increasing the field that's pulling the relay >> in. When the generator slows enough for the current to reverse, the >> series and shunt fields buck instead of adding, and the relay drops out. > > According to the "everything is on the web" rule: > > http://www.gus-stories.org/november_1968.htm > >> So why did the Karmann-Ghia generator work after I pulsed the cutout by >> hand? Hint: think residual field. > > For it to work in the first place, the genertor has to supply some > voltage with no field current. There is enough "permanent magnet" > magnetization in the generator to do that. The first time you power > it up the other way, it will reverse that field.Right so far. If you turn the generator the other way, it "builds down, instead of building up. Connecting the battery willy-nilly by pushing the current cutout reversed the residual magnetization -- polarizing the field the other way -- so it will henceforth build up instead of building down.>> Those DC generators needed not only the reverse-current cutout, >> but also voltage and current regulators. The three relays in >> the box performed one function each. An alternator needs at >> most a voltage regulator, and can get by without even that. > >> How come? > > The battery will keep the voltage pretty constant, until it > dries up. Excess current electrolizes more water, making more > hydrogen and oxygen, and less water. The chemistry of older > batteries wasn't so good, and it was normal to add water every > so often. Now, many are sealed with no water caps.Yup! The same reason that the taillight bulb on my bicycle burns out if the headlight is disconnected. My generator (no diodes) delivers about half an amp whatever. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
