Current can flow from lower voltage source to higher one?

[freepenguin]

The car alternator charges the battery. Everybody knows it.
The rotor becomes electromagnet, rotate inside the stator, generate 14+ volts with current to supply electricity to car. The left-over electricity charges the battery. When the alternator output below the amount of car's electricity, current starts from the battery. That's my understanding.

It's intriguing to me. In close circuit, the voltage drop is zero by K... law. Sorry for not typing the correct word K... I don't understand it how the current from the battery flows into the rotor when the alternator is running because the alternator produces higher voltage than the battery.


My question is can the current flow from lower potential (voltage) source to higher one?

[Alien509]

There is a reduction in current when the voltage is adjusted. When the electromagnet in the center of the alternator reduces in it's field strength there is a resulting reduction in current and voltage as well. The energy comes from spinning the electromagnet and inducing alternating current into the winding- the speed of the alternator spinning is the voltage output of the alternator. That's where the voltage regulator comes into play, if the alternator is spinning faster it will limit the current to the electromagnet in the center which will reduce the field strength and shrink the magnet field radius. Most people view field strength and radius of the resulting magnetic field in terms of voltage being the radius only and current being the density of the field within that radius. The actual magnetic field strength is dependent upon the resistance of the materials that the current is flowing through. The current from the battery is transformed into a higher voltage potential which then crosses into the winding where it is transformed back into dc energy. The regulator monitors the dc addition to the battery source and when it becomes equal to the two volts then the field to the electromagnet is reduced.

The actual voltage across the coil (which has three windings wound together and 1/3 out of phase) may be a few hundred volts because the alternator utilizes a multiphase diode array. The efficiency of using a multi-pulsed field is much higher than a linear field. The multiphase winding also keeps the voltage from dropping to zero if the alternator came under load suddenly. The alternator stator field actually self oscillates internally because of the diode array and change in field strength through the revolutions of the stator, with induction into the multiple windings of the coil.

So to combine it all- the belt gives the higher voltage potential fields which automatically allow the alternator to induce energy into the simple 12volt battery. The multiphase induction, then can multiply the instantaneous power output of the alternator. To answer your question simply: current can be transformed into a higher voltage at any theoretical power rating which can overcome the voltage source of any other system.

[mscoffman]

1) Current flows from the battery if the voltage from the alternator is less than the voltage
from the battery, current flows from the alternator if the voltage from the alternator is greater.
The diodes protects the alternator from a *reverse* current flow if the alternator voltage
is less than that of the battery.

2) Current from the battery also flows through the regulator and activates the field coil
of the alternator as much as possible/or as much as is needed. The alternator is a
variable sized machine controlled by the amount of field current sucked from the battery.
The alternators mechanical shaft then acts a a brake to the engine. The more
power or energy needed by the alternater the harder the shaft is to turn. So the alternator
uses mechanical energy to multiply the field current to get the alternator output current.
The regulator tries to get the alternator just above the battery charging voltage of +14.4Vdc
If it can. It does not get "upset" if it can not. If it can, it is suppling the current to run the car
and charge the battery. If it can not then the battery current is running the car and discharging
the battery. If you've ever had an alternator fail while you are driving your car you know that the
battery can supply the car, but for only so much time...the lights get dim ect. Eventually
the battery voltage drops below that required to run the ignition and the car stops.

Whether the field coil spins and the output coil sits still fixed to the frame, or
vice versa doesn't really matter is just their relative motion. It's slightly easier
to inject the lower power field current through a slip ring rather than the higher
output current, so that is what is done. The Alternator is AC but is easily converted
to Dc current by the diodes.

:S:MarkSCoffman
 

[Yucca]

Just some thoughts on this: Of course a current can be made to flow from a lower to a higher voltage source, for example:

1)
In a Van de Graff machine a current can be forced to flow from ground to the charge store at the top. The rubber belt moves X coulombs of charge per second (amps) by physical means. The charge store can become tens of thousands of volts higer than the ground.

2)
Can induce a current elecromagnetically and it will flow in a conductor from lower voltage source to higher voltage source.

3)
Can also force electrons uphill by exposing them to a force, for example centrifugal force from spinning causes electrons to migrate toward the rim in a metal wheel.

There must also be many more ways to do it and encourage it. The secret to OU might be combining two or more different methods into one system and taking advantage of some asymetry that might crop up.