Power flow in AC circuit.

You may be remembering bits and pieces of a discussion about reactive power. In AC circuits some circuit components can *temporarily* store energy during part of each cycle and then return that energy back to the system that supplied it. Inductors store energy temporarily in their magnetic field for example. So as the energy flows back and forth, it is a form of 'power flow' to / from the generator. This reactive power is a significant issue with AC power systems but doesn't exist in DC systems.

daestrom

AC current flows alternately in both directions. It flows in one direction for some period of time, then flows in the opposite direction, then repeats.

It is possible that the load can store energy that it receives from a generator when the current flows in one direction and return that energy to the generator when the generator's current flows in the other direction. This doesn't happen with "resistive" loads like incandescent light bulbs and toasters, for example. But it can happen with "reactive" loads that include capacitors and inductors. Capacitors and inductors are energy storage devices.

DC current flows in one direction only.

When you turn on a DC generator into a resistive load, the load consumes power from the generator. When you turn the generator off, the load stops consuming power. BUT when you turn on a DC generator into a purely reactive load, the load consumes energy from the generator for a short time as it stores that energy. The load eventually stops consuming energy. When the generator is turned off, the stored energy is returned to the generator.

So the same phenomenon happens with both AC and DC. With AC and reactive loads, energy is repeatedly stored in the load and returned to the generator. With DC and reactive loads, energy is stored in the load once at turn-on and returned to the generator once at turn-off. With with either AC or DC and purely resistive loads, energy flows in only one direction: to the load. A resistive load dissipates the energy it receives from either an AC or DC generator.

Because purely reactive loads return the energy that they store, they consume no power. Resistive loads, on the other hand, do consume power; nothing is returned to the generator.

There are two aspects to this power flow.

One aspect, pointed out by other responders, is about reactive power in ac systems. Over a quarter cycle power stores energy in the capacitive and inductance of the circuitry. That power get returned in a quarter cycle. This happens, then, two times per cycle.

Some energy is also stored in the capacitance and inductance of a dc circuit. Over a short term, that usually will be significant during the switching on and off times. On long dc transmission lines, the stored energy can be significant and must be considered for the design and operation of the line.

A second aspect is that the load is used to store energy deliberately. That energy is then returned to the generator.

The most common example these days is the use of regenerative braking in electric automobiles. Motors convert electrical energy to mechanical kinetic energy. When it becomes necessary to slow or stop the vehicle, the mechanical energy is converted back into electrical energy and usually fed back into a battery. These days, modern electronics converts from dc to ac or back so that the distinction has lost much of its importance.

"Negative" energy is usually referring to the direction of power flow. There is no such thing as literally adding negative energy to body in order to cool it down. Think of a storage battery, such as the one in your car. Positive power flow is when you draw down the battery to do something like run your radio. Negative power is when you recharge your battery.

Bill

Here are some examples: At the Healy Coal mine in Healy Alaska they use a very large shovel powered by 7200 volts AC. When the shovel is allowed to move down by gravity the reverse power is sent to a large fly wheel that stores the energy until the shovel is raised then the fly wheel energy is sent back into the electrical grid. Also at the Grand Coulee Dam several of the pump/generators work as pumps during the night and pump water into a large reservoir. During the day the water is run through the pump/generator to generate electricity. Also, when calculating fault current motors are considered contributing factors in that when a fault occurs motors contribute energy to the grid and add to the available fault current.

As other contributors have said (mainly Salmon Egg) there are situations where the load can deliver power to the generator (both for AC and DC). For DC, if a true DC generator supplies an active load such as a motor or a battery, a drop of the generator voltage below the motor or battery internal voltage will cause a reversal of current and of power flow. The only essential difference between a generator and a motor is in the direction of the power flow- any generator can be a motor and any motor can be a generator. Practical limitations occur such as motoring a fossil power plant won't turn electricity into coal. Study the books a bit more- this is covered in basic circuit theory.

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

| Once I studied in a book that in AC circuit the power can flow from | Generator to the load and also flow from the load to Generator. But in | DC it is not possible. In the book they termed it as negative power | flow. I dont know how could power flow from the load to the generator.

Simply because the voltage is changing, any voltage state stored in a load (such a capacitor or inductor or just a rotating motor) could feed back at times when the source voltage drops below the load voltage. DC does not have such changes so there would be no feedback unless something about the load cause cause its voltage to be higher. But I can envision some odd circumstances in which voltage can back feed with DC.

From the outside, it is difficult (when you cannot resolve commutator ripple) to distinguish an ideal dc generator (or motor) for a secondary battery. If the internal emf exceeds the load voltage, (conventional) current will flow out of the positive generator terminal terminal into the load terminal connected to the generator. If the load has a higher voltage than the generator, current will flow from the load into the generator's positive terminal.

In a typical secondary battery, that is the equivalent of the positive terminal changing from anode when current flows in (charging) into a cathode when current flows out (discharging). Whether the battery is charging or discharging, it still is the same device.

Bill