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.
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.
On 1/5/08 2:55 AM, in article
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
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.
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
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.
| 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.
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
On 1/6/08 10:36 AM, in article email@example.com,
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
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.
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