Electrical Engineering

An AC generator is an electrical machine which produces alternating current electricity. It must be turned by a prime mover which can be an internal combustion engine - driven, usually, by diesel oil or gasoline - or can be a turbine, driven either by superheated steam or by water falling from a reservoir. (The latter is known as "hydro-electric power generation").

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Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. That kind of system is called an AC generator or an "alternator".

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An a.c. generator, which is usually called an 'alternator', is a machine which converts kinetic energy into electrical energy. Its output is an alternating voltage.

For more information see the answers to the Related Questions shown below.

DC, in DC motor, is an acronym for Direct Current, which means that it works on direct electrical supply by a battery. In an AC motor, the motor runs on Alternating Current Supply from other sources other than batteries, such as electricity from your house.

No, d.c. is not an acronym. It is simply an abbreviation.

"a word formed from the initial letters or syllables taken from a group of words"

http://www.thefreedictionary.com/acronym

Yes, a word. "dc" is not a word.

"Ohms Law" defines resistance (R) as the the ratio of voltage (V) to current (I).

R = V/I

If you move those variables around, you can get the formula:

I = V/R

So you can see that when resistance increases, current flow will decrease.

Resistance is most definitely not defined as 'the ratio of voltage to current', although that ratio may tell you what it happens to be.

Resistance isn't a variable in the Ohm's Law equation. It is a constant because it is unaffected by either current or resistance.

by using the step down transformer we can easily reduce the voltage.if we step down means the current(amps) icrease its vry useful for us

As transformers are a.c. machines, you cannot use a transformer for this purpose. You will need to create a voltage dividercircuit -essentially a pair of resistors, of appropriate value, connected in series across the supply.

Reactive power is an odd topic in AC (Alternating Current) power systems, and it's usually explained with vector mathematics or phase-shift sinewave graphs. However, a non-math verbal explanation is possible.

Note that Reactive power only becomes important when an "electrical load" or a home appliance contains coils or capacitors. If the electrical load behaves purely as a resistor, (such as a heater or incandescent bulb for example,) then the device consumes "real power" only. Reactive power and "power factor" can be ignored, and it can be analysed using an AC version of Ohm's law.

Reactive power is simply this: when a coil or capacitor is connected to an AC power supply, the coil or capacitor stores electrical energy during one-fourth of an AC cycle. But then during the next quarter-cycle, the coil or capacitor dumps all the stored energy back into the distant AC power supply. Ideal coils and capacitors consume no electrical energy, yet they create a significant electric current. This is very different from a resistor which genuinely consumes electrical energy, and where the electrical energy flows continously in one direction; moving from source to load.

In other words, if your electrical appliance contains inductance or capacitance, then electrical energy will periodically return to the power plant, and it will flow back and forth across the power lines. This leads to an extra current in the power lines, a current which heats the power lines, but which isn't used to provide energy to the appliance. The coil or capacitor causes electrical energy to begin "sloshing" back and forth between the appliance and the distant AC generator. Electric companies must install heavier wires to tolerate the excess current, and they will charge extra for this "unused" energy.

This undesired "energy sloshing" effect can be eliminated. If an electrical load contains both a coil and capacitor, and if their resonant frequency is adjusted to exactly 60Hz, then the coil and capacitor like magic will begin to behave like a pure resistor. The "energy sloshing" still occurs, but now it's all happening between the coil and capacitor, and not in the AC power lines. So, if your appliance contains a large coil induction motor, you can make the motor behave as a pure resistor, and reduce the current in the power lines by connecting the right value of capacitance across the motor coil.

Why is reactive power so confusing? Well, the math is daunting if not entirely obscure. And the concept of "imaginary power" puts many people off. But this is not the only problem. Unfortunately most of us are taught in grade school that an electric current is a flow of energy, and that energy flows back and forth in AC power lines. This is completely wrong. In fact the energy flows constantly forward, going from source to load. It's only the charges of the metal wires which flow back and forth.

Imagine that we connect a battery to a light bulb. Electric charges already present inside the wires will begin to flow in the circle, and then electrical energy moves almost instantly to the light bulb. The charge flow is circular like a belt, but the energy flow is one-way. Now imagine that we suddenly reverse the connections to the battery. The voltage and current will reverse... but the energy still flows in the same direction as before. It still goes from battery to bulb. If we keep reversing the battery connections over and over, we'd have an AC system. So, in an AC system, only the voltage and current are "alternating," while the electrical energy flows one-way, going from source to load. Where AC resistive loads are concerned, electrical energy does not "alternate." To understand energy flow in AC systems, it's critically important that we understand the difference between charge flow (current, amperes) and energy flow (power, watts.)

What is imaginary power? Simple: it's the unused power which flows backwards and forwards in the power lines, going back and forth between the load's coil or capacitor and the distant AC generator. If your appliance was a pure capacitor or inductor, then it would consume no electrical energy at all, but instead all the flowing energy would take the form of "sloshing energy," and we'd call it "imaginary power." Of course it's not actually imaginary. Instead it's reflected by the load.

What is real power? Even more simple: it's the energy flow which goes continuously from the AC generator and into the appliance, without any of it returning back to the distant generator.

Finally, what is "apparent" power? It's just the combination of the above two ideas: it is the continous-forward-moving or "real" energy flow, combined with the sloshing or "imaginary" energy flow.

A tungsten filament does follow Ohm's Law at any instant of time. You may be confused in that the filament resistance changes from its "cold" state to its "hot" state. When cold the resistance is about 1/15 the resistance of what it is when the filament heats up, which happens very quickly. At any instant Ohm's Law holds. When the voltage is applied you have an initial current draw that exceeds the steady state current draw based on the change in resistance.

Ohm's Law either applies, or it does not. It cannot apply 'at an instant of time' -a change in current is either proportional to a change in voltage, or it isn't!

A tungsten filament does not obey Ohm's Law, because the current flowing through the filament does not increase in proportion to the applied voltage. This is because the resistance changes due to the filament's increasing temperature as the applied voltage increases. This is why Ohm's Law specifies that current is proportional to voltage, provided the temperature remains constant.

Although tungsten doesn't obey Ohm's Law, the so-called Ohm's Law equation applies whether a circuit obeys Ohm's Law or not. This is because the formula is really derived from the definition of the ohm, and not from Ohm's Law itself, which makes absolutely NO reference to resistance!