Understanding How Generators Work

Don't despair over the simple drawing you see... we will get more complicated further on.

The idea here is to show you how to identify what TYPE of generator you have, so that you can wire it properly for residential use.

Simple Generator

It all starts with an electromagnet.  An electromagnet is a soft iron bar that has wires wound around it. The wires are called windings or coils.  When you apply electricity to the wires, the bar becomes magnetic:

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A simple generator is an electromagnet that has a magnetic field pass over it.  The magnetic field induces the flow of electricity.  Here is a simple generator:

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There is not much difference between a simple generator and a motor.  If we were to energize the windings we could also force the magnet to turn.  We would need to alternate the polarity of the above coil once every half revolution to induce rotation.  How often we alternate polarity - the frequency - would determine how fast the motor rotates.  If it's a generator, how fast it rotates determines the frequency of the electricity generated.  Sixty times a second (3600rpm) would give us 60-cycle or 60hz, which is the same as residential AC power.

Also note that I am turning the magnet, the windings are stationary. Moving the wire and keeping the magnet stationary is the same as moving the magnet and keeping the wire stationary.

This is a horrible generator, however.  You would only see one in a science fair.

The drawback is that the voltage generated will vary with rpm and load.  There are other factors - like the strength of the magnet and the number of windings - but for any combination of the two the voltage will still vary with rpm and load.  If we attach a higher load, we must rotate faster to compensate.  Since frequency is also tied to rpm, this is a big problem.  Too bad we can't vary the strength of the magnet.

But wait, we can.

A Better Generator

How about if we replace the magnet with an electromagnet?  This way we can keep the generator running at a constant rpm (and frequency) and control voltage by changing the strength of the electromagnet:

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This is confusing though, we have two coils, either one could be the electromagnet.  How about if we call the rotating one the rotor, and the stationary one the stator?  Whichever one we are going to energize and make an electromagnet out of, we'll call the exciter.

That's a basic generator.  They don't look like that in real life because what you see there is terribly inefficient.  You are only generating power for the short time that the bars pass by each other.  A real generator (and motor) has very complex windings to maximize the amount of time the fields interact with each other.  Here's a great image of a complex rotor/stator interaction:

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graphic from http://www.ansoft.co.uk/products/em/max3d/overview.cfm

Since I can't afford that software, I'll stick to the simplistic graphics.

So where do we get the power to run the exciter coil?  The most obvious answer is we can steal some of the power that's being generated and send it back in to generate some more.  This is not as simple as it sounds.  The more voltage we give the more we get, the more we get the more we give.  So let's isolate the part we use to create exciter voltage, we'll call this part the alternator.  

The term alternator is very confusing.  (You have one in your car, by the way.)  By definition an alternator is a generator that makes alternating current - AC - instead of direct current - DC.  But, you say, the alternator in my car makes DC.  Well.... yes and no.  The alternator in your car is actually a three-phase AC generator with built-in diodes that convert the output to DC.  This is the accepted definition of an alternator today.  Smaller residential generators have the alternator built into the generator head.  You can usually identify the alternator by the diodes wired into it.  Larger units actually have an alternator just like a car (except they're usually 24v) belt-driven on the front of the motor.  Many smaller generators also have a 12v or 24v DC output connection, this is a tap on the alternator.

We would still need some kind of battery to jump-start the process.  If we have no battery, we could just build a permanent-magnet generator right into the system to give us our initial DC power.  The result would be a multi-stage generator, which is very common where no battery is present:

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The permanent-magnet generator - referred to here as a magneto - produces enough DC power to get our alternator going, which in turn makes DC  to power the main exciter field of our generator, which then gives us our desired AC output power.

Adding PHASES to our Generator

We still have one more problem if we want to use this to power our house: phases.

Residential power is split-phase, so we need to get there somehow.  The way the power company does it is with a center-tap transformer:

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There are generators out there with a transformer like this.  The only reason for such an arrangement would be if the generator head has only one output coil.

Why?  Because it requires a big heavy transformer capable of carrying the entire load.  Why don't we just generate the proper phases?  All it takes is two coils opposite each other, this is a 3-wire generator.  The connection between the coils can take place inside or outside the generator head:

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Again, it doesn't really matter what is rotating.  Our rotor could be the center coil, or the two outside coils.  A multi-coil generator usually rotates the single coil though - less and smaller brushes are needed to complete the circuit on the moving part.  I'm going to shrink the exciter coil in my next graphic.

With the addition of one more coil, we have a three-phase generator:

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We can connect the coils together inside or outside the generator head, this would make a 4-wire or 6-wire generator.

The most common large generators are 12-wire.  With a 12-wire you have six coils and all wires come outside the generator head so you can configure the generator in a variety of ways.  Series the opposing coils and you have doubled the voltage, parallel them and you have doubled the current:

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The coils shown above are in series, so this is called a high wye or star configuration and it outputs 240v/416v in a normal generator.  How do I know the voltages?  Well, besides being told by some reliable sources, I can check the math very easily myself.  If I draw this out as just lines on a piece of paper I can measure the difference between the short lines going to center and the long lines around the outside.  I find that the long lines equal 173.3% of the short lines.  240 X 1.733 = 416 .  How about that. ( 120 X 1.733 = 208 )  NOTE THAT ANY PARALLEL COILED DRAWING CAN BE ACHIEVED WITH A 6-WIRE GENERATOR

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These voltages are unsuitable for household use though.  We need 120v/240v.  The next type of wiring scheme is called a delta - for the triangle it makes.  Note that the low delta has no neutral and only one voltage, the high delta can have a side "center tapped":

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The double delta and high delta produce perfect voltage and phase for household use.  They are also very difficult to wire if you don't know exactly which coil is which.  Many exciter circuits will not work with them either.  Worse of all, they won't work with a 6-wire generator.  The final configuration is called a zig-zag, and is most commonly used when residential 120v/240v is needed from a 3-phase generator:

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Notice that the SUM of the coils between L1 and N is equal to the coils between N and L2.  The sum coil is shown as a "ghost" coil in fuchsia.  All I had to do to achieve the zig-zag wiring was disconnect the L1 coils from neutral (N) and connect them to the L3 position.  L3 is just a splice now, I don't put load there.  I can get the same results by moving L2 to L1 or L3 to L2.  In a series or high configuration, I'd have 240v/480v.

The hardest part of re-configuring a generator like we've seen above is figuring out which of the wires coming out of the generator head are connected to which coils.  Without a manufacturer's documents, your best bet is determining how the generator is currently wired and working backwards from there.

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