[SOLVED] How Alternator Generates Electricity

I have a confusion about the alternator. We know that voltage is induced only with a magnetic filed. But as we know there is no permanent magnet in alternator (portable generator alternators) and electromagnetic is only magnetized with electricity. So how does alternator produce electricity with non magnetized coils.

In Wikipedia it is said,
"The rotor's magnetic field may be produced by induction (as in a "brushless" alternator) or by a rotor winding energized with direct current through slip rings and brushes. The rotor's magnetic field may even be provided by stationary field winding, with moving poles in the rotor"

I have a portable power generator. It has an alternator with slip ring rotor and stator. But I didn't see any supply to rotor so that it can induce voltage into stator. An AVR is installed which takes input from stator coils and outputs to the slip rings of the rotor. But question arise here how does stator produce electricity itself.

You have posted an automobile alternator.Well I was talking about generator alternator. Alternator mechanism is described as, The rotor induces voltage into stator. But we know rotor is electromagnet then how it induces voltage into stator without any supply. Because there is no additional battery for alternator rotor. So how does it get electricity to its slip rings?

There is alternator with permanent magnets of course, they are used as wind generators.

Standard car alternator also can have rotor with permanent magnets, but control of voltage production is based on RPM.

Classic alternator rotor must use power from battery to generate magnetic field. Usual they are all wires, regulator regulate current in rotor windings, this give us stronger or weaker magnetic field according to needed voltage in stator. Current is drained from stator, this is obvious way. Ususally car is three phase, but for winding generator can have more.

Alternator with permanent magnet on rotor dont consume current from battery to make magnetic field, but classic alternator use, and when you want to drain more current from it, they give higher resistance and you need to give more power to rotate rotor (this spent battery power and fuel also, and decrease HP from engine.

My question is that where does stator get electricity. I make my question simple for you. We know both stator and rotor are electromagnet. If we rotate the rotor nothing will happen because both the stator and rotor are just coils. In order to generate electricity stator or rotor must be magnetized. And in portable power generators I don't see any battery so that it can magnetized on of the coils. So how does stator or rotor get magnetized?

Read this link for portable generators :

https://en.wikipedia.org/wiki/Excitation_(magnetic)

Self excitation

Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a residual magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage.

Starting

Self-excited generators must be started without any external load attached. An external load will continuously drain off the buildup voltage and prevent the generator from reaching its proper operating voltage.

Field flashing

If the machine does not have enough residual magnetism to build up to full voltage, usually provision is made to inject current into the rotor from another source. This may be a battery, a house unit providing direct current, or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called "field flashing". Even small portable generator sets may occasionally need field flashing to restart.

The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed.

Yes you right. You are telling me the mechanism of alternator described in books. But my confusion about alternator comes from manual start generators because there is no battery in these generators.

Read post #6 I update it, I need some time to write post.


Residual Magnetism

http://www.eiseverywhere.com/file_uploads/63356affaf81e30a2410dc337476805b_Residual_Magnetism.pdf



Residual Magnetism Example :

**broken link removed**

**broken link removed**

If the coil is wound on a core that has been hardened, the hardened core will retain some of the magnetic strength of the electromagnet. This remaining magnetism is referred to as residual magnetism.

**broken link removed**

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I have one interesting question for you and others, I will start thread about this soon.

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Thread name is :

Planet Earth Magnetic Field
https://www.edaboard.com/threads/260931/

Well, I try to explain how I see it.
- If we assume, that there were some dc running in the rotor, that would be magnetic, and thus then there would be a voltage generated to the stator, right?
- So, if the alternator would generate power, for its rotor, it would generate for its outputs as well.

The trick is thus to understand how it all STARTS, as there has to be some way of generating the magnetization of rotor first place! We don't have a battery, and magnetization of rotor has to be maintained by current from the stator, which is initially zero. Then I thought: Which was first, an egg or a chicken? You don't get chickens without eggs, and eggs without chickens.

However, we have here a solution of type Deus Ex Machina (See https://en.wikipedia.org/wiki/Deus_ex_machina, if you want to know what that Latin phrase means. I think the explanation on top of that article is quite fitting):

Surprise surprise, there is something called remanence (or residual magnetism) in the (rotor's) iron: It is NEVER totally un-magnetized. So there is weak magnetism in the rotor, which during start of mechanical motion generates a weak current to the stator, which is rectified and fed to the rotor, making its magnetism a bit stronger, and thus stator is generating more current ... etc. And suddenly everything works as it should, the alternator is generating plenty electricity for its main output, too!

Does this story help you?
-Ted

Then I thought: Which was first, an egg or a chicken? You don't get chickens without eggs, and eggs without chickens.

Click to expand...

This is solved this days, we get egg and chicken from lab in any order. :grin:

Probably the similar and more advanced lab is used on begining of begining.

Yeah, we live in a big long-term lab project, and try to understand all kinds of stuff we are made of and surrounded by.

Often things are confusing as we build a good theory which seems to work well, except for the initial condition: How it is achieved?

In case of alternator, there is that seemingly impossible equation of having to generate magnetization current for generating magnetization current, which can't be generated without current in the first place.... And then the not-so-obvious factor of residual magnetism in supposedly soft magnetic material, solving that never-never loop.

If only some other initial conditions had been equally easy to understand and experimentally confirm. A top notch example of our limitations is: If there was that "big bang" where everything was created from nothing, including the space itself, then what was there before the bang? What was there instead of space, if space was not created yet? And who/what triggered it and how? Why just then? If you don't say "G** made it to happen, and we don't quite know how", then we have to wait for another, better, explanation with great excitement and likely for a long while, and an empiric proof even longer.

Oh, well, maybe this entry is sliding quite much outside the scope of this thread. But it is Friday evening, isn't it? One should for sure be forgiven becoming a bit philosophical, then, especially when the main point is already answered and explained by so many.

So let's crank more of that alternator another time, if necessary. And I hope all the explaining has been useful for tahir4awan, who asked the original question here!

-ted

Test your assumption that there is no magnet in the alternator with a piece of thin iron wire or a compass.
There must be some residual magnetism.

more details of all types;
http://avstop.com/ac/apgeneral/alternators.html

I have a question: In typical small consumer-level portable generators, what is the most common way to reflash the coils if necessary? Or do you usually have to service them if the coils get demagnetized?

You must go to service. This can happen when generator rest long time.


How to Flash the Field of a Brushless Generator
https://www.ehow.com/how_7987906_flash-field-brushless-generator.html


How to Demagnetize a Magnet
https://www.ehow.com/how_5071154_demagnetize-magnet.html



Better small portable generators have small battery for taster cranking and for this.