Hello,
I need to design a small ac generator.
There are two types of designing it.
1. magnet rotating
2. magnet stationary.

In the 1st method, brushes are not necessary as i want to use permanant magnets. The output of the generator should be atleast 3v. the current rating will be [200ma - 500ma].

Now i have two types of magnets.
-bar magnet
- disk shape magnet.
can i use disk shape magnet as a rotor? What will be the polarity of the inner periphery of the disk shape magnet?
In the middle hole of the magnet, i am going to place the armature conductors. That conductors should be cyclically cut by the altenate north and south poles. But here i dont know the polarities of a disk shape magnet.
please help me.

 

A disk with a hole in the center is more commonly called a "ring."

A disk magnet or ring magnet can be magnetized axially or diametrically. An axially magnetized ring or disk (magnetized through the axis) will have N on one face and S on the other. A diametrically magnetized disk or ring (magnetized through the diameter) will have N and S 180 degrees apart on the outer circumference.

Example of diametrically magnetized rings: http://www.emovendo.net/magnet/12-x-14-x-18-diametric-rings.html (Other vendors sell them too, but this page had the best photo.)

Use an ordinary compass to determine where the poles are.

 

Does the disc shaped magnet repel another on it's face, or edge?

If it is on it's face, one side is north, one south, so spinning it wouldn't create a moving field that would generate electricity.

Do you have ceramic (greyish brown) or "neo" (silver/gold metallic) magnets?

To get an idea of what is needed for magnets/turns you can do some quick experiments if you have some motors laying around. My results are below for this test, conducted out of curiosity while writing this post. Try a small permanent magnet motor as a generator. Put a 10Ω 10W "sand bar" resistor across the outputs, and measure the output voltage with a DMM while turning the shaft.

Power out (Volts * Amps) would be V^2/R, with R being the 10 Ω, and V being what was measured.

Your goal is about 750mW or 3/4 Watt output. Using the 10 Ω load above, 2.74 V with a current of 274mA would be 750mW.

Reason for the much larger resistor is to keep from getting burned in case something excellent is tested or built.

Test Motor #1 is an older one needing a tuneup, from an R/C car or plane. It is labeled as a Trinity "Speed Gems 2 Quartz" - 19 turn Double Wound (2 wires in parallel for each turn), . Can size is about 1" diameter x 2.5" long. Weak Rare Earth magnets. Stator/rotor gap very close and uniform: Standard Paper is too thick to slide between rotor and stator. I wasn't able to get over 0.2V unless I disconnected the load, or ran it with another motor at 4000 rpm, which bumped the output to 0.6V. This is due to the windings being in parallel instead of one long wire, giving more torque at a given voltage, or current at a given RPM. Typical motor price: $25 US

Test Motor #2: "Hobby Motor", Ceramic magnets. Rated 1.5-3V use, Reversible, 0.80 oz. in. of torque at 6000rpm, drawing 320mA. Diameter is about 3/4" (15 mm), length is just under 1" (about 22mm), weight, about 1/4 oz total. Same test as above, couldn't get voltage over 0.15 V when driven at 4000rpm (no load). Spinning by hand produced barely measureable output (through a 1:4 thumbwheel gearbox). Typical motor price: $3 US

Test Motor #3: Bipolar Stepper Motor. 1.2° Steps. By far the best output if frequency/phase isn't a concern. Results are in "Car Battery Charger" Range. Output could be series for voltage, or parallel/shunt for current. Typical Cost varies wildly with construction quality and power, This one is in the mid range from a CNC axis drive. Apparently used in some tiny scale home-windpower projects Shown here .

As you can see, I had a bit of extra time on my hands today and had some motors out already. Winding motors is fun, though tedious. One large difference between motors and dynamos is brush timing, which can make a big difference in output, this is the reason I chose the reversible motors, with no obvious benefit.

Terminology: Generally, "Generator" has a fixed magnetic field, and an alternator has a rotating magnetic field. Either type can produce DC (using brushes) or AC, with Permanent magnet DC generators referred to as "Dynamos".

Some good places for info on winding a motor/generator:
http://www.physclips.unsw.edu.au/jw/electricmotors.html
Calculating turns for desired output with known magnet strength:
http://www.6pie.com/faradayslaw.php
Alternator made from wood, dirt, magnets, and wire (similar to stepper):
http://www.otherpower.com/woodmill.html
Excellent source of information about winding, topic is brushless DC motors, but theory applies to all motors/generators, sources for magnets, wire, designs, etc:
http://www.rcgroups.com/forums/showthread.php?t=240993

 

thanks for u both.
I got your concept. I need to check whether it is diametrically magnetized or on axis.
thank you again

 

hello thatoneguy....
still iam hanging around ur links.
i have a doubt here.
in this link
http://www.otherpower.com/woodmill.html
under the stator figure and just above the magnetic dirt figure, it is mentioned that "The coils are wound in opposite directions".
see the attachment.
what does that mean? how should i make the turns? i need a little bit explanation.

and one more thing,
Shall i replace the magnets with four electro magnets? i know that i need to use brushes. but what should be the guage and the turns. On which material i have to make the windings?

 

The windings need to be made to match the size/number/orientation of the magnets.

For a standalone alternator, I'd suggest using high power magnets, instead of a brushed rotor. Harder to build, and you need a voltage to start generating power. This is why car alternators won't put out any power if they have none to start with. Also, no brushes make for a MUCH simpler design. Essentially, one of these CD Rom Stepper motors, rewound, with stronger magnets, and what was the rotor becomes the stator.

This is a good tutorial on winding a 3 phase motor. To make a single phase motor, wind the same way, but with one length of wire instead of three. It will make more sense with pictures.
http://www.gobrushless.com/kb/index.php?title=Brushless_101_-_Chapter_1

--ETA: "Opposite Directions", one pole is wound clockwise, the wire crosses the axis, and the pole across from it is wound counterclockwise. This is shown in the link above, but it isn't as clear due to much of it describing 3 phase function. The core should be laminated steel or ferrite for best results. Hard to make a good rotor and stator form from scratch without a mill and lathe, and the average budget.

 

hello,
i need to build the generator shown in this link
http://www.otherpower.com/woodmill.html
The magnets shown in that pages are not available here in my area.
I want replace them with electro magnet. I'll look after the brush arrangement. Can anybody suggests me about the electro magnets. I need the similar magnetic strength. I'll give the DC voltage from a rectifier through the brushes. How many turns and which gauge i have to use? There is no alternative for earth magnets here.
It is easy or tough, i need to design the artificial magnets.
please help me.

 

You could get an old "Universal Motor" that runs on both AC and DC, old power tools have them, some can openers, sewing machines, and other small appliances.

I'm not sure of another way to arrive at a commutator/brush/winding/core setup capable of the output you are looking for.

 

I hope you are not powering your generator with a hamster.