# Thread: Looking for DIY alternator formulas

1. ## Looking for DIY alternator formulas

Is here anyone that can give me some links or documentation on alternator circuits? I'm looking for formulas (not permanent magnet versions though).

I want to build a prototype alternator that can be switched over and be used as an engine when needed.
When alternating it will charge big capacitors, maybe 100 V or higher.
When I need the power I'll switch and push it back through the alternator as an engine.

What I want to do is an electrical hybrid "retainer" (is it called recuperation?) that works as the electrical cars do. They take the energy back when braking and deliver it again when accelerating. The idea is to replace the flywheel on an engine to an alternator-engine. The big diameter would make the torque high enough.

Regards,
Sven

2. Most alternators are three phase units - remove the diode pack, apply power to the rotor field, and drive the stator with a three phase supply.

Aaron

3. 070208-1913 EST USA

pastera:

Are you confusing the words alternator (an AC generator) and a DC generator. Without some switching system a capacitor will not work for long term energy storage. A resonant L C circuit tuned to the excitation frequency could store useful energy for a few cycles.

A DC generator-motor and a battery or large capacitor bank may be what you are refererencing.

If your AC motor-alternator is the electromechanical device, then you would need AC-DC conversion between the battery or capacitors for useful energy storage.

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4. The OP asked about using an alternator as a motor with reference to an automotive application. Automotive alternators are 3-phase generators with bridge rectifiers on the output. If you remove the bridge and replace it with a 2 quadrant drive, the alternator could be used to power the vehicle or charge the battery.

I made no metion of capacitors as a storage solution. At low voltages, the super capacitors can store large amounts of energy but are still a higher cost and lower density solution than good old lead-acid batteries.

Aaron

• pastera:

Sorry. My message was meant to be addressed to svenakela.

My question to him is he really talking about an an alternator vs a brush type dc motor (generator).

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• I'm talking about an alternator, with brushes. How else would it make the stator/field magnetic? But it will be turned around and used as three phase engine when needed.
I checked my old formula collection and couldn't find enough about the windings. How many turns of wire, how thick etc.

Regards,
Sven

• svenakela; What you seem to be thinking about appears to be functionally equivalent to what I discussed with Adobe and Ninerseventango in posts 73, 74, 75, 76, 77, 78, 80, 81, 82, 84, 86 in this thread.

Alt fuels... & a Good stock pick

Capacitors do have the advantage over batteries that you can cram energy into them quickly. But as pastera says they are expensive and have a low energy density. This is why I favor flywheel energy storage because it has the ability to store rapidly at an energy density greater than batteries.

• Are you looking at Field on the stator or rotor field (automotive style)?
As pointed out if you store the resultant rectified AC as DC, you will need an inverter to convert back to AC to drive the alternator as a motor.
Also If you have a rotor field (DC two brushes), when used as a motor you have the choice to make it run as an induction motor (no DC field) or by connecting DC to the rotor field when close to synchronism will make it a synchronous motor.
Al.

• 070211-1058 EST USA

svenakela:

Your can build a 3 phase AC machine with the 3 phase coils on either the rotor or the stator. Generally these are the stator because this puts the high power dissipation near the outer surface of the machine.

With the 3 phase coils as stator the rotor can be either a permanent magnet, or a wound rotor with DC excitation. If it is a wound rotor, then there are two continuous slip rings.

A so called DC brushless motor is basically an AC alternator or synchronous motor with a permanent magnet rotor. Additionally there is position sensing circuitry.

Formulas: it is probably easiest to scale from an existing unit. If you want to double the voltage then double the turns. Obviously the fixed window area for copper will define wire size. The maximum flux density is most likely set by the saturation of the core material, but maybe by thermal considerations in some cases.

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• ...Also keep in mind that if you are using an automotive Alt. as a design guide, they are grossly overrated, for .e.g. a 100amp automotive Alt. is rated for very intermittent operation 100a, if you compare the size of a continuous rated 100amp Alt. they are approximatley double in size.
Al.

• Glad you guys got interested.

I do know how different configurations work, and I do now cap's in that size are expensive. But I can get 'em for free so money isn't an issue.
The alternator/motor is going to be wide, as wide as the flywheel to be precise, so there will be a lot of torque to give when it's needed. I'm not the first one doing this, I just want to test it out by myself and if its efficient enough.

We've got tests on commuter buses here to retain energy. As Geof stated it can be done with flywheel energy but that is both heavy and hydroscopic. Another test that went really well was retaining the energy by starting up a compressor that stored the energy as compressed air. The bus was smooth to drive as the energy reserve "faded out" nicely. Actually, I don't know why they went on with it.

But I want to go the electric way, as all EV's do!

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