# Question About Design of Low-Speed Alternators

I understand why the conductors of low-speed alternators need to be large and, therefore, expensive. But why aren't there units
that work around the problem.
For example, couldn't a unit have a rotating field?
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| I understand why the conductors of low-speed alternators need | to be large and, therefore, expensive. But why aren't there units | that work around the problem. | | For example, couldn't a unit have a rotating field?
Are you asking this about a unit that the mover rotates the windings, so it can also make alternating contacts to produce pulsed DC? If so, then rotating the field would disrupt the DC output because the cycling rate of the induced voltage on the windings would be out of sync with the brushes.
But if this is an AC output device, wouldn't the rotor have the magnets and the stator having the windings to avoid any brushes at all? Are you wanting to rotate both in opposite directions?
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I am wondering, for example, about a unit with a 3-phase stator coil producing a rotating field such that an armature*//* rotating at 20 rpm would appear to be rotating at, say, 1800 rpm relative to the stator field.
snipped-for-privacy@ipal.net wrote:

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| I am wondering, for example, about a unit with a 3-phase stator coil | producing a rotating | field such that an armature*//* rotating at 20 rpm would appear to be | rotating at, say, 1800 rpm | relative to the stator field.
You could do that. Are you trying to produce 30 Hz AC with a 1/3 Hz mover? You'd need some 29-2/3 Hz AC to drive the rotating stator field. Where are you going to get that from?
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No, I'm trying to get power out of a 10 - 30 rpm (ave ~20 rpm) mover
snipped-for-privacy@ipal.net wrote:

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Use a pulley system or gear box to match your alternator. It is a matter of mechanical impedance matching.
Bill
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| |> No, I'm trying to get power out of a 10 - 30 rpm (ave ~20 rpm) mover | | Use a pulley system or gear box to match your alternator. It is a matter | of mechanical impedance matching.
An alternative to that would be a 240 pole alternator (or some big number in that range). That would be quite an incredible alternator design, but it might work. But the gearbox would certainly be the practical and obvious approach to a simple setup.
Imagine a large rotor with 240 magnet poles of alternate polarities, and around that rotor 240 windings, also in alternate polarities. Every 1/120 turn would complete a full wave cycle. At 30 RPM, you'd get 60 Hz. That mover will need a lot of force behind it. A variation of that would make for 3-phase power, if needed.
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snipped-for-privacy@ipal.net wrote:

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On a practical point, in order to get that many coils in a circle, you would need a pretty large diameter stator. Something like the large hydro generators used at Niagara are still much fewer poles than that and those are something like 12 - 14 feet in diameter.
Or use really, really teeny, tiny wires ;-)
daestrom
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wrote: | snipped-for-privacy@ipal.net wrote: |> On Tue, 19 Aug 2008 14:51:46 -0700 Salmon Egg
| <snip> |>> Use a pulley system or gear box to match your alternator. It is a |>> matter |>> of mechanical impedance matching. |> |> An alternative to that would be a 240 pole alternator (or some big |> number in that range). That would be quite an incredible alternator |> design, but it might work. But the gearbox would certainly be the |> practical and obvious approach to a simple setup. |> |> Imagine a large rotor with 240 magnet poles of alternate polarities, |> and around that rotor 240 windings, also in alternate polarities. |> Every 1/120 turn would complete a full wave cycle. At 30 RPM, you'd |> get 60 Hz. That mover will need a lot of force behind it. A |> variation of that would make |> for 3-phase power, if needed. |> | | On a practical point, in order to get that many coils in a circle, you would | need a pretty large diameter stator. Something like the large hydro | generators used at Niagara are still much fewer poles than that and those | are something like 12 - 14 feet in diameter. | | Or use really, really teeny, tiny wires ;-)
At 12 feet diameter that's less than 1.89 inches per pole around. Given that 12 feet is really impractical for a home projects, this is going to be a much smaller figure.
But I'm thinking it would not need very many turns on each pole since all 240 poles would be in series. So at 1/2 inch each, this would need a stator at least 3.2 feet diameter. It could be done. Whether it is worthwhile, only the OP can say. I have no mover of such a speed. Maybe a waterwheel?
Note to the OP: If you choose to build such a beast, be sure to NOT wire the poles in series _sequentially_ but instead, wire half of them in one direction around the stator, then half back the other way around. That way you don't end up with a big giant inductor.
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What is the application you have in mind?
Bill