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Is a dynamo really a dynamo?
Hi Folks,
Bear with me here as I'm not electrically qualified, hence my question. I want to mount dynamos on both sides of a wheel, and asked an electrical engineer friend if it would work (i.e. if I could drive one of them the wrong way). He said it would be OK if it's a true dynamo, and not an alternator. The result (as long as the drive mechanism can take it) would just be to reverse the polarity of the output terminals. So my question is: Are bottle dynamos true dynamos? Tim |
dy·na·mo
ˈdīnəˌmō/ noun noun: dynamo; plural noun: dynamos
by that definition, they're all dynamos. He must be using some other definition, probably this one with a commutator. Most bottle generators do not have commutators or rectifiers and therefore generate A/C, and so you cannot put them in series or parallel (electrically) without problems. You could put them in parallel if you could make sure the phase matched -- but there's no way to do that with a bottle generator. That said, you certainly could use two of them simultaneously if they powered totally separate lights -- that would be fine. You could also use a rectifier or the equivalent and convert to DC and then you could put them into parallel, but that's probably not what you're after. They ought to run just fine in reverse, however -- they just can't be electrically linked with others. |
Thanks for your answer.
Here's my friends statement in full: The eBay advert [for Soubitez dynamos] doesn't give much information. If the unit really is a dynamo then it should be delivering Direct Current, for which some form of rectification would be built in, using either a solid state diode bridge or low tech armature and brushes. The output statement: "3w" appears to confirm this (technically, an alternator would be rated 3VA) but probably shouldn't be relied on. What I actually want to do is run the dynamos simultaneously into a battery to charge it, does that change the situation?I am guessing that the unit uses permanent magnets for a field, rather than using a self- excited field coil. The latter can be re-polarised to enable reverse rotation by "flashing" the field terminals with a battery. Regarding the permanent magnet type: assuming that the mechanical drive within the unit can accept reverse rotation, all that happens when (e.g.) a L/H device is rotated R/H (whatever that means) is that the polarity of the output terminals is reversed. This needs to be noted and taken into account before two units are wired together in parallel. However, if the unit is really just a simple alternator (bearing in mind that incandescent light bulbs can run on either AC or DC with no issues regarding polarity) then wiring a pair of them in parallel is likely to introduce insoluble phasing problems. |
Using his engineer nomenclature, it's generally a magneto rather than a dynamo and delivers AC instead of DC.
However, you could add a bridge rectifier to each generator (extremely easy to do) and then put the output of those (each generator needs its own bridge rectifier due to the phase problems he mentioned) of any number of those into parallel and use them to charge your battery or whatever else as long as the device doesn't mind "noisy" DC (it'll vary between 0 and full voltage, after all) and the voltages are acceptable. (And some simple more circuitry can make the DC less noisy and control the voltage better.) Do be aware that these things are known to blow their bulbs when you go really fast down hills, so that tells me that they don't very effectively limit their voltage -- so you'll need to make sure that whatever you add to it can handle voltages that are higher than they say they deliver. Exactly how much higher you have to worry about ... I do not know. |
There are charger circuits for bike dynamos. They only recently came out, and they are rather expensive. The task is harder to tackle than you might think. Output voltage from the dynamo varies a lot, and it is A/C, so you have to rectify it (convert it to DC) to make a battery charger out of it.
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Engineers are prone to trying to prove they're the smartest person in the room. I love your friend's independent analysis of the possible operation of a simple, common device that's been around for 100+ years with abundant supporting literature. :thumb:
So anyway, since one can buy 12V, 6W bicycle dynamos, I'm guessing you're trying to charge a 24V battery? What bicycle borne application do you have that requires 24V battery power? On the other hand, if this is a stationary set-up, there are better ways to move electrons than little bottle dynamos. |
Originally Posted by tcs
(Post 16505134)
So anyway, since one can buy 12V, 6W bicycle dynamos, I'm guessing you're trying to charge a 24V battery? What bicycle borne application do you have that requires 24V battery power?
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some Hub motors for front wheels may do that . at a weight penalty..
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Originally Posted by TimEarl
(Post 16515544)
I'm aiming for a regenerative braking system that will use the harvested power to charge a battery. The battery will then be used to power lights and/or an audible warning device.
This is a more complicated issue than you may have considered. It makes sense with an electric bike as the motor can generally be turned into a generator through circuitry, and it already has the battery pack ... but regenerative braking makes far less sense on a non-electric bike. A standard bicycle generator setup charging a modest battery all the time with a few watts of power would be a lot simpler. Or skip the battery entirely and have the generator power your lights directly, perhaps with a capacitor to allow the lights to remain on for a minute or two after you stop moving. |
Originally Posted by dougmc
(Post 16518414)
A standard bicycle generator setup charging a modest battery all the time with a few watts of power would be a lot simpler. Or skip the battery entirely and have the generator power your lights directly, perhaps with a capacitor to allow the lights to remain on for a minute or two after you stop moving.
If I conclude correctly from this discussion that bicycle "dynamos" are actually alternators, and therefore produce AC, and (in electrical terms) could rotate either way, then why are they labelled as LH/RH? Is it something about the drive mechanism that will only rotate in one direction? |
LH/RH refers to which side of bicycle that dynamo can be mounted (to which direction it swings, when switching it on)
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Originally Posted by TimEarl
(Post 16515544)
I'm aiming for a regenerative braking system that will use the harvested power to charge a battery. The battery will then be used to power lights and/or an audible warning device.
1) Is it going to be able to produce enough power ( just going down hills ) during the average ride to be worth the time and trouble not to mention the expense.? 2) Is it going to be practical enough ( from all aspects; weight, size, output, expense..etc ) to make sense using one vs. using a standard dynamo? Anyway, I feel these are questions for the engineers to answer. Not saying it can't be done but I don't think it's going to happen and still make practical sense. |
Originally Posted by tcs
(Post 16505134)
Engineers are prone to trying to prove they're the smartest person in the room.
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yes, you can
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Originally Posted by yyy
(Post 16543177)
yes, you can
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Originally Posted by TimEarl
(Post 16543172)
I appreciate the efforts to help, but I would really appreciate an answer to my original question: Can I drive a cycle dynamo in the opposite direction to its stated "handedness"?
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problems may arise if the threaded parts used to hold the drive drum
on the end of the bottle dynamo, unscrews if rotated the other way .. though I have none anymore to look , so IDK , B&M slides on with a hex . |
Originally Posted by fietsbob
(Post 16544049)
problems may arise if the threaded parts used to hold the drive drum on the end of the bottle dynamo,
unscrews if rotated the other way .. |
if im thinking what youre thinking, its glued
and have you considered one front one back? |
got rid of my last bottle dynamo when I sold the Brompton it was attached to ..
instructions on the hubs I own, now, say 'plug on the fork right side, only'. |
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