Advertise on Bikeforums.net



User Tag List

Results 1 to 20 of 20
  1. #1
    Junior Member
    Join Date
    Oct 2013
    Posts
    19
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    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

  2. #2
    Senior Member dougmc's Avatar
    Join Date
    Jul 2008
    Location
    Austin, TX
    My Bikes
    Giant OCR2, Bridgestone RB-T, Bike-E, Vision R-40, Novara Safari
    Posts
    3,014
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    dy·na·mo
    ˈdīnəˌmō/
    noun
    noun: dynamo; plural noun: dynamos

    • 1.
      a machine for converting mechanical energy into electrical energy; a generator.


    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.

  3. #3
    Junior Member
    Join Date
    Oct 2013
    Posts
    19
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

    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.

    What I actually want to do is run the dynamos simultaneously into a battery to charge it, does that change the situation?

  4. #4
    Senior Member dougmc's Avatar
    Join Date
    Jul 2008
    Location
    Austin, TX
    My Bikes
    Giant OCR2, Bridgestone RB-T, Bike-E, Vision R-40, Novara Safari
    Posts
    3,014
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    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.
    Last edited by dougmc; 02-17-14 at 11:19 AM.

  5. #5
    aka Tom Reingold noglider's Avatar
    Join Date
    Jan 2009
    Location
    West Village, New York City
    My Bikes
    too many
    Posts
    20,486
    Mentioned
    99 Post(s)
    Tagged
    1 Thread(s)
    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.
    You don't read my signature anyway, do you?

    Tom Reingold, noglider@pobox.com
    Residences: West Village, New York City and High Falls, NY
    Blogs: The Experienced Cyclist; noglider's ride blog

  6. #6
    tcs
    tcs is offline
    Palmer tcs's Avatar
    Join Date
    Mar 2007
    Location
    Parts unknown
    Posts
    4,291
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    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.

    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.
    "When man first set woman on two wheels with a pair of pedals, did he know, I wonder, that he had rent the veil of the harem in twain? A woman on a bicycle has all the world before her where to choose; she can go where she will, no man hindering." The Typewriter Girl, 1899.

    "Every so often a bird gets up and flies some place it's drawn to. I don't suppose it could tell you why, but it does it anyway." Ian Hibell, 1934-2008

  7. #7
    Junior Member
    Join Date
    Oct 2013
    Posts
    19
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by tcs View Post
    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?
    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.

  8. #8
    coprolite fietsbob's Avatar
    Join Date
    Jun 2010
    Location
    NW,Oregon Coast
    My Bikes
    7
    Posts
    2,446
    Mentioned
    41 Post(s)
    Tagged
    0 Thread(s)
    some Hub motors for front wheels may do that . at a weight penalty..
    Last edited by fietsbob; 02-21-14 at 03:34 PM.

  9. #9
    Senior Member dougmc's Avatar
    Join Date
    Jul 2008
    Location
    Austin, TX
    My Bikes
    Giant OCR2, Bridgestone RB-T, Bike-E, Vision R-40, Novara Safari
    Posts
    3,014
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by TimEarl View Post
    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.
    In general, a generator that's large enough to generate enough resistance to replace your brakes for relatively gentle stopping (you'll use your existing brakes, non-regeneratively, for more forceful braking) will be as large as a motor used to power a bicycle, and unless you can make some sort of transmission that can completely disengage when not used, it'll slow things down even when not activated. And then you'll need a battery pack large enough to accept 250 watts of charging power for a few seconds here and there.

    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.

  10. #10
    Junior Member
    Join Date
    Oct 2013
    Posts
    19
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by dougmc View Post
    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.
    But that's not the problem I'm trying to solve. If it was I'd just buy (or make) a system such as you describe. This project is as much about the intellectual challenge as the practicality, but I needed some technical info about types of generators, so I came to the experts.

    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?

  11. #11
    yyy
    yyy is offline
    Newbie
    Join Date
    Dec 2012
    Posts
    4
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    LH/RH refers to which side of bicycle that dynamo can be mounted (to which direction it swings, when switching it on)

  12. #12
    Senior Member 01 CAt Man Do's Avatar
    Join Date
    Dec 2012
    Location
    Columbia, Maryland
    My Bikes
    Mountain bike & Hybrid tour bike
    Posts
    556
    Mentioned
    2 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by TimEarl View Post
    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.
    While I applaud the concept and the willingness to be inventive I have to agree with what DougMC had to say about it. Even if you could build such a thing and manage to keep the weight to a certain level you have to consider the practical issues such as;

    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.

  13. #13
    Junior Member
    Join Date
    Oct 2013
    Posts
    19
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by tcs View Post
    Engineers are prone to trying to prove they're the smartest person in the room.
    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"?

  14. #14
    yyy
    yyy is offline
    Newbie
    Join Date
    Dec 2012
    Posts
    4
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    yes, you can

  15. #15
    tcs
    tcs is offline
    Palmer tcs's Avatar
    Join Date
    Mar 2007
    Location
    Parts unknown
    Posts
    4,291
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by yyy View Post
    yes, you can
    So as stated in post number one, you (the OP, not yyy) want to buy two identical dynamos and mount them to opposite sides of 'a wheel'...instead of buying two dynamos, one with a LH mount and one with a RH mount (they're probably available LH & RH, but you haven't wanted to share with us the particular hardware you're considering). Following your purchase and mounting plan, that will put one on its mount in the proper 'trailing' position and the other in a leading" position. As has been learned over the last 100 years of cyclists using dynamos like this, if anything should cause the leading mounted dynamo to lock, freeze or go into a high drag position, it can subsequently lock the wheel. If it's your front wheel, you'll add a few feet to your frequent flyer miles account.
    Last edited by tcs; 03-03-14 at 08:15 AM.
    "When man first set woman on two wheels with a pair of pedals, did he know, I wonder, that he had rent the veil of the harem in twain? A woman on a bicycle has all the world before her where to choose; she can go where she will, no man hindering." The Typewriter Girl, 1899.

    "Every so often a bird gets up and flies some place it's drawn to. I don't suppose it could tell you why, but it does it anyway." Ian Hibell, 1934-2008

  16. #16
    tcs
    tcs is offline
    Palmer tcs's Avatar
    Join Date
    Mar 2007
    Location
    Parts unknown
    Posts
    4,291
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by TimEarl View Post
    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"?
    Your original question was in fact, "Are bottle dynamos true dynamos?"
    "When man first set woman on two wheels with a pair of pedals, did he know, I wonder, that he had rent the veil of the harem in twain? A woman on a bicycle has all the world before her where to choose; she can go where she will, no man hindering." The Typewriter Girl, 1899.

    "Every so often a bird gets up and flies some place it's drawn to. I don't suppose it could tell you why, but it does it anyway." Ian Hibell, 1934-2008

  17. #17
    coprolite fietsbob's Avatar
    Join Date
    Jun 2010
    Location
    NW,Oregon Coast
    My Bikes
    7
    Posts
    2,446
    Mentioned
    41 Post(s)
    Tagged
    0 Thread(s)
    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 .
    Last edited by fietsbob; 03-12-14 at 11:31 AM.

  18. #18
    aka Tom Reingold noglider's Avatar
    Join Date
    Jan 2009
    Location
    West Village, New York City
    My Bikes
    too many
    Posts
    20,486
    Mentioned
    99 Post(s)
    Tagged
    1 Thread(s)
    Quote Originally Posted by fietsbob View Post
    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 ..
    I don't believe they are screwed on. Never heard of this problem.
    You don't read my signature anyway, do you?

    Tom Reingold, noglider@pobox.com
    Residences: West Village, New York City and High Falls, NY
    Blogs: The Experienced Cyclist; noglider's ride blog

  19. #19
    Senior Member italktocats's Avatar
    Join Date
    Oct 2013
    Location
    Netherlands
    Posts
    214
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    if im thinking what youre thinking, its glued


    and have you considered one front one back?

  20. #20
    coprolite fietsbob's Avatar
    Join Date
    Jun 2010
    Location
    NW,Oregon Coast
    My Bikes
    7
    Posts
    2,446
    Mentioned
    41 Post(s)
    Tagged
    0 Thread(s)
    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'.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •