How many magnets & poles do i need to generate generate 5v dc?

Discussion in 'General Electronics Chat' started by Chris Oremus, Aug 17, 2018.

  1. Chris Oremus

    Thread Starter New Member

    Aug 16, 2018
    10
    1
    I am building a kind of torque converter exercise machine that I invented and I would like to add the ability to generate power.
    I've got a 100mm rotor 30mm wide spinning at between 1000 and 5000 rpm with Neodymium magnets pointing outward radially. I would like to generate 3+ volts at 1 to 5 amp to charge lithium batteries.
    Could I get your advice about a few choices I have to make?
    How many magnets & poles should I use?
    I don't need 3 phase power so wouldnt a single phase be easier to make?
    How big should the magnets be & what diameter wire should I use? Diameter of the wire loops and number of turns? I believe I'd like to make the coils flat like silver dollars to fit into a tight space but how would that work? Is there a way to make it without using iron or steel to simplify construction and keep it lightweight?

    There are so many complicated variables to consider so I appreciate any guidance or knowledge you would like to share. Thank you for your time.
     
  2. crutschow

    Expert

    Mar 14, 2008
    20,055
    5,646
    3-phase is used because its power output is constant whereas single-phase is pulsating.
    You have to use a magnetic material to make an efficient generator.
    Determining the design of such a generator is not a simple task and requires a high degree of magnetic circuit knowledge which few but experts in generator design have.
     
    -live wire- likes this.
  3. oz93666

    Senior Member

    Sep 7, 2010
    184
    37
    You say you already have this rotor with magnets pointing out ??? You do not need too much power , so this is very achievable ...

    You need to create or obtain an enameled copper wire coil that's the appropriate size for the magnets , preferably with an iron core , get this as close to the spinning magnets as possible and it will induce a voltage pulse each time a magnet passes ,the output from the coil needs to be rectified so it's DC , perhaps a small capacitor , and then fed into a charging regulator so that the battery is not charged too high .... eBay is a good place to find all you need ... if one coil does not deliver enough power then you add more ...

    Take it step by step and we can guide you through it ...put "magnet wire" in ebay and you'll get the enameled copper wire you need , about 0.5mm dia is appropriate .... make a coil of this around a nail or iron bar which has the same dia as the magnets ....You can do this without the iron core but it will not generate much power , so you may need more coils , which defeats the purpose .There are many ways to do this , all will generate power ... You can have a flat coil , but then you will need many of them .

    Without knowing the size of your magnets I cannot say how many coils you will need , if the face of the magnets is a rectangle so should your coil be (for maximum efficiency)
     
    Last edited: Aug 17, 2018
  4. Chris Oremus

    Thread Starter New Member

    Aug 16, 2018
    10
    1
    Thank you for your advice
     
  5. Audioguru

    Expert

    Dec 20, 2007
    10,584
    1,178
    You probably need a voltage regulator because a slow generator produces a low voltage and a fast spinning generator produces a much higher voltage.

    Of course you also need a lithium battery charger IC.
     
    -live wire- likes this.
  6. Chris Oremus

    Thread Starter New Member

    Aug 16, 2018
    10
    1
    I am designing the magnets to be set into the rotor. I haven't made the rotor yet but wanted to describe what I'm working with.
    I got some iron rod and .5mm enameled magnet wire now. I'm kinda blindly guessing that 4 magnets with 4 poles in a single phase would be a good choice here?
    I have a voltage boosting battery charger that will run off 1v to 5v to charge my lithium batteries.
     
  7. oz93666

    Senior Member

    Sep 7, 2010
    184
    37
    It's very hard to know ... the crucial thing is getting the coil as close as you can to the rotor , the electronics is easy , the first thing is to establish how much power is produced by one coil , I would try first with 4 magnets ... , if all the magnets have the same pole facing out a DC pulse is induced ,you will not need a rectifier ... putting a capacitor across the coil wires will smooth out the pulse , then you can see what constant voltage and current you have and adjust things accordingly.
     
    Last edited: Aug 17, 2018
  8. -live wire-

    Active Member

    Dec 22, 2017
    828
    72
    Also, make sure to have a snubber of some sort, too.
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
    5,685
    3,243
    Why?
     
  10. -live wire-

    Active Member

    Dec 22, 2017
    828
    72
    If you suddenly disconnect your load, or significantly reduce it, there may be large inductive spiking, which can destroy semiconductors and other things.
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
    5,685
    3,243
    Most just use a simple diode to redirect the overshoot, not a snubber.
     
  12. -live wire-

    Active Member

    Dec 22, 2017
    828
    72
    A flyback diode can be a snubber though, right? And what if you spin the motor in the other direction? Wouldn't you want to use that power instead of heating up your diode?
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
    5,685
    3,243
    Usually at least the way I learned it, A flyback or free wheel diode just redirects the voltage. And a snubber dissipates the voltage to a lower, safe voltage. One is just a diode, the other is a network of diode/resistor or capacitor/resistor. A diode is used for DC and a snubber for AC.
     
  14. -live wire-

    Active Member

    Dec 22, 2017
    828
    72
    But actually, if you want to allow the motor to spin both ways, or if it generates AC, you will need not just a diode but a more complex snubber.
     
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
    5,685
    3,243
    What motor? This is for some kind of treadmill/exercise equipment.
     
  16. Reloadron

    Distinguished Member

    Jan 15, 2015
    3,619
    1,625
    This is what snubber circuits are all about: RC Snubber Circuits Importance – Design & Usage
    This is what a flyback diode (or your favorite name) is all about: Freewheeling or Flyback Diode Working and Their Functions.

    I see no need for either on a generator or alternator design? My truck has an alternator which is a three phase generator using six diodes to convert the generated AC to a usable DC. In addition to maintaining the battery that alternator supplies both resistive loads like headlights for example and also inductive loads like all the relays and blower motor. There are no snubbers nor are there any flyback diodes. I see no need for either unless I am missing something? A snubber is typically an RC network placed in series with the load across a switch like relay contacts or a thyristor.

    Below is an example of a rotating magnet
    . Magnets 1.png

    Note how the poles are labeled N, S, N, S, and all 12 poles of the magnet rotate. The reason this magnet is scrap is because of the missing metal chips. There was no way it would ever achieve a dynamic balance on a shaft rotating at about 15,000 RPM and while I doubt the thread starter plans to rotate his generator that fast anything which rotates requires consideration for balance.


    Ron
     
    shortbus likes this.
  17. -live wire-

    Active Member

    Dec 22, 2017
    828
    72
    The generator. Most motors can act as generators, and visa versa.
     
  18. oz93666

    Senior Member

    Sep 7, 2010
    184
    37
    I would remind again if all the same polls are facing outwards only DC is generated and even if direction of rotation is reversed the polarity of the current is NOT reversed ...all you need is a electrolytic capacitor across the coil wires ,somewhere about 1,000uf 25V not at all critical ....but important to check polarity of pulse before connecting cap ... cap must be connected the right way round.
     
  19. -live wire-

    Active Member

    Dec 22, 2017
    828
    72
    If I’m not mistaken, there would be no way to avoid reverse polarity across the capacitor. Either it is reversed on normal operation, or when the inductor must dissipate the energy, the polarity is reversed. So don’t you also need a diode? Additionally, just a capacitor could lead to resonance.
     
  20. Sensacell

    Moderator

    Jun 19, 2012
    1,982
    613
    Just buy an off-the-shelf 3-phase brushless permanent magnet DC motor.
    There is no difference between a motor and generator in this configuration.

    You can find them in "frameless" configuration, where it's just a rotor and stator without housing or bearings.
     
    -live wire- likes this.
Loading...