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How to control the field current of an alternator by using PWM?

Discussion in 'General Electronics Discussion' started by Xavier Pacheco, Mar 24, 2017.

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  1. Xavier Pacheco

    Xavier Pacheco

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    Feb 12, 2017
    Hello,

    How can I use a PWM to control the field current of an alternator? That is, a regulator to vary the strength of the magnetic field?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You vary the pulse width (or more strictly the mark/space ratio) to vary the average current.
     
  3. Xavier Pacheco

    Xavier Pacheco

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    Feb 12, 2017
    So, do I just need to build a normal PWM and supply it with the "B" Terminal (Output voltage) of the Alternator?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Well, rather than supplying DC, you supply a PWM signal.

    Obviously you will need to monitor the current if the absolute value is important, and provide a current path for when the signal is off.

    As a guess you'd probably want to operate the field coil in continuous mode.
     
  5. Bluejets

    Bluejets

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    Oct 5, 2014
    B terminal is the main battery connection.
    What type of alternator do you have.
    Most these days have an internal regulator.
    If an older type with external regulator, field regulation is through 1 and 2 via an external regulator designed for the job, no pwm required anywhere.
     
  6. Xavier Pacheco

    Xavier Pacheco

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    Feb 12, 2017
    I have attached the alternator that I have. I just need to control its magnetic field strength, that is why I thought of PWM.
     

    Attached Files:

  7. Minder

    Minder

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    Apr 24, 2015
    The modern regulator uses PWM now, the first style that replaced the old electro-magnetic type was an electronic version of it that produced radio interference, hence the transition to PWM.
    To control with your own PWM you have to remove the internal regulator if fitted and output the new PWM to the slip ring terminals.
    M.
     
  8. Xavier Pacheco

    Xavier Pacheco

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    Feb 12, 2017
    Thank you. I think you got the point. I have attached additional pictures about what I have.
    This is the point:
    That's a fitness bike system which has a display and an alternator control board. The display has many functions. Imagine you just have the alternator, and you want to control its magnetic field to make it useful so you can use it to exercise. As you said, all I need is to output my new PWM to slip ring terminals and change the strength of magnetic field by a potentiometer.

    I have attached a PWM circuit. So, I just need to replace the 6V by the alternator output voltage, right?
     

    Attached Files:

  9. Minder

    Minder

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    Apr 24, 2015
    The alternator shown appears to not have an internal regulator so you should be OK feeding the field with a PWM signal, I assume you are increasing the voltage out into a fixed load in order to increase the effort needed to turn it.
    M.
     
  10. Xavier Pacheco

    Xavier Pacheco

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    Feb 12, 2017
    What do you mean by "I assume you are increasing the voltage out into a fixed load in order to increase the effort needed to turn it"?

    Sorry, but I am still confused. Do I need to include the resistor load shown in the picture attached below? The alternator output voltage is Ok for supplying the PWM?
     

    Attached Files:

  11. Chemelec

    Chemelec

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    Jul 12, 2016
    Using just PWM can reduce the Output Current, but it will not regulate the output voltage.
    It would be better to use a Voltage Regulator that has a Current Limiting of some kind.
     
  12. Minder

    Minder

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    Apr 24, 2015
    I don't see a need to regulate the voltage if the alternator is feeding a fixed (e.g.resistive) load of some kind the load (effort) will increase with voltage for a given peddling rate.
    M.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm pretty sure this is another one of those threads which starts with:

    How do I suck eggs​

    And where, given that no other information is offered, the only response that is not a plea for more information is:

    You apply suction to an egg
    Further discussion reveals that the person asking the question doesn't actually understand "suction", have much understanding about "eggs", and seems unsure why they even need to "suck an egg" (perhaps someone else suggested it).

    So now we get to the point we are at now where the person posing the question is taking about baking a cake. Sure, we've heard about eggs used in cakes, and we've (bravely) suggested that maybe the sucking was meant to get the egg out of the shell (in which case there are far better methods). But no, we are assured it's nothing about that, and why are you trying to confuse me.

    The problem is that we don't seem to be dealing with "sucking" or "eggs" any more, we know that the might be a cake involved somehow. But details are being kept from us. Is there a problem with the icing? Did the file keep sinking to the bottom of the pan? Are we getting warmer?

    It is obvious now that the answer to the first question should have been to ignore pretty much everything that was asked.

    So the real question becomes:

    @Xavier Pacheco, what are you actually trying to do? Please describe the problem, NOT what you think the solution is.​
     
    CDRIVE likes this.
  14. Xavier Pacheco

    Xavier Pacheco

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    Feb 12, 2017
    I am sorry. My intention was not to confuse any of you. I am not an expert in the topic, that is why I appreciate your effort to give me a hint. Now, let me try to explain to you the problem again.

    I have an alternator, which I have attached some information above. I want to control the strength of its magnetic field to use it as an exercise bike. That is, while pedaling I want to adjust its magnetic field by using a potentiometer to make the pedaling either stronger or softer. That is why I talked about controlling field current. Let me know if you got the idea. This is something I am working on by myself. I am not pretending to get all the job done by you all. I am just trying to get started. If you need another detail, don't hesitate to let me know. Thank you for taking a time to answer me.
     
  15. Minder

    Minder

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    Apr 24, 2015
    It has already been answered, here and other forums, if you want to control the voltage via PWM with a fixed load on the output, then do you need more?
    M.
     
  16. Chemelec

    Chemelec

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    Jul 12, 2016
    What is the Output of your Alternator Connected to?

    No Load and it will turn fairly Easy regardless of the field current, High Current load and Much Harder to turn.
    But it is also about RPM as alternators don't put out much at low RPM.
     
  17. Minder

    Minder

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    Apr 24, 2015
    I think you will find the opposite is true, One reason why alternators replaced DC generators in auto's is the fact they have the ability to relatively high output at low rpm.
    M.
     
  18. Chemelec

    Chemelec

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    Jul 12, 2016
    Yes but Not That Low.
    Even at an Idle, of 800 RPM, the Alternator is turning about Twice That.
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, I know :)

    That's still how...

    And so is that. But at least we can extract from your description what the "WHAT" is:

    You want to make an exercise bike where you can adjust the effort required to pedal it. Presumably you have the bike bits and some way to connect it to an alternator that you already have.
    The resistance to rotation of the pedals is due to energy lost in the system. If there is no loss, once you start the pedals they will keep going for ever. You're almost certainly interested in having a variable amount of resistance felt when keeping the pedal rpm constant.

    Ignoring small losses in bearings, etc., the resistance you feel is due to the energy being extracted from the alternator. You want to be able to vary the amount of energy extracted from the alternator both as speed changes, and for a given speed.

    Because of this, the alternator has to drive a load. If there is no load, the energy that the alternator could be generating doesn't go anywhere, and thus there is no significant resistance to turning.

    You might start with the field windings being powered from either the output or a fixed voltage source. The former is easier, but relies on residual magnetic field in the alternator to work -- and you may not have this. Either way though, you then need to load down the alternator (the field coils, if powered from the output ARE a load, but typically not a large one). You should probably start with a purely resistive load. You need a load which has an appropriate resistance to give you the *MAXIMUM* resistance you ever want to feel, and capable of dissipating the amount of power dissipated when the pedals are going as fast as you can even imagine they will be. It could be 10Ω and 500W (but I have no idea what it will be!).

    Now you can reduce the effort required for a particular RPM by reducing the current in the field windings (you can also do it by varying the load resistance, but variable resistances at high powers are really expensive, and I'm not sure of the voltage and other characteristics so I won't recommend a semiconductor solution). There are a number of ways of varying the current through the field windings. The first is to vary the voltage. If you're using external excitation, this might be a good option. If you're using the output to drive the field windings then adding some series resistance might do the trick. Finally there is PWM and this can be applied in either circumstance.

    Given that the current through the field windings is significantly smaller than that generated by the alternator, series resistors have to dissipate less power, and will be cheaper.

    However, you really need to know many more of the characteristics of your system before we can make firm suggestions. For example, if the field current is only a couple of amps, a cheap variable switchmode regulator could be used to provide a variable voltage to the field windings. If they require 10A, that's less likely.
     
    BobK likes this.
  20. daberbaber

    daberbaber

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    Feb 2, 2012
    If you want to generate electricity for later use, maybe this makes sense but if you want this to cause a varying degree of pedaling resistance, any electricity will have to be dissipated as heat. My old stationary bike has felt pads similar to brakes on a bike that are adjusted to do the same thing.
     
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