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Loading / Rapid de-loading of DC motor

  1. Feb 25, 2017 #1
    HiI have a device comprising a dc motor connected to a body and connected to the motor are four rotor arms. As the rotor arms are rotated they pass a point on the body that applies load to the motor then rapidly de loads the motor. So that every quarter turn the motor is loaded and then de loaded. The device is suspended by a length of string so that there is no contact with the ground. What I am observing is that as the rotor arms accelerate and decelerate every quarter turn there is no counter rotation of the body. Am I to understand that in the de loading process that the flux density in the motor decreases rapidly so that the speed must increase at the expense of torque. This acceleration with out ( or with very little ) torque may explain the lack of torque reaction observed. Hopefully someone can help explain the process better?.
     
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  3. Feb 25, 2017 #2

    Baluncore

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    Welcome to PF.
    Is the motor connected to a fixed voltage?
    What are the rotor arms connected to, blades?
    Do the rotors have the same axis? vertical or horizontal?

    We really need a sketch of the configuration.
     
  4. Feb 25, 2017 #3
    Hi thank you for your reply. Hope the sketch helps. As you can see the load is produced by the magnets being forced together and when the poles are facing the magnet in the tube fires down the tube de loading the motor. The voltage is fixed so the rotor arms are moving very slowly when the tube magnet fires down the tube so the change in angular momentum will be minimal. The repelling magnet fields in the loading process explain the absence of torque reaction for half of the interaction but the lack of it in the acceleration phase has up to now puzzled me. Hopefully you can explain this to me as I am not an expert on dc motors Since the body is not moving in the opposite direction you can turn the motor off as the rotor arms begin to accelerate and the following rotor arm magnet will collide with the body magnet imparting its momentum onto the body causing the body to move in the direction of the rotor arms. I welcome any further questions you may have.
     

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  5. Feb 25, 2017 #4

    Baluncore

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    This is certainly an intriguing device. It has all the ingredients found in designs for perpetual motion machines, and a motor.
    What returns the sliding magnets to the outer ends of their tubes after passing that repulsive body magnet?
    What is the purpose of the machine?
     
  6. Feb 25, 2017 #5
    Centrifugal action returns the magnets to the outer edge ( they are fleeing from the center ) as the rotor arms accelerate after each interaction and the purpose of the machine is to convert electrical energy into momentum or in this case angular momentum. Just so we are clear this is no perpetual motion device, power must be supplied to the motor to enable the rotor arms to rotate.
     
  7. Feb 25, 2017 #6

    jim hardy

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    Is your motor of a permanent magnet or wound field construction ?
    Former is for all practical purposes constant flux.
    Latter has flux in proportion to field current.
     
  8. Feb 26, 2017 #7
    Hi The simple answer is I do not know what type of motor it is. I bought the motor off ebay about 5 years ago. If it helps it was sold as a motor for opening and closing garage doors.If you could explain the de loading events in the wound field construction type I would be very grateful and please remember my knowledge of dc motors is some what limited at the present time
     
  9. Feb 26, 2017 #8

    jim hardy

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    DC motors with brushes are described by these two equations

    Counter EMF = K X Flux X RPM ,
    Torquein ft-lbs = (same)K X 7.04 X Flux X Armature Current
    where
    K is an emprical constant for an individual motor
    Flux in a permanent magnet motor is a constant namely the strength of field magnet,
    Flux in a wound field motor is a variable set by field current.

    If it's a permanent magnet type, or wound field with constant field current , you can lump Flux and K into one constant that I call KΦ,
    so the formulas become
    Counter EMF = KΦ X RPM
    Torque = 7.04 KΦ X Iarmature

    In a proper lab we measure Counter EMF by spinning the motor at known RPM's with fixed flux and measuring how much voltage it generates. Plot values at several speeds, slope of that line is KΦ.

    Anyhow, to your de-loading
    Assume initial condition is the motor is a constant flux type and is running steady state ;
    constant speed with some value of torque for load
    reducing external torque demand leaves excess torque available to accelerate the motor's rotor and whatever parts are affixed to it
    so they accelerate increasing RPM
    Counter EMF goes up proportionally with RPM, opposing applied voltage, causing armature current to decrease
    less armature current results in less torque produced
    when torque produced (7.04 KΦ X Iarmature) matches external torque demand, acceleration ceases and you've reached new equilibrium speed.

    Proper treatment includes inertia and writes differential equations of motion just like in your university dynamics course.

    I'm guessing your motor is a permanent magnet type because they're so much easier to manufacture than wound field.

    Look it over. Nowadays it's common to find task specific microcontrollers embedded. If it's brushless it certainly has one and all bets are off.

    But i'll wager you find a simple brushed DC permanent magnet motor not much different from the one in your car's electric windows.(Caveat- last one of those i had apart was a 1998 model)

    Is above headed in the right direction for you?

    good luck, and keep us posted?
     
  10. Feb 26, 2017 #9
    Thank you for the information I was looking at a self excited series wound motor in particular at small current, speed is inversely proportional to flux etc but as I have said I have no way of knowing motor type. The speed of the motor is around 60 rpm so every quarter second the motor decelerates to a near standstill and then accelerates into the next repelling interaction slowing once more and then repeating the process as long as power is available to the motor. On the point of excess torque available is the motor accelerating to seek balance ie in the loading process the system becomes unbalanced, or is it conserving as in an ice skater bringing their arms in so that they accelerate without pushing against the ice.The puzzle here is how the rotor arms are able to accelerate without pushing on the body.
     
  11. Feb 26, 2017 #10

    jim hardy

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    series wound motor has field in series with armature, so flux is proportional to armature current.

    yes, Kirchoff dictates that applied voltage must get balanced.
     
  12. Feb 26, 2017 #11

    jim hardy

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    Thought experiment

    were it frictionless you could turn the motor off and it should keep turning forever.

    Will body oscillate as a magnet in a vane approaches then recedes from the magnet on the body ?
     
  13. Feb 27, 2017 #12
    Hi I take it both of these thought experiments are in relation to the device? To answer the first ,the momentum is transferred to the body that must rotate through its environment and so interact with it. The time that it is able to do so is dependent upon the density of this environment. So to put a time scale on things if in deep space a long time , if through treacle not so long. On the subject of forever it would be better to look at the formula supplied with the sketch. If flux goes to zero then speed must be infinity. Assuming there is a rate of acceleration it is reasonable to conclude that it will take an infinite amount of time for the motor to accelerate to infinity or in other words forever. The second is the word in and not on. If the magnet were ON then as it passes the body magnet the repelling magnetic interaction will push against the body magnet pushing the body backwards whilst the rotor arms will also get a push . I would have thought that the body would step backwards as the rotor arms accelerate With the magnet IN this interaction can not take place as the energy build up in the collision phase is released by the momentum of the magnet travelling down the tube and the body does not get its magnetic push backwards. On the motor I am using a ELVI code 102.567/FCE type. A person has asked for specs on this motor on the internet so when someone gives an answer I can supply this to you. If you feel the answers are vague in any way please contact and I will attempt to be more clear .
     
  14. Feb 27, 2017 #13

    jim hardy

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    Where i was heading is
    there's two pieces
    the arm-body magnets
    and the motor

    if the motor tries to maintain constant speed which with constant applied voltage it will,
    will it apply to the body torque that's equal and opposite to that of the first repelling then attracting magnets as vanes sweep past ?
     
  15. Feb 27, 2017 #14
    Hi Jim sorry I misread the questions. My sketch may be misleading, The body magnet has the north pole facing inward towards the end of the rotor arms. The magnets in the tubes are all north pole facing outward so that as the tube magnets attempt to travel across the body magnet they face each other and the body magnets are propelled down the tubes so they are no longer influenced by the body magnet. The motor is now in a no load situation and is free to accelerate. The following magnet which is at the end of the tube rotates into the body magnets repelling magnetic field and as the distance between them decreases the repelling magnetic fields build until the sudden release so there are no attracting magnets as the vanes sweep past. Apologies for any confusion caused. On the body torque in the repelling phase , the body magnet wants to push away from the oncoming rotor arm so it pushes back against the torque reaction locking the body in place.If a tube magnet sticks in the tube and misses a repelling interaction, the rotor arms will accelerate and the body will start to counter rotate when the tube magnet becomes unstuck the system goes back to its acceleration and deceleration mode with no counter rotation of the body. You are right that there are two systems here and I am reasonably certain it is what happens in the motor that is the key to this.
    Regards Roger
     
  16. Feb 27, 2017 #15

    jim hardy

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    okay

    it needs a diagram showing the forces
    the word picture leaves me confused

    i'll guess you're describing a continuous process where rotor and motor slow down as magnets approach one another and speed up as they retreat
    and you are asking why it doesn't wobble back and forth when hung from a string ?
    Good question, my ceiling fan visibly torques against its mount when starting .

    Sum of torques = I dω/dt , I = moment of inertia

    action-reaction pair , torques get applied to both rotor and body by the motor , each has its moment of inertia and that of rotor changes when magnets move
    motor has its own moment of inertia too
    whole assembly has its own moment of inertia
    since whole thing hangs from a string like a mobile it seems natural to sum torques
    but they don't look easy to quantify.

    The motor i suspect is permanent magnet field which with constant voltage will try to run at constant speed
    how fast it accelerates and decelerates depends on its internal design
    some fast servo motors use a thin brass cylinder for their rotor because such an armature has a very small moment of inertia so can change direction quickly.
    Yours i doubt is anything that exotic.

    Gonna take some algebra , i think.


    old jim
     
  17. Feb 27, 2017 #16

    Baluncore

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    Conservation of angular momentum would suggest that the body should counter rotate when the motor drives the rotor arms. If the body remained static when the rotors were driven, then you will have invented a helicopter that does not need a tail rotor.
    I cannot see why this configuration is being contemplated, or why it might be built.
     
  18. Feb 28, 2017 #17

    CWatters

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    +1

    My guess is he hold the body stationary until the motor is up to speed then releases it. If he does than then the body only spins when released because the rotor experiences air resistance rather than because of conservation of angular momentum.
     
  19. Feb 28, 2017 #18

    CWatters

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    Averaged over long periods (say one revolution) there is no change in the momentum of the rotor. So over the same time period you wouldn't expect any change in the angular momentum of the body (due to conservation of angular momentum).

    Over shorter periods you might expect to see the body oscillate back and forth. However if the body has much higher mass than the rotor (where is the battery?) then the motion of the body might be quite small or barely detectable. (eg If I run laps around my garden there is no obvious counter rotation of the planet).

    Edit: The motor is probably operating in a constant speed mode so it will be trying to smooth out any changes in velocity caused by the magnets. So are you sure the rotor really is accelerating/decelerating?
     
  20. Feb 28, 2017 #19

    Baluncore

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    I suspect that the force that pushes the magnet towards the rotor centre and so slows down the rotation during that process, also speeds up the rotor because the radius of the mass is being reduced. Once the arm has passed the fixed magnet the process is reversed.
     
  21. Feb 28, 2017 #20
    I totally agree with Jim that the word picture is confusing. My daughter has skype on her laptop so if one or all of you gentlemen have the same I can demonstrate the device to you in real time.I am in the UK but I am certain a mutual time or times can be arranged. I have no contact with the device I simply turn the power on and turn the power off after a few revolutions. The end result is that the body rotates in the same direction as the rotors were moving , from a stationary start. It works every time so there will be no excuses and I will demonstrate multiple times for your satisfaction. The body moves very visibly and you will be in a better position to comment if you see it in operation.
    Best regards to all Roger
     
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