Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

EMF Pinewood Derby

  1. Mar 23, 2016 #1
    I'm taking physics 2 and was told to create a practical project utilizing electromagnetic force. Starting thinking about Lenz's law (which I just read in a text book for the first time a week ago) and I thought it would be fun to send a pinewood derby car uphill instead of down.
    The sprinkler pump had a bad capacitor from standing water and a lot of other connections were rusted up pretty good. I haven't tested the windings yet but I assume they are okay. The idea is to hook the well pump's coil to a battery and use the induced field to accelerate a magnetic car down a track.
    I took the windings from an A/C motor but that shouldn't matter right? I can just hook them directly to a D/C battery? Should I just plug it into the wall? I don't want to start a fire but I don't know my boundaries.

    Am I a crazy person? Will this work? Thoughts/criticism/safety concerns/prayers?
    See two attached photos of current progress. This is my first post on Physics Forums so I'm not 100% on how image posts work here.

    Help or advice of any kind is appreciated, even if it's mean. I understand I'm playing with fire and could seriously hurt myself, so don't don't sugar coat it.

    Attached Files:

  2. jcsd
  3. Mar 23, 2016 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    welcome to PF :smile:

    that should produce a magnetic field. Do you have a test meter with Ohms ( resistance ranges ) ?
    if not, you need one
    I would want you to measure the resistance of the coil windings before connecting it to anything

    that is very dangerous, please don't do it

    finally you really shouldn't be doing this without some direct supervision from someone who
    understands electricity and circuits

  4. Mar 23, 2016 #3


    User Avatar
    Science Advisor

    It's a lot more complicated than you think. The problem is aligning the magnetic fields of the poles in the track and switching them in the correct sequence to induce linear motion in the little 'rotor' car.

    A simplified DC version would look something like this:
    Linear motor coils.

    Your motor coil (from what little I could see) was designed to use the rotational energy of the changing AC field (with a phase shift capacitor) to produce this coil switching effect in a continuous manner around the circle of field poles.

  5. Mar 23, 2016 #4
    The motor is a National Pump Company SFLP150. I was looking for wiring schematics but their website is down. I understand they have been bought out. My plan for the stator is to dismantle the windings and keep them in separate coils. This will be switched on individually by a rotating dial similar to what you would find on a clothes dryer. I don't have the dial yet, but the concept is for it to have a sequential switch, in order, for every coil used. They will be powered by the same battery (as yet undetermined) and pull the car in, switching to the next coil before the car passes through.

    There are 4 matching sets of wires coming out of the still assembled motor. Measured in the lowest 200 ohm setting:
    White/black stripe = 0.2
    Purple = 0.2
    Orange = 8.5
    White = 0.9

    There are also 2 blue wires, which I believe ran to a capacitor and they read 0.0
    A solo yellow wire has continuity with the white wires, and reads 0.9 ohms from

    Attached is the picture of my plan, which I have a week to work out and is open to change. The currently intact stator is just a convenient source of magnetic wire which will be broken up into separate coils, and I could add more if I can find the raw material.

    Attached Files:

  6. Mar 23, 2016 #5
    NSAspook, you mentioned linear motor coils which appear flat. I assume those would be placed underneath the track, energized, and create a current in a small electric motor driving a wheel in the car itself? Is that how monorails work?
    That sounds awesome but I'm not sure how I would get the parts. If it isn't obvious I'm basically dumpster diving.
  7. Mar 23, 2016 #6


    User Avatar
    Science Advisor

    Maybe a better idea would be a EMF launcher for the car using the motor coils to make a short linear activator pusher for the car instead of a drive track.
  8. Mar 23, 2016 #7
    That's basically the same idea but one giant coil, right? Bigger initial push for a drag race.
  9. Mar 23, 2016 #8


    User Avatar

    Staff: Mentor

    Mentor Note -- this thread is being monitored for safety. Please do not consider hooking anything to the AC Mains with your level of experience. Thanks.
  10. Mar 23, 2016 #9
    I'm not hooking anything into a wall, that was a bad joke.
  11. Mar 23, 2016 #10


    User Avatar

    Staff: Mentor

    Good, thank you.

    So to propel a small car along a track with magnetic force, you would use coils spaced along the track, not a motor rotor or stator, it would seem...?
  12. Mar 23, 2016 #11
    The motor stator is just scrap. It's meant to be canabalized and components (already formed insulated copper loops) repurposed. You're right about my inexperience though. I'm not pretending to have it all figured out. That's why I'm doing it! The how is up for debate.
  13. Mar 23, 2016 #12


    User Avatar
    Science Advisor

    Imagine unrolling a circular stator into a flat stator. Then lengthen it by continued duplication. That will make a moving “carpet” of magnetic field that might drag a conductive plate or metal car along the stator in the same way that an induction motor drags around the conductive armature.

    To get the moving magnetic carpet of phase you will need to make AC. That can be two phases at 90°, or it could be three phases at 120°.
  14. Mar 24, 2016 #13
    I agree the idea of a linear motor seems superior at face value, I'm just not sure how to put in practice. Converting DC to AC sounds like a project of its own which I was avoiding, but I'm starting to believe it would be silly not to. My circular coil idea involved me manually alternating current via a switch anyway. Lots of reading to do on the subject tomorrow. Thank you all for the repeated suggestion of a linear motor. The car doesn't have to run an obstacle course. If it moves a reasonably measurable distance due in whole to electromagnetism, and furthers my understanding of how this all works, I'll call it a success.
  15. Mar 24, 2016 #14


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    A DC motor (linear or otherwise, will need some form of commutator and an AC motor works because of the different phases of the fields, which make the armature 'chase' the fields around in the same way, in principle, that the DC is switched by the commutator. If you want to supply the system with DC then you need to switch the fields on and off, according to where the armature (the projectile, in this case) happens to be, in such a way that the 'next' coil is the one that's excited and the one it's actually passed is switched off so it's not pulled back. This is what a DC commutator does as the motor rotates. If the car had a 'shoe' on the bottom, that shoe could connect the supply to the next coil in the chain. So all the coils would be connected to the + supply and the shoe would connect the next coil to the Earth return and activate it. The coils from your AC motor would do the job and it wouldn't be hard to wire them so that each one goes to a contact via the shoe at the right time. Some reasonable amount of iron in the system should increase the force on the projectile. A set of brass pairs of contacts would be easy enough to fit to a wooden 'rail' and the shoe on the car would complete the circuit as it passes by. The 'pulses' that the car receivers would be nicely synchronised as it moves along the track.
    You could improve on it by using a magnet on the car, which could activate a reed switch as it goes past, to switch each coil on and off.
  16. Mar 24, 2016 #15
    That was a beautiful - common sense description, and I thank you for it.

    I'm a fan of the "shoe" idea. Is that how bumper cars at the county fair work? I'm going to go read some more and I'll try to come back with a plan that makes sense.
  17. Mar 25, 2016 #16
    I drew some quick sketches to try and further my understanding:

    This is why I can't send a magnet over a bunch of coils in a row.
    1st coil pushes magnet forward, good
    2nd coil pulls magnet forward, good
    2nd coil pulls magnet backward, bad
    3rd coil pushes magnet backward, bad
    Magnet falls into equilibrium and stops

    4th coil would never be reached, but hypothetically it would create another equilibrium.
    I (think) I understand why this doesn't work, unless I'm over simplifying it.
    | can't-do-magnets-tough-coils.jpg

    I understand the concept of a shoe to activate coils at desirable points, but I'm not sure why it's necessary if the coils are perpendicular to the track.
    The momentum of the car should force the magnet over the center of the coil. At this point, polarity is "reversed" and the coil which was attracting the north end of a permanent magnet is now repelling the south end of the same magnet, while the next coil down the line simultaneously pulls on the north end. That chain reaction just keeps going until you run out of coils.

    I think this understanding is wrong, but I don't understand why it's wrong. Could someone set me straight? I'm trying to figure out the best orientation for coils in the track. Hope to build a working prototype this weekend. Scored a brand new spool of magnet wire to make the coils with, so I should be okay on materials. I suppose I'll use big bolts as iron cores for the coils.
    Last edited: Mar 25, 2016
  18. Mar 25, 2016 #17
    Well now I understand. I built a test coil powered by a D battery. It picked up iron things fine, but I'm seeing that overpowering the magnets in the car in an effort to repel them is futile. The magnets will attract to the iron core in the coil and no amount of feasible electricity will move them away. Non-permanent magnet attractants in the car and a system to disable coils seems perfectly logical now that I see it in action.

    Realizing I probably won't use permanent magnets in the car being pulled by the emf, what is a good alternative? Nails? Long rods? A flat base on the bottom of the car?
  19. Mar 25, 2016 #18


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I'm assuming that the orientation of coils and magnet are appropriate. (Like the motor only laid flat).
    Without any use of switches of some sort, there will be no net momentum transferred to the magnet and your first description of things is correct. True, you could push it fast enough to go past all the coils - but then you could do without any coils and get the same result. It's essential that, wherever the magnet is, it needs some repulsion one end and / or attraction the other (even starting from stationary). This is the whole basis of how a commutator motor works; always push/pull in the same direction and no chance to reach equilibrium.
    There are many options for layout I think but the way the coils are arranged is not how you have drawn in the second diagram (at least not the way the poles of the field are arranged). Poles should be presented to the magnet in NS pairs, I think. Your way would not start.
    I think this arrangement could do what's needed
    NS SN NS SN NS (as in your first diagram)
    and two coils would be activated when the magnet position is
    and off
    and the next pair on
    I haven't thoroughly checked this but you could do it on paper first.
    To make sure it works from stationary, the phase of the commutating segments would probably need to be adjustable.
  20. Mar 25, 2016 #19
    Thank you so much for your help setting me straight. I really do appreciate it.
    In the second image above I was imagining a bolt core like under the track and pulling in the magnet at each node. So the first electromagnet gives enough momentum to reach the 2nd's field but turning off before the car reaches it and stops. I made one to test with a D battery and it functioned to my delight, even if it wasn't very strong.
    I have two 12v disposable lantern batteries for the real run, and also have access to a 12v 580cca car battery, though I'm skeptical of using it. power-latern-battery-2x.jpg

    I've decided not to use permanent magnets inside the car due to them possibly overpowering the EMF and stopping on the coil cores due to regular magnetic forces. I found some hardened welding steel (realizing non-harded would be more ideal) and am considering using it for both the attractant in the track car and the core for the coils.
    Instead of a bolt sitting directly under the car creating an EMF sucking everything in though the bolt head, I would cut the welding steel to fit in the track sideways with the coil surrounding it. I'm still debating if that's a better idea or worse. Here is my newest design, which I've actually drawn while writing this response:
    It incorporates your shoe design, activating the oncoming coils one at a time. My hasty drawing doesn't shoe that process to scale. The rear of the car's shoe would leave the next coil just as the welding rod embedded in the nose of the car reaches it. Still a work in progress but I think this is my best design yet. Since it doesn't incorporate permanent magnets, energizing two coils at a time for both push and pull wouldn't work anymore. That was an excellent suggestion though, and permanent magnets aren't completely off the table. I'm just trying to figure out the best course of action. Hope to make a working prototype over the weekend.
  21. Mar 25, 2016 #20
    I could have a second shoe and magnetized rod toward the back of the car, offset to trigger at distances midway between the first bar, but far enough away to have no measurable effect on the nose rod. The car is a little over 9 inches long. Maybe I could even do 3 of them. That's a sudden eureka moment just from looking at my own drawing. I love those.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: EMF Pinewood Derby
  1. The Carrier of an Emf (Replies: 7)

  2. Electrical Derby (Replies: 1)

  3. EMF calculation (Replies: 5)

  4. Measuring the EMF (Replies: 19)