EMF Pinewood Derby: A Practical Physics Project

[Stinson]

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 textbook 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.

 

[davenn]

Hi
welcome to PF

I can just hook them directly to a D/C battery?

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

Should I just plug it into the wall?

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 circuitsDave

 

[nsaspook]

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.

 

[Stinson]

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.

 

[Stinson]

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.

 

[nsaspook]

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.

 

[Stinson]

That's basically the same idea but one giant coil, right? Bigger initial push for a drag race.

 

[berkeman]

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.

 

[Stinson]

I'm not hooking anything into a wall, that was a bad joke.

 

[berkeman]

I'm not hooking anything into a wall, that was a bad joke.

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...?

 

[Stinson]

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.

 

[Baluncore]

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°.
https://en.wikipedia.org/wiki/Linear_induction_motor

 

[Stinson]

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.

 

[sophiecentaur]

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.

 

[Stinson]

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.

 

[Stinson]

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.
|

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.

 

[Stinson]

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?

 

[sophiecentaur]

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'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
NS SN NS SN NS SN NS SN
S--N
and off
NS SN NS SN NS
S--N
and the next pair on
NS SN NS SN NS SN NS SN
S--N
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.

 

[Stinson]

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.

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.

 

[Stinson]

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.

 

[Stinson]

And a brain fart moment. The drawing is wrong. Because the coils are surrounding the welding bar horizontally in the x-plane of track, the induced magnetic field would still flow above and under the track in the y-plane. I had that idea right when I first thought of the concept but drew it wrong in the diagram , then described the diagram as I typed as drawn. That was a big oops, but this is a learning process.

 

[Stinson]

Instead of a straight bar would a U-shaped bar with the poles angled toward the iron bumper of the car be better?
I suppose my biggest grey area is shape, placement, and configuration of the coils.
Activating them at specific intervals and reversing polarity in a NS SN NS fashion seems straight forward enough.

 

[Stinson]

I also have 3 thin square magnets which could be placed in front of the car via a hacksaw groove. They could be placed in front of the car passing though alternating NS SN NS SN coils which would be activated in pairs via shoes as you described above. I think I'll try to build that in the morning unless someone chimes in that it's a silly idea for whatever reason.

 

[sophiecentaur]

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'm afraid that doesn't make sense. Using a magnet on board means that you can have repulsion and attraction going on at the same time, which is why electric motors are made that way. There is no disadvantage in this. Your ideas about "EMF" are not valid (I don;t actually understand what you are worried about there).
Having two shoes and a more complicated set of contacts could be a distinct advantage and modifications could be made retrospectively.

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.

Again, this means absolutely nothing to me. You don't "suck" EMF. It's not like that. You have a problem here, if you know very little about Electromagnetism and you must avoid jumping to conclusions, as you seem to be doing. It will waste your time. The relatively simple ideas which your early diagrams (and my crude one) are quite enough to get a result, as long as you can build the thing (the biggest problem for many people. ).
Things to remember:
1. A magnet in the car is the only way to get good propulsion. Iron just gives attraction, which is not good enough.
2. More than one shoe could improve the timing of the propulsive pulses.
3. Study the 'logic' of the switching with diagrams. A movable strip of paper, with the magnet and brushes drawn on it and a row of coils for it to move past should be easy to arrange and would give you a good model. All you need to remember is that NN and SS will repel and NS and SN will attract. (Stating the obvious but a strict rule to apply)
4. A Car battery would be a good source of power BUT it is essential that you use a FUSE in series with it or you could easily have a good old fire on your hands. A car battery is virtually an INFINITE source of power. Be prepared to use up a lot of (say 10A) fuses in your tests.

 

[Stinson]

My way of thinking was like a junk yard magnet crane. Instead of a single crane attracting some iron and moving it, there would be a series of cranes very close together attracting some iron but letting the next one down the line take over before it actually connects. There would only be a pull, no push, and the ride would probably be jumpy, but I thought it would work. I was jumping to conclusions, like you said though. Experimenting with magnets now.

 

[sophiecentaur]

A crane doesn't need to move things. A motor does. Think about the way a motor works and extend it to linear motion. A successful motor uses Magnets (electro or otherwise) for both rotor and stator. Trust me. (No one on PF will disagree - if they're reading)
If you can find a single simple motor diagram on the web that does't use magnets, I will be gobsmacked.

 

[Stinson]

I have 180 winding electro magnets which will pick up paper clips on a D-battery but they won't repel my permanent magnets. The core itself is more drawn to the magnet than the coil can push away. I guess it's time to up the current.

 

[sophiecentaur]

I have 180 winding electro magnets which will pick up paper clips on a D-battery but they won't repel my permanent magnets. The core itself is more drawn to the magnet than the coil can push away. I guess it's time to up the current.

Yes. A stronger magnet will 'beat' a weaker one and cause attraction to the core, in spite of N-N repulsion. BUT, if you restrain the attractive force, that's towards the magnet, you will still find a lateral force, due to the imbalance from one pole to the next. You should look at the way a motor works. The spacing in a rotary motor is controlled by the rigid stator and the bearing of the rotor shaft. You have to satisfy the same condition if you want to see the Motor effect above the simple attractive effect. I have already made this point. The construction is important if you want a practical linear motor. A bar magnet will be attracted and repelled in a lateral direction even when there is a force towards the stator.

 

[Stinson]

I don't think this is what you are saying but this was my logic. *I don't think this will actually work because what is going to move the magnet from one side to the next unless the electromagnets are super strong? Originally I thought they would be but now I'm not so certain.
I'm going to try to draw something better right now.

I'm going to think about it some more. Actual magnet and coil pictured, drawing artificially darkened to improve clarity
Each row is a representation of time when the permanent magnet crosses an electromagnet
Electro magnets would be switched "off" and back "on" the moment the permanent magnets pass over.

 

[Stinson]

I have physical movement of the car in both directions from a single magnet, so that's neat. Still a lot of work ahead but I have real tangible baby steps.

 

[Stinson]

Also, the car battery isn't blowing the 10A fuse (very little time connected) but the smaller lantern battery seems to result in more motion. That is unexpected.

 

[sophiecentaur]

Also, the car battery isn't blowing the 10A fuse

It's encouraging that you are getting some motion.
If you have a DMM, you could measure the resistances in the circuit. That would give you an idea of the actual current flowing. It looks as if your coil circuit has a resistance of more than 1Ω, if 12V doesn't blow the fuze. That's fine. The fuse is there to protect against the effect of shorts and my 10A guess must be about right. However, why the car battery is not performing better is a surprise. Poor connections or a high resistance fuse holder, perhaps?
How are you supporting the moving magnet, to deal with the problem of attraction towards the stator? Have you understood the point I am making about that and how it's dealt with in a normal motor?

 

[Stinson]

Yes I am understanding, and I thank you so much for your patient and persistent support. It turns out my error in theory on paper was a missunderstanding of how the permanent magnet would react to the flux of the electromagnet. Now that I can physically see the permanent magnet does not travel from the "N" to "S" end of the electromagnet, but instead achieves equilibrium dead center, reversing polarity at that point to to push/pull half way though a single electromagnet is clear. I just wasn't getting that on paper looking at endless flux diagrams.

So the car starts at one pole, is pushed to the middle, polarity reverses, car continues on to the next magnet, repeat repeat.

I have 4 working electromagnets and enough magnet wire for 10 more, so I can keep this up for a good long while. I haven't used any brand new magnet wire yet, only what I have salvaged used. It made sense to use the old stuff to work the kinks out of wrapping the windings before I use the brand new spool. There were some noticeable blemishes on the used wire with close inspection, but it checked out on the ohm meter.

I currently have the magnets secured to the nose of car with painter's tape. Once I feel satisfied I am out of the orientation experimentation phase, I will cut a hole in the nose of the car and bondo the magnet into the frame.

The fuse is attached with some small alligator clips I picked up at that auto parts store when I got the fuses. Jumper cables to battery, small alligator clip from positive jumper cable to fuse, alligator clip from other end of fuse to electromagnets (solder), and then I'm just touching the ground to the negative alligator clip for now. Haven't messed with shoes and permanent placement yet, though I did pick up a piece of aluminum sheet metal to make the shoes and track with.

I figure I'll have 2 wooden rods like chop sticks or popsicle sticks coming out the sides of the car in line with the magnet. I'll fold a piece of paper to function as a spring and hot glue it facing down from the stick. Then hot glue some sheet metal to the paper. One side will be initial coil initiation and the other side will be polarity change. To figure out where to put the sheet metal switches for the show, I'll simply run the car. When it is close enough to the magnet to pull in, start the track. When it stops, stop the track and start the polarity change. I'll bondo the gaps smooth so the shoe doesn't physically realize it isn't on the sheet metal.

 

[Stinson]

Just got back from church and got to try out the double wrapped coil I finished just before I had to take a shower and leave 5 minutes late. That makes a BIG difference, wow. I expected it to, but it exceeded those expectations. I plan to step it up as I go down. The first 3 coils have 90 wraps, the next two will have 180, then 270, then 360

Just for the lolz. I expect the bigger difference I'm seeing from 180 is because whatever friction retardation or anything else holding the first 90 wraps back are already taken care of. It's more than double the distance.

I started with the single row coils because the worst shape material I had required that recepie. As I go up in rows I'll also be using brand new wire for the first time. The double coil I just made was actually in the worst shape of all.

 

[sophiecentaur]

This seems to be going well. You should try to 'tune' it by subtle changes in the phases and the spacings of the shoes. It could be impressive (but no promises!)