Building a Linear Induction Motor | Tips & Tricks

[Flash27]

I am currently trying to create a linear induction motor for fun and am having some trouble getting it to start oscillating or move at all. I am using this video as a reference...



I am using 3D printed PLA as the structure for the copper to wind around, 26 GA Craftware USA copper wire, 5/8" cold-rolled steel round from the hardware store, neodymium magnets each 0.5T, and a DC power supply at ~5V and ~0.4A (I have a breadboard with a 10ohm resistor in series to allow for the higher voltage).

The spool for the copper to wind around is 0.1" thick and has 6 layers of 250-280 turns of wire.

I was thinking of first increasing the layer count and maybe change to a smaller diameter rod.
I also have a flux sensor on the way so I currently do not know the strength of the induced field.

If someone could give some advice or a flux model that captures the setup, it would be greatly appreciated.
Thanks!

 

[berkeman]

Welcome to PF!

So you are not getting the magnetic attraction shown in the video? It may be your choice of the pin material:

5/8" cold-rolled steel round from the hardware store

Ideally you would use a rod from a regular solenoid that you can buy, since presumably it will be of a high magnetic permeability material. Do you have access to any solenoids that you can borrow the center pin from?

https://www.newark.com/productimages/standard/en_US/20M1028-40.jpg

Can you post a picture of your setup versus the one in the video?

 

[Flash27]

This is currently what it looks like.

 

[256bits]

1. are your magnets attached with the correct polarity - the two right hand ones should be repelling.
2. when the coil is energized, the right hand one should be attracted to the coil and the one on the left should be repelling the flux.
3 the base holding the coil has to be correctly positioned so that the rightwards extension of the rod closes the switch, activates the coil, which provides the flux to push out the left magnet, and at the same time overcome the repulsion of the left stationary magnet.
4 setup is important. It looks simple. but as a resonant circuit isn't

 

[Flash27]

1. are your magnets attached with the correct polarity - the two right hand ones should be repelling.
2. when the coil is energized, the right hand one should be attracted to the coil and the one on the left should be repelling the flux.
3 the base holding the coil has to be correctly positioned so that the rightwards extension of the rod closes the switch, activates the coil, which provides the flux to push out the left magnet, and at the same time overcome the repulsion of the left stationary magnet.
4 setup is important. It looks simple. but as a resonant circuit isn't

I am also wanting to find the right material for the core. I was trying to find a fully iron core, but all the stores around me only sell stainless steel. Will that work?

 

[256bits]

I am also wanting to find the right material for the core. I was trying to find a fully iron core, but all the stores around me only sell stainless steel. Will that work?

Depends upon the stainless steel.
Two types - austenitic ( not magnetic ) and ferritic, martensitic ( magnetic ).

Take a small magnet to the store and check the attraction.
https://www.scientificamerican.com/article/why-dont-magnets-work-on/

Isn't your cold rolled steel rod adequate. It should be ferromagnetic.

Maybe your rod is too long and the flux generated by the coil is not strong enough to push the end magnets if they are so far out there - from looking at the picture ).

 

[Flash27]

Depends upon the stainless steel.
Two types - austenitic ( not magnetic ) and ferritic, martensitic ( magnetic ).

Take a small magnet to the store and check the attraction.
https://www.scientificamerican.com/article/why-dont-magnets-work-on/

Isn't your cold rolled steel rod adequate. It should be ferromagnetic.

Maybe your rod is too long and the flux generated by the coil is not strong enough to push the end magnets if they are so far out there - from looking at the picture ).

I also thought that would be the problem, but once my flux meter arrived I found a different problem. The flux field emitted from my coil was none existent. I took some time to think about it and I was wondering if it had something to do with the fact I was using uncoated copper wire. Do you think that could be the reason?

This is the flux meter I am using:
https://www.amazon.com/dp/B07SSZXCFG/?tag=pfamazon01-20

 

[sophiecentaur]

I am also wanting to find the right material for the core. I was trying to find a fully iron core, but all the stores around me only sell stainless steel. Will that work?

I use eBay for various bits of metal. Ferrous and non-ferrous are available in small 'offset' quantities and it's usually free postage. (Just avoid overseas sources as that can introduce a long delay although the cost is sometimes lower).

 

[sophiecentaur]

The flux field emitted from my coil was none existent.

Was that with or without the steel rod inserted?

Also, you would get better results with 'soft iron' which may not be readily available. If your steel is cold rolled, its permeability may be improved by annealing it. To do this, remove the magnets(!) and bring it to cherry red with a blow torch and remove the heat over a period of a few minutes, to slow the rate of cooling it. Depending on the strength of flame you can get, you could bury the rod in some sand (in a tin) for the process, which will slow down the cooling further. Perhaps a PF expert could confirm this may be worth while. If you have a blow torch, it may be worth trying the experiment.

PS Have you checked that current is actually flowing in the circuit (sorry if that's very obvious)? Brushing wires together should give tiny sparks and you should expect the wires to get warm if there's significant current flowing. Contacts could be poor.

I was using uncoated copper wire. Do you think that could be the reason?

I just read this. The wire has to be insulated (enamelled) or you will have a short circtuit where the wires touch at the start of the coil - no current through the coil. Use enamelled wire and clean off the ends.

 

[256bits]

I was using uncoated copper wire

I just read this. The wire has to be insulated (enamelled) or you will have a short circtuit where the wires touch at the start of the coil - no current through the coil. Use enamelled wire and clean off the ends.

Seems to be bare wire is right.

26 GA Craftware USA copper wire,

https://craftwireusa.com/collections/copper-wire-spools

That would be a problem for sure.

 

[256bits]

Here is a list of the permeability of various materials.
https://www.engineeringtoolbox.com/permeability-d_1923.html

On the chart you can compare the relative permeability of the material to a vacuum ( = 1. )
If you can get some pure iron, that would be excellent.
Not sure why the carbon steel is so low - maybe due to the additional elements that are added that can change the crystal structure.

But notice that as per @sophiecentaur , the annealed versions perform better.

 

[sophiecentaur]

@Flash27 eBay again for your enameled copper wire.

 

[Flash27]

@Flash27 eBay again for your enameled copper wire.

Thanks! I'll try it out. I've just always had a hard time trusting eBay.

 

[Flash27]

Here is a list of the permeability of various materials.
https://www.engineeringtoolbox.com/permeability-d_1923.html

On the chart you can compare the relative permeability of the material to a vacuum ( = 1. )
If you can get some pure iron, that would be excellent.
Not sure why the carbon steel is so low - maybe due to the additional elements that are added that can change the crystal structure.

But notice that as per @sophiecentaur , the annealed versions perform better.

Nice thanks for the web page that helps a lot!

 

[Flash27]

Was that with or without the steel rod inserted?

Also, you would get better results with 'soft iron' which may not be readily available. If your steel is cold rolled, its permeability may be improved by annealing it. To do this, remove the magnets(!) and bring it to cherry red with a blow torch and remove the heat over a period of a few minutes, to slow the rate of cooling it. Depending on the strength of flame you can get, you could bury the rod in some sand (in a tin) for the process, which will slow down the cooling further. Perhaps a PF expert could confirm this may be worth while. If you have a blow torch, it may be worth trying the experiment.

PS Have you checked that current is actually flowing in the circuit (sorry if that's very obvious)? Brushing wires together should give tiny sparks and you should expect the wires to get warm if there's significant current flowing. Contacts could be poor.

I just read this. The wire has to be insulated (enamelled) or you will have a short circtuit where the wires touch at the start of the coil - no current through the coil. Use enamelled wire and clean off the ends.

Well, when the rod was in it only read the flux value of the rod itself while the current was passing through. Without the rod, it was nonexistent. Yeah, I definitely messed up buying noncoated wire. Also, I'll try annealing it in the meantime because I do have some soft iron on the way.

 

[sophiecentaur]

Also, I'll try annealing it in the meantime because I do have some soft iron on the way.

Looking at the data in the above link, soft iron should make a big difference. BUT if you had used the right wire from the start, you would already have a working motor, I think and you wouldn't have to be talking to PF.
Just put it down to experience. You are not the first to make that mistake. About 50 years ago, a teacher brought me a Crystal Set that a pupil had tried to build but it didn't work. He'd wound the coil with tinned copper wire - same problem as yours - and half a century ago. (Jeez that makes me feel old). The really sad thing about that episode was that the teacher, herself hadn't sussed out the problem.
Electrical Engineering is VERY HARD cos there's such a lot to learn. Stick at it and you'll get there.

 

[sophiecentaur]

Thanks! I'll try it out. I've just always had a hard time trusting eBay.

That's a point of view. I would say however, that I have used ebay to access dozens of small retailers for small quantities of stuff. I could count failures on the fingers of one hand and eBay give you your money back. I have also bought items for several hundreds of GBP and even stuff from China and India. They have always arrived (after the sort of delay you'd expect with free postage from across the world). Same with PayPal. They want / need your business and they are well enough organised not to spoil that relationship.
A couple of decades ago, the local shops started to close down for useful hardware (I don't mean plumbing and DIY, of course) and electronics bits. I really have had no alternative but it's worked for me OK.