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### Topic: Linear three phase induction motor (Read 351 times)previous topic - next topic

#### futurenow

##### May 21, 2018, 04:28 am
I'm trying to make a linear three phase motor like this but on a bit smaller scale. So I think I understand the concept of how it all works now just trying to figure the wiring. So I will probably have 4 sets, two on each side(able to change). This is run on AC current but I want to use a dc battery not wall power. So I have read I need an inverter(not entirely sure what that means other than turns dc to ac. I have also discovered that the idea is to have as close to a perfect sine wave as possible rather than just full on blocks(each wire has same sine wave but spaced out by 120 degrees). Also I would like to be able to adjust the frequency and by means of code rather than a bulky external device. Furthermore, I have read many similar things that need to turn 12v to like 120v or more for their application but if not mistaken i don't think i would need to step-up(i think the term is) unless ohms law does not work the same in AC as it does DC because I think Ill just use somewhere from 20-30awg wire and in pretty small space so 120v would mean super high current. Each coil may only be 1-10ohms I would think. and current would need to be <5amps prob hopefully on the lower end to decrease heating of wires. So the voltage of my dc battery should be fine unless ac doesn't apply the same principles.

So basically I'm wondering how to make the setup. What parts and the wiring setup. Then ultimately the code if I cant figure it out. As in all things I think there are multiple ways of doing this. As I'm not an expert simpler and more compact is always easier to build and understand.

I made these diagrams to help illustrate the plan. There one attachment is a more detailed look at only one side. The other side will be the same thing although maybe opposite polarity(I'll have to think about that more/ just test it). Please ask any questions. Thanks!

#### allanhurst

#1
##### May 21, 2018, 04:36 am
I presume it's like a normal 3-phase squirrel cage motor but with coils in a line.

The demo shows a  metal plate moving - aluminium?

I suspect this will be very inefficient - see the cooling fan on the side.

Is this just a demo? , or do you have a real-world application, for which other approaches might be better.

Allan

#### futurenow

#2
##### May 21, 2018, 04:42 amLast Edit: May 21, 2018, 04:43 am by futurenow
Yes that is aluminum and it is the same principle of an induction 3 phase motor accept flattened out. I am just interested in this concept based on lenz law and the aluminum works for this even though it is not magnetic it is conductive so copper works better but aluminum is cheaper. Tesla cars run on induction motors and I like the idea of creating a track out of aluminum kind of like a rollercoaster. I am not sure on the efficiency or anything but I would think its pretty good since tesla cars use this concept.

#### allanhurst

#3
##### May 21, 2018, 04:48 am
Almost ALL normal 3 - phase motors are of the induction type -TESLA's included,  but not like this.

I suspect he uses BLDC types.

The video shows something which I doubt approaches 1% efficiency.

Allan

#### futurenow

#4
##### May 21, 2018, 04:53 am
I meant tesla cars but ya. I find it more interesting that there aren't moving parts in this rather than the efficiency. Also just learning through the process of making this is interesting to me

#### allanhurst

#5
##### May 21, 2018, 05:09 am
Using the eddy currents in a conductor is a terribly inefficient way of creating an induced magnet.

Most of the energy goes into heat. It's basis of eg magnetic braking systems on lorries/coaches.

Normal motors use segmented silicon iron or permanent magnet rotors and can acheive 95%+ efficiency.

OK for fun, but there'd have to be jolly good reason to try and make a motor this way.

Allan

#### futurenow

#6
##### May 21, 2018, 05:25 am
Ya but in a real world senario would this be ideal because it would be way too expensive to make a track out of neodymium magnets but aluminum would be cheaper. It all got me thinking from this video u can just skip to around 15:30 as well as like 18:00 and they say their idea is very cheap but i was trying to think of a cheap less moving parts improved version of it. Thoughts?

#### allanhurst

#7
##### May 21, 2018, 05:32 amLast Edit: May 21, 2018, 06:16 am by allanhurst
You're not the first to go down this route. Laithwaite in the 60's tried to make a train based on these ideas, as did the Japs.  I saw Laithwaite's  forlorn scrap at Cranfield in the 70's.

They got <30% the efficiency of conventional motors, and a used lot of complicated expensive unreliable electronics, so the idea has been quietly dropped.

Suggest you do the same.

You're never going to get round the terrible efficiency of the induced magnetic loop. Whatever you do , the gaps have to be large, which is a BAD idea. A train trundling along at 100mph is unlikely to be able to maintain a <1mm gap, even if the track is perfect. And what about curves? How will you upgrade existing track and install your driving strip to those accuracies? What about distortion in extremes of heat and cold?And any disgruntled yob could ruin it with a few decent thumps with a sledgehammer.

A linear BLDC might be worth a second look - with a segmented steel track and variable frequency drive. But to what gain over much cheaper and more efficient conventional systems?

Especially when we're all meant to care about energy consumption......

Linear motors do have a place - for example in 'flying faders' for mixdown on music recording consoles. It's been done. But not for transport.

Allan

#### futurenow

#8
##### May 21, 2018, 06:38 amLast Edit: May 21, 2018, 06:40 am by futurenow
Is it that bad? this video shows that tesla cars use induction motors. I suppose the gap is very small <1mm. Is that the difference? I saw charts saying this is very efficient and also at all speeds rather than a certain speed. BLDC is the same thing except with magnets in the track(if it were linear). correct? except they use dc power. idk why they must be a bit more different then. What are you suggesting is most efficient and also real world cost effective (not building on train railroad tracks or anything, starting from scratch). I think those maglev trains in china are this bldc approach. I think they are very expensive.

So is the induction method cheaper to make but less efficient meaning electricity would cost more to run the cars/trains

wheras bldc is more expensive initially but more efficient so less costs to run the car/train

wait but then why wouldnt tesla cars use bldc. I feel like theres something I'm missing here

#### futurenow

#9
##### May 21, 2018, 07:02 amLast Edit: May 21, 2018, 07:08 am by futurenow
I'm not super familiar with this just yet but upon further research it seams this switched reluctance motor may be a more efficient alternative. Curious to hear your input on this. I'm going to go try and read more but figured I throw this out there to you for now

so far its construction as well as its workings appear to be much simpler though

#### MarkT

#10
##### May 21, 2018, 11:21 am
Using the eddy currents in a conductor is a terribly inefficient way of creating an induced magnet.
The efficiency can be high if you use enough material - large industrial induction motors are
efficient.  The cost of permanent magnets in ton quantities means large AC motors are all
induction types, steel is a lot cheaper that magnets.  The time constant of the induced magnet
can be several seconds in a very large motor, meaning the energy is dissipated slowly compared
to the working power going through the device.

But yes, small induction motors are poor compared to good permanent magnet BLDCs
Quote
Most of the energy goes into heat. It's basis of eg magnetic braking systems on lorries/coaches.

Normal motors use segmented silicon iron or permanent magnet rotors and can acheive 95%+ efficiency.
Induction rotors are laminated silicon iron too, with copper or aluminium squirrel cage embedded,
the laminations are essential to good efficiency in any cored motor.
Quote
OK for fun, but there'd have to be jolly good reason to try and make a motor this way.

Allan
The reason is the rail can be cheap as it doesn't need magnets.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### TomGeorge

#11
##### May 21, 2018, 01:21 pm
Hi,
What you have to realize is with a linear induction motor, the field is open in a flat plane, with a rotary induction motor the field is in a circular plane.

In a circular system you enclose the field and can make clearances extremely tight, due to bearings.
The motor only has to use its energy to rotate  the load.

In a linear system like transport, you have an open field that is not just propelling the load but supporting it to obtain ride clearance.
Any advantage from mag lev to reduce friction is used to levitate.

It is surprising how efficient a train with steel wheels on a steel track, with proper wheel and rail profiling can be.

Tom...
Everything runs on smoke, let the smoke out, it stops running....

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