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Discussion Starter #1
I "rediscovered" this motor in my searches and it seems to be making a comeback because electronic controls are now ubiquitous and the design has many advantages and few downsides. As a point of reference this was touched upon here:
http://www.diyelectriccar.com/forums/showthread.php?t=75088&highlight=reluctance

Here are some links I posted elsewhere but I'll consolidate them here:

http://machinedesign.com/article/the-switch-to-switched-reluctance-1211
http://www.youtube.com/watch?feature=player_embedded&v=jt4Fa4H43Iw (simple VR motor)
http://www.youtube.com/watch?v=nQ8G5wnH5sc&feature=player_embedded (variable reluctance test motor 3p stator 4p rotor)
http://www.youtube.com/watch?v=b3hmkehrcUg&feature=player_embedded (SRM test)
http://www.youtube.com/watch?v=G2qS2TxU9KY&feature=player_embedded (first run)

I made a drawing of a proposed design for a switched reluctance motor:



Here is a video of a simple SR motor and controller I just made:

From the description:

This is an experimental switched reluctance motor which uses only external electromagnets in the stator, and no windings or permanent magnets in the rotor. Thus it is a very simple, rugged, and low cost design. The expense and complexity may be in the controller. The principle is essentially using electromagnets to align a piece of steel and then switching the excitation to adjacent magnets to achieve motion. I think I need to work on the design of the rotor and the pole pieces so that the force aligns the rotor at an exact point of rotation, which means wider stator pole pieces or more narrow rotor tips.

I may make some changes and see if I can get significant improvement, especially for self-starting. I think I will need to use a full three-phase H-bridge so the pole pairs can be driven both positive an negative (N and S). I also found that it helps to drive two sets of poles together to get higher torque and smoother transitions.
 

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how is this different from a squirrel cage reluctance motor using aluminum bars.
If I understand it correctly, an induction motor is based on slip of the rotor versus the stator to give power whereas a SR motor needs "exact" positioning, because the rotor aligns to minimize magnetic "resistance". Right?
 

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Discussion Starter #4
The induction motor relies on the rotating magnetic field and the slip to cause current flow in the conductive squirrel cage and a back EMF.

The switched reluctance motor works on magnetic forces only so it is totally synchronous and can work with full torque down to zero RPM. The rotor has only magnetic losses so it runs much cooler.

I need to do more thinking and tinkering, but I think it is possible to achieve a lot more torque from the SR motor.

Here is a diagram of how I plan to drive the stator to achieve rotation:

 

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Discussion Starter #5 (Edited)
I found a good explanation and animation of a variable reluctance motor (just another name for the same):

http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU14208

I think these motors are commonly used for disc drives.

Yesterday I took apart a 1050 RPM 1/7 HP shaded pole blower motor. It has six pole pieces and a rotor with 33 slots and a skew of about 16 degrees or 1.5 slot pitch. I was able to cut the conductive ends off the rotor (with a hacksaw) but the bars of the squirrel cage could not be removed and I was unable to rearrange the rotor laminations to eliminate the skew in an attempt to make a SRM. I also tried, unsuccessfully, to mill or cut slots in the rotor, so I gave up on that attempt. However the stator may be useful.

Now I plan to make a rotor from several steel plates. I have some transformer laminations that I might be able to press together tightly enough to be able to machine the outer edges to match the ID of the stator. I may make just a two pole stator rather than four pole as was the experimental motor in the video. I think it will work just as well, and perhaps better.

My first idea was to use a pattern of two coils and four coils as shown for the 4 pole rotor:



But I realized that I should be able to get much better magnetization and more torque by using all six coils for multiples of 60 degrees:



I found an interesting article about motor efficiency that indicated a reduced skew can increase efficiency:
http://www1.eere.energy.gov/buildings/appliance_standards/commercial/pdfs/tsd_sem_nopr_ch4.pdf

And here are other references to skew and other related topics:
http://www.rle.mit.edu/media/pr152/15_PR152.pdf
http://masters.donntu.edu.ua/2010/etf/allagulova/library/article_8.pdf
http://www.lmphotonics.com/InductionMotor/MCog.php (cogging and crawling)
http://www.scs-europe.net/conf/ecms2011/ecms2011%20accepted%20papers/eee_ECMS_0107.pdf (induction motor analysis)
 

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Discussion Starter #6
I copied the following from a discussion on wheel motors:

I am thinking that the switched reluctance motor may be capable of multi-speed operation and a very high torque. Consider the following:



The operation shown requires six different states for a single revolution. But as the motor gets up to speed it may be possible to drive it by simply applying two states, one for 0 degrees, and the other for 180 degrees. But the speed is really only limited by how fast the drive electronics can switch the fields, so this may not really matter. And I think such a motor, with all of the fields fully magnetized, would have enormous torque compared to other motors where at any given moment only about 1/3 of the windings are being driven. This motor could be driven to "lock" in any of the positions shown, and the drive current could be varied using PWM techniques so that it would only pull whatever power is actually needed to maintain that position. And intermediate positions could be obtained by applying different PWM values to adjacent poles (much like microstepping of a stepper motor). That's what this is, actually, but without magnets. ;)
 

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Discussion Starter #8 (Edited)
Thanks for the link. It has a good basic explanation of the principles of the variable reluctance motor (as they call it), and I definitely realize the challenge of the electronics which I have not (yet) implemented in my experimental design. Looks like a shaft encoder of some sort will be needed. I look forward to the challenge! :D

I found more information from Microchip:



This comes from http://www.microchip.com/pagehandler/en-us/technology/motorcontrol/motortypes/sr.html

Inerestingly, they say that the polarity of the pole does not matter. But for the implementation I am going to try, it does matter, and I think it may also result in a more powerful (or torquey) motor.

Here is a section on their motor control module for the PIC24F series:
http://ww1.microchip.com/downloads/en/DeviceDoc/DS39735.pdf

Freescale has some similar information. http://www.freescale.com/webapp/sps/site/overview.jsp?code=WBT_MOTORSRTUT_WP I like their animated GIFs of the SR motor and ACIM (and stepper and BLDC):

SR motor:
ACIM:
Stepper:
BLDC:


And some more from Freescale: http://www.freescale.com/webapp/sps/site/application.jsp?code=APLSWRMOT

This seems to have the most detail on motor characteristics and control theory: http://cache.freescale.com/files/microcontrollers/doc/ref_manual/DRM032.pdf
 

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Hi, I'm a long time lurker and this is my first post.

I've been researching the SRM for about the last six months and now getting ready to build one. Your the first guy I've found thats also into these. I can give some links to some good information, if you would like. Both on drive circuits and types of SRM's that are being made.

As far as self starting they need to be of at least three phases. More than three gives less noise and cogging torque, but ups the complexity of the drive. There are two types of commutation for them, sensored and sensorless. The sensored is the easiest to make.

I'm modifying a single phase 36 slot stator for my prototype. It is going to be a three phase 9 pole, with a 6 tooth rotor. Also it will be a short flux path type. This type is a lower temperature due to the back iron basically on being magnetically active in the individual poles. The magnetic circuit also is more localized in both the stator and rotor.

Please let me know if you want some links to more information.
 

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Discussion Starter #10
I would like any information you may have. You may post the links here or you can PM me, but let me know if you want any of these links kept "private". I believe in the free distribution of information but if you are seeking a patent or trade secret then I'd need to know that. Good luck with your build and I look forward to more discussion.
 

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The links are all from the searching I've done. I too am all about sharing information and encouraging innovation. I'll dig up a few links and post them.

Maybe you can help with my latest problem on this. I have looked and can't find a chart giving acceptable amperage levels for stator windings. The windings in a motor or transformer are different than ratings for open air. Motor windings carry more amps than the open air amps, but can't find a guide for choosing what gage to use. Any information would be much appreciated.
 

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Discussion Starter #13
Very interesting and valuable references! :) I've only just skimmed them and the mathematical analyses are freankly beyond what I can comprehend. I'm a practical experimenter sort and I have some ideas that do not seem to have been explored in these papers or anywhere else. If you notice, I am proposing a wide pole face on both rotor and stator, which I think may produce more torque and/or smoother operation. Also, I propose to drive more than one pair of poles at a time in a sequence. And I want to use full H bridges for each pole pair to reverse the field. So I'm building the circuit and plan to try it on my simple prototype first. Then I have a fan motor that has six pole pieces and I want to make a rotor with either two or four poles. Lacking ability to cut thin laminations I plan to use a stack of steel washers a bit larger than I need and then turn them to size on a lathe and cut the notches for the poles.

As to your question of the wire size and heating of the stator windings, it depends largely on the current you need and the duty cycle and the maximum temperature of the wire insulation. You will need higher voltage as the speed and frequency increase because of the coil inductance, and that will vary depending on alignment. You want more current when out of alignment in order to create rotation torque, and that is when the inductance is low. At alignment, inductance is higher, but you don't need much torque except as holding torque for low speed and servo type use.

You will need to determine the parameters from the physical construction of the motor, and probably just fill the winding space as efficiently as possible. The wire size will determine number of turns and in conjunction with speed will determine the voltage. You may need to take a first shot by an educated guess and then taking measurements when you apply power. Then you can determine exactly what you need. And just add a thermal sensor to detect temperature and increase power until it reaches a level you're comfortable with. That's what I would do! :D
 

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Yeah, my math is very bad too. I haven't worried about the math part either, just the practical part of the design.

Hope you don't mind me sharing some thoughts on what your trying to do. Not trying to be negative or stop you from experimenting, just some things during my research I found about the beast -

The wide more than one pole turned on at a time doesn't seem to work. The SRM principle is as the unaligned rotor pole becomes aligned, it stops moving. So if you turn on more than one adjacent pole thats aligned it is then a brake. Because of the magnetic attraction.

The poles don't benefit from 'bipolar switching'. Because there is no magnetism in the rotor poles. But they need to have the opposing teeth of a pole pair wound to give a north and south pole. This is to complete the magnetic circuit. The mag circuit forms similar to two D's with the flat sides together. It goes through the rotor and around both sides of the stator back iron. If the poles aren't opposite polarity theres no circuit.

All of this is why I decided to go with a "short flux path" motor. Only the back iron and the immediate edge of the rotor under the stator tooth thats 'on' are involved in the magnetic circuit. The stator pole is made of two adjacent slots of the stator. One tooth wound as a north the other as a south. The rotor can then be made from either the original induction rotor or a solid piece of steel. Because there is no real eddy currents involved in it. And the shorted turns of the induction rotor aren't affected by the magnetism.

Sorry for the long post, but this is the first time I've had anyone to talk to about this stuff:) And again I hope you don't take offense to what I said about your design.
 

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Yeah, they have a pretty flat torque level for the whole speed range, are capable of very high speeds and take no magnets. They operate at lower temperatures. They've been around since the late 1800s but have just now become practical due to mosfets and IGBTs for the inverters. With the right controller they can operate in all four quadrants of motor control.

The new Dyson motor is a SRM, and a lot of washing machines are now using them.
Mining equipment has them as wheel motors, and the actual digging power for the machines.
 

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Discussion Starter #18
According to at least one of the articles cited, commercial acceptance and the need for special drives makes the SRM a specialized item that will likely not see extensive commercial and industrial use anytime soon. Perhaps if the drive electronics could be incorporated inside the motor so that it could be used as a simple line start/run motor, it might become more widely used. I'm excited about its possibilities for EV use but practically speaking, it's probably much more practical and less expensive to use off-the-shelf ACIMs and drives if they can be obtained used and surplus. ;)

I'm interested in the "short flux path" design. It makes sense to minimize the path which should reduce the amount of iron and it will probably have more torque with less weight by eliminating much of the iron in the center of the rotor. It is also probably possible to design a "partial stator" which can be placed around a rotor similar to a brake caliper on a disk brake. It would also work as a linear drive similar to MagLev vehicles. The track would be simply a steel rail with notches filled with a non-magnetic material to maintain a smooth surface.

Another idea I had was to eliminate the air gap entirely and instead use roller bearings to carry the flux, rather than air. The motor would then not need bearings on the shaft as the surfaces of the rotot and stator would be a large roller bearing. For a linear motor it might require something like a recirculating ball bearing mechanism as used in linear actuators. More food for thought! :)
 
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