---> The Ideal Motor Solution?
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safe
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SR - Switched Reluctance
http://www.srdrives.com/technology.shtml
http://www.members.cox.net/rdoctors/motworld.html
"What type of motor shall I use? For most application the induction motor is a great choice, with the DC brushless a close second, although the SR or switched reluctance will be seen more and more as the electronic controls are getting inexpensive and the SR doesn't need magnets. The induction motor is simple, cheap and reliable. For sizes over 1 HP it's much better to go to a three phase motor. Do you need variable speed, what is the target cost, what power do you need, torque and RPM, size and shape, duty cycle, new product or replacement?"
This type of motor is still of the "Inductance" category (which means that it prefers full power) and it has no permanent magnets. There are no natural backEMF issues to worry about and the efficiency can get into the 94% range as long as you are using full power.
So here's what appears to be the "ideal" motor solution. It's computer controlled and uses NO sensors. For the American 750 watt law you could set the power limit to exactly 750 watts and be able to get that power from zero rpm to full speed. (whenever you hit the wall of air resistance)
While this doesn't help the people who ride at a "steady state" all day long it does provide the ideal solution for anyone who wants "sport" performance that complies with the law. A hub motor that was built using this design could accelerate from zero to full speed and always get about the same power and not have efficiency problems. When you back off the throttle and run at partial settings then the efficiency drops. If you've ever ridden one of those now out of date two stroke motorcycle road racers (like an old RD400) this was how they behaved... if you rode too much at partial throttle you would "foul" your spark plugs and the motor would stop working. You literally were required by design to keep the rpms high or the bike would break down.
When buying hub motors one might be asked:
"Would you like our 'sport' model or our 'cruising' model?"
...which would be either an SR Hub Motor (best at peak power) or a DC Brushless Hub Motor (best at high rpms and low power).
http://www.srdrives.com/technology.shtml
http://www.members.cox.net/rdoctors/motworld.html
"What type of motor shall I use? For most application the induction motor is a great choice, with the DC brushless a close second, although the SR or switched reluctance will be seen more and more as the electronic controls are getting inexpensive and the SR doesn't need magnets. The induction motor is simple, cheap and reliable. For sizes over 1 HP it's much better to go to a three phase motor. Do you need variable speed, what is the target cost, what power do you need, torque and RPM, size and shape, duty cycle, new product or replacement?"
This type of motor is still of the "Inductance" category (which means that it prefers full power) and it has no permanent magnets. There are no natural backEMF issues to worry about and the efficiency can get into the 94% range as long as you are using full power.
So here's what appears to be the "ideal" motor solution. It's computer controlled and uses NO sensors. For the American 750 watt law you could set the power limit to exactly 750 watts and be able to get that power from zero rpm to full speed. (whenever you hit the wall of air resistance)
While this doesn't help the people who ride at a "steady state" all day long it does provide the ideal solution for anyone who wants "sport" performance that complies with the law. A hub motor that was built using this design could accelerate from zero to full speed and always get about the same power and not have efficiency problems. When you back off the throttle and run at partial settings then the efficiency drops. If you've ever ridden one of those now out of date two stroke motorcycle road racers (like an old RD400) this was how they behaved... if you rode too much at partial throttle you would "foul" your spark plugs and the motor would stop working. You literally were required by design to keep the rpms high or the bike would break down.
When buying hub motors one might be asked:"Would you like our 'sport' model or our 'cruising' model?"
...which would be either an SR Hub Motor (best at peak power) or a DC Brushless Hub Motor (best at high rpms and low power).
safe
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Switched Reluctance Motors
Switched reluctance motors (SRMs) are similar to electronically commutated permanent magnet (ECM) motors except ECM motors are usually fractional HP and have permanent magnets. SRMs are offered in the fractional horsepower range and go into double-digit horsepower. They are always variable speed and come with a special controller just as ECMs do. They may be three or more phases but that is of little relevance to the user because the input to the controller is always single phase or three phase. They are challenged with torque ripple and acoustic noise, but recent articles keep reporting advances that improve those performance characteristics. They produce high low-speed torque and allow precise speed control because they are synchronous machines.
Due to their attractive torque characteristics, SRMs have been investigated for use in electric vehicle traction drives. Some so-called servo motors are SRMs. I don't know the speed ranges, but I think they can be made to have quite high speed because they have a simple rugged rotor with no cage or magnets and controllers that can be made to switch as fast as necessary. They probably create the same sort of line-side harmonics as conventional adjustable speed drives (ASDs) do because they start with a diode rectifier.
SRMs are sometimes used in original equipment manufacturer (OEM) equipment such as appliances and are more common in Europe. SRMs are actually a very old motor technology, predating other electrical motor types. Advances in power control technology have resulted in these early motor designs being revisited. Modern SRM technology was developed and licensed by Leeds and Nottingham Universities in the UK. One application in the U.S. (originally promoted by Emerson Electric, U.S. Motors) is an agitator drive motor for front-loading washing machines. Emerson Electric at one time reported that they would launch a line of integral horsepower SRM VSD motors for industrial applications. I haven't seen any data on cost, but based on how they are made, it seems they should have the potential to cost less than the combination of an induction motor and ASD because there is no cage to cast or fabricate.
I don't know why they are not bigger in North American markets than they are, but maybe (as Austin Bonnet of U.S. Motors points out) the U.S. motor industry is heavily invested in tooling to produce motors in a certain way and the mere appearance on paper of a better design does not mean it's immediately easy to manufacture. Permanent magnet brushless DC motors may be another competitor in larger variable speed applications.
Rob Boteler of U.S. Motors reports that a SRM motor may cost less than a standard induction motor but the drive may cost a bit more. Apparently, U.S. Motors bought out SR Drives of Leeds, England, so has the rights to produce switched reluctance motors in the U.S. Rob indicates that you wouldn't purchase a SRM for cost savings, but might specify such a motor for its high torque, high speed, or precise speed control qualities. Rob indicated that these motors may not have a higher full-load efficiency than competing products but may hold their high efficiency over a wider speed range. From a marketing standpoint, U.S. Motors is targeting original equipment manufacturers with specific product applications. They would essentially design and then build a motor for an application that would require high volume production. Rob indicated that producing one or a small quantity of industrial motors would be cost-prohibitive.
I have read articles that claim that SRMs are quite efficient, but have never found any good articles or manufacturer's literature that discusses exactly how high their full-load efficiency is, or what the potentially achievable efficiency is. Some SRM proponents predict that they will eventually compete with induction motors even in fixed speed situations. That seems to be quite a challenge since you don't need a controller for an induction motor in a fixed speed situation. I suspect SRM controllers are (or potentially are) cheaper than conventional ASDs because they do not have to be pulse-width modulated (PWM).
http://www.energyideas.org/default.cfm?o=h,g,ds&c=z,z,3961
Characteristics
No I2R loss in the rotor.
Inert rotor. No permanent magnet. No backEMF.
Compact size and low weight.
Low cost.
Efficiencies greater than 90% possible.
Inexpensive and easy to manufacture.
Lowest construction complexity of any motor. Many stamped metal elements.
High reliability (no brush wear). Rugged construction.
High efficiency.
High start-up torque and high speed operation possible.
So the only negative is electrical complexity... you basically need a computer to make it work.
Switched reluctance motors (SRMs) are similar to electronically commutated permanent magnet (ECM) motors except ECM motors are usually fractional HP and have permanent magnets. SRMs are offered in the fractional horsepower range and go into double-digit horsepower. They are always variable speed and come with a special controller just as ECMs do. They may be three or more phases but that is of little relevance to the user because the input to the controller is always single phase or three phase. They are challenged with torque ripple and acoustic noise, but recent articles keep reporting advances that improve those performance characteristics. They produce high low-speed torque and allow precise speed control because they are synchronous machines.
Due to their attractive torque characteristics, SRMs have been investigated for use in electric vehicle traction drives. Some so-called servo motors are SRMs. I don't know the speed ranges, but I think they can be made to have quite high speed because they have a simple rugged rotor with no cage or magnets and controllers that can be made to switch as fast as necessary. They probably create the same sort of line-side harmonics as conventional adjustable speed drives (ASDs) do because they start with a diode rectifier.
SRMs are sometimes used in original equipment manufacturer (OEM) equipment such as appliances and are more common in Europe. SRMs are actually a very old motor technology, predating other electrical motor types. Advances in power control technology have resulted in these early motor designs being revisited. Modern SRM technology was developed and licensed by Leeds and Nottingham Universities in the UK. One application in the U.S. (originally promoted by Emerson Electric, U.S. Motors) is an agitator drive motor for front-loading washing machines. Emerson Electric at one time reported that they would launch a line of integral horsepower SRM VSD motors for industrial applications. I haven't seen any data on cost, but based on how they are made, it seems they should have the potential to cost less than the combination of an induction motor and ASD because there is no cage to cast or fabricate.
I don't know why they are not bigger in North American markets than they are, but maybe (as Austin Bonnet of U.S. Motors points out) the U.S. motor industry is heavily invested in tooling to produce motors in a certain way and the mere appearance on paper of a better design does not mean it's immediately easy to manufacture. Permanent magnet brushless DC motors may be another competitor in larger variable speed applications.
Rob Boteler of U.S. Motors reports that a SRM motor may cost less than a standard induction motor but the drive may cost a bit more. Apparently, U.S. Motors bought out SR Drives of Leeds, England, so has the rights to produce switched reluctance motors in the U.S. Rob indicates that you wouldn't purchase a SRM for cost savings, but might specify such a motor for its high torque, high speed, or precise speed control qualities. Rob indicated that these motors may not have a higher full-load efficiency than competing products but may hold their high efficiency over a wider speed range. From a marketing standpoint, U.S. Motors is targeting original equipment manufacturers with specific product applications. They would essentially design and then build a motor for an application that would require high volume production. Rob indicated that producing one or a small quantity of industrial motors would be cost-prohibitive.
I have read articles that claim that SRMs are quite efficient, but have never found any good articles or manufacturer's literature that discusses exactly how high their full-load efficiency is, or what the potentially achievable efficiency is. Some SRM proponents predict that they will eventually compete with induction motors even in fixed speed situations. That seems to be quite a challenge since you don't need a controller for an induction motor in a fixed speed situation. I suspect SRM controllers are (or potentially are) cheaper than conventional ASDs because they do not have to be pulse-width modulated (PWM).
http://www.energyideas.org/default.cfm?o=h,g,ds&c=z,z,3961
Characteristics
No I2R loss in the rotor. Inert rotor. No permanent magnet. No backEMF. Compact size and low weight. Low cost. Efficiencies greater than 90% possible. Inexpensive and easy to manufacture. Lowest construction complexity of any motor. Many stamped metal elements. High reliability (no brush wear). Rugged construction. High efficiency. High start-up torque and high speed operation possible. So the only negative is electrical complexity... you basically need a computer to make it work.
safe
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Observations and Thoughts
A DC motor uses permanent magnets. Having permanent magnets forces a certain shape to the motor powerband and pushes the efficiency into a specific location relative to voltage. No matter how hard you try you can't escape the effects of the permanent magnets.
So the permanent magnets are the true original "problem".
But it's possible to remove the permanent magnets and use induction to create the magnetic fields instead. This was "breaking news" in 1887. :wink:
So then we have to think about the various types of induction motors and their advantages and disadvantages. The typical AC induction motor is either one phase or three phase. The three phase is better, but you're still more or less stuck in the "old world" of analog technology.
The switched reluctance motor frees up the control logic so that all of the control is within the software of the controller. You don't use Hall Effect sensors or anything of the sort because you measure the way the electricity is behaving and from that you can retroactively figure out what is going on. In effect the computer creates a "virtual motor" in software and runs the motor with very little direct mechanical involvement. The idea here is to make the physical parts "simple" and the software part "complex".
The switched reluctance motor is the perfect match for the world of computers...
A DC motor uses permanent magnets. Having permanent magnets forces a certain shape to the motor powerband and pushes the efficiency into a specific location relative to voltage. No matter how hard you try you can't escape the effects of the permanent magnets.
So the permanent magnets are the true original "problem".
But it's possible to remove the permanent magnets and use induction to create the magnetic fields instead. This was "breaking news" in 1887. :wink:
So then we have to think about the various types of induction motors and their advantages and disadvantages. The typical AC induction motor is either one phase or three phase. The three phase is better, but you're still more or less stuck in the "old world" of analog technology.
The switched reluctance motor frees up the control logic so that all of the control is within the software of the controller. You don't use Hall Effect sensors or anything of the sort because you measure the way the electricity is behaving and from that you can retroactively figure out what is going on. In effect the computer creates a "virtual motor" in software and runs the motor with very little direct mechanical involvement. The idea here is to make the physical parts "simple" and the software part "complex".
The switched reluctance motor is the perfect match for the world of computers...
safe
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Some guy actually built a simple Switched (Variable) Reluctance motor from scratch...
http://www.thekeeser.com/Projects/variable_reluctance_motor.htm
The Video:
http://www.thekeeser.com/Projects/Electronics%20projects%20files/variable%20reluctance%20motor.wmv
http://www.thekeeser.com/Projects/variable_reluctance_motor.htm
The Video:
http://www.thekeeser.com/Projects/Electronics%20projects%20files/variable%20reluctance%20motor.wmv
dirty_d
10 kW
im pretty sure there will always be a backemf in an electric motor, when the stator poles switch on the magnetic field around the poles create an opposite magnetic field in the iron rotor poles(i think), thats where the attraction comes from, the rotor is spinning and has a magnetic field so its going to create a backemf in the stator windings. there should also be RI^2 losses in the rotor since its made of iron and is spinning in a magnetic field.
cadstarsucks
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There would be I^2R losses but they are not in the rotor-that is an iron rotor and a magnetized stator. On the other hand, there is a miniscule hysteresis loss in both the rotor and the stator.dirty_d said:
Dan
dirty_d
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oops, by RI^2 losses i meant the eddy current losses in the rotor.
cadstarsucks
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True there would be that as well in the solid bulk iron.dirty_d said:
Dan
VR motors are interesting. I see they use them in some washing machines now. I think their maximum efficiency cannot beat a permanent magnet motor, but they do have other advantages.
One advantage is for a 3 phase version, you only need 3 FET switches instead of 6 for a permanent magnet motor. This means the controller is less expensive to make.
I don't think you can run it sensorless. I think it would need some kind of shaft encoder to send speed and position feedback to the controller.
Lower cost and simplicity are the main advantages.
One advantage is for a 3 phase version, you only need 3 FET switches instead of 6 for a permanent magnet motor. This means the controller is less expensive to make.
I don't think you can run it sensorless. I think it would need some kind of shaft encoder to send speed and position feedback to the controller.
Lower cost and simplicity are the main advantages.
safe
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fechter said:
No, that's not what I'm reading. Apparently what they do is "sense" the electrical conditions that are going through the motor and create a sort of "virtual motor" in software. With this abstract modeling you eliminate the need for sensors because the electrical flow itself becomes the sensor.
The efficiency is very high under full load (in the 94% range) but that efficiency falls off with less load. However the rpm does not significantly effect the efficiency, only the load does, which means that if you crank the throttle wide open from zero rpms all the way up to full speed you will get full power AND full efficiency all the way up. All Inductance motors they like to be ridden hard this way. It's "upside down" in it's behavior compared to a DC motor where full load is usually a bad situation to be in.
I think the "Tesla" approach is the future and the ultimate is a fully computer controlled "Switched Reluctance" (Variable Reluctance) motor. They are so simple... basically you just have a hunk of metal as the rotor and then you have a number of coils arranged around the outside.
No sensors No brushes...and there are even some prototypes in the lab that use:
No bearings (they levitate the rotor with magnetic levitation)But in order to get all this to work you need to dig deep into the software side and create that "virtual motor". If this type of motor came into existance in a big way it would have to be done with a tightly integrated computer system that had been highly refined.
This is NOT the kind of trivial task like building a typical DC motor, but the final result could set all kinds of new records for performance in a racing situation.
If I was racing in NEDRA and had a million dollar budget (okay, how about $100,000 budget?) then this looks to be a quick way to "turn a large fortune into a small fortune".
This is what we would call "progress" in the electrical motor world... (do a search on the internet, there are all kinds of recent college papers and patents getting filed on this stuff... this is where the "action" is right now 8) )
safe
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Tidalforce Electric Bike : The Ideal Motor Solution?
This appears to be VERY close to being "it". The Tidalforce electric bike appears to have had a similiar design to the Switched Reluctance Inductance motor, but somehow they decided to go with the permanent magnets rather than relying on Inductance.
So it's a very "near miss" to having got it just right in my opinion. Any time you resort to using a permanent magnet you force a shape onto your powerband. If there is a shape then there are tendencies toward "steady state" behavior. Inductance motors like full load... DC permanant magnets prefer "steady state" and low load. This is NOT a "sport" motor, but for casual riding it would have worked well.
Nice try though... 8)
This appears to be VERY close to being "it". The Tidalforce electric bike appears to have had a similiar design to the Switched Reluctance Inductance motor, but somehow they decided to go with the permanent magnets rather than relying on Inductance.
So it's a very "near miss" to having got it just right in my opinion. Any time you resort to using a permanent magnet you force a shape onto your powerband. If there is a shape then there are tendencies toward "steady state" behavior. Inductance motors like full load... DC permanant magnets prefer "steady state" and low load. This is NOT a "sport" motor, but for casual riding it would have worked well.Nice try though... 8)
That's cool if they can make it sensorless. I don't see how it would handle a stall condition sensorless though. Having sensors is not a big deal anyway. Perhaps it would only need a single shaft encoder.
If the efficiency drops at light load, the power consumption is less anyway so the total amount of energy wasted will be less.
High efficiency at high load is more important that high efficiency at light load for most vehicle applications.
One drawback (not a serious one) is that VR motors are reportedly a bit more noisy than other motor types.
We should be seeing more of these motors show up in home appliances. If the volume of production gets high enough, the price will drop.
If the efficiency drops at light load, the power consumption is less anyway so the total amount of energy wasted will be less.
High efficiency at high load is more important that high efficiency at light load for most vehicle applications.
One drawback (not a serious one) is that VR motors are reportedly a bit more noisy than other motor types.
We should be seeing more of these motors show up in home appliances. If the volume of production gets high enough, the price will drop.
fechter said:One drawback (not a serious one) is that VR motors are reportedly a bit more noisy than other motor types.
We should be seeing more of these motors show up in home appliances. If the volume of production gets high enough, the price will drop.
Yes. VR motors were used in older version of a well-known brand front load washer. But the brushless motors are used in the newer version. If the VR motors in the old version fail, the replacement will be the brushless motors and the associated controller boards.
fechter said:
For 3-phase version, one could use 3 FET switches or 6 FET switches for a BLDC motors.
3 switches only supplies power to one phase at one instant.
6 switches will supply power to two phases at one instant and so the motor winding is more effectively used than that of 3 switches.
Refer to unipolar and bipolar drive in the Introduction of:
safe said:Tidalforce Electric Bike : The Ideal Motor Solution?
This appears to be VERY close to being "it". The Tidalforce electric bike appears to have had a similiar design to the Switched Reluctance Inductance motor, but somehow they decided to go with the permanent magnets rather than relying on Inductance.
Nice try though... 8)
Ahh, good in theory and makes great marketing hype, but...
In actual testing a Crystalyte hub motor outperformed the Tidal Force in terms of Wh/mile at a given speed.
The7 said:
I guess my point was that with a permanent magnet motor, you need a full bridge output so the windings can be energized in both directions. In a VR motor, the magnetic attraction does not depend on the polarity of the winding.
You could make a VR motor with as many phases as you like, but 3 is typical on larger motors.
safe
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fechter said:
"Torque Ripple" seems to be the phrase I've heard. Apparently the torque tends to get created in very strong pulses and so if you had one it might feel like you were riding a Harley Davidson motorcycle... hmmmm... that's not such a bad idea...
For some reason people want their motors completely quiet and the vibration that comes from the Torque Ripple means that it's not going to seem as smooth compared to the other types of motors. I doubt it's going to be much worse than a motorcycle engine which tends to vibrate a great deal.
There seems to be some debate / dispute about the terminology too.
Is the name of this type of motor:
1. SR - Switched Reluctance 2. VR - Variable Reluctance...there are people using both phrases and I don't know which is more dominant than the other. There's even one company that names itself for the "SR" phrase. (so they've committed to the name)
It's a little like our "re-rating" verses "over-volting" controversy... which phrase is going to dominate as the accepted one?In response to my own question let me suggest that "over-volting" is a "subset" of "re-rating". Because it's possible to over-volt something like a hub motor and never gear it down afterwards. The re-rating process always has a speed increase of the motor coupled with a gearing reduction to the rear wheel. So they are similiar and can mean the same thing, but they are also separate in that re-rating can mean not a change in voltage but for AC current a change in frequency. So they really are separate and unique, though often similiar to the point of being the same and can be in special cases the very same.
The torque ripple is at a fairly high frequency, so you won't feel it much in the frame of a bike, it just makes a loud whining noise. The power used to create the noise is dissipated (wasted), so anything to smooth out the power delivery ususally results in higher efficiency.
By shaping the waveform of the drive, you can make the torque output nearly constant. Waveform shaping would take a bit of extra processing, but with a decent DSP processor, it should not be that hard to implement.
Similar waveform shaping could be helpful with a permanent magnet motor.
Most of the references I've seen in the past call them VR motors. I don't really care what we call them as long as everybody knows what we're talking about.
By shaping the waveform of the drive, you can make the torque output nearly constant. Waveform shaping would take a bit of extra processing, but with a decent DSP processor, it should not be that hard to implement.
Similar waveform shaping could be helpful with a permanent magnet motor.
Most of the references I've seen in the past call them VR motors. I don't really care what we call them as long as everybody knows what we're talking about.
xyster
10 MW
fechter said:
As my grandpa used to say, "you can call me anything you want -- just don't call me late for dinner."
cadstarsucks
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It does take a bit of doing but is not that complicated...a table, a 16 bit compare in a timer, and a cpu fast enough to set it up before the next switch event.fechter said:
Dan
safe
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Apparently there is a 6/4 (six outside coils and 4 inside rotor edges) configuration and higher like 8/6 and 10/8. The higher the number the more it smooths out the torque ripple, but also the more you increase the complexity of the motor.
The 6/4 seems about as simple as you can go before the torque ripple becomes unreasonable. Trying something like a 4/2 would likely not produce very good results.
The 6/4 seems to be more a less the "standard" at this point.
The 6/4 seems about as simple as you can go before the torque ripple becomes unreasonable. Trying something like a 4/2 would likely not produce very good results.
The 6/4 seems to be more a less the "standard" at this point.
cadstarsucks
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The other way to smooth torque ripple is to feed the thing as AC instead of trapezoidal.safe said:
Dan
safe
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Message to RockyMountainTechnologies a manufacturer of Switched Reluctance Induction motors of about 1-2 horsepower:
Dear Safe,
For the initial testing you may wish to use some of our standard
"off-the-shelf" products. You may wish to look at the SR130S
or SR130M and the SRTD2550. I have attached our standard
price list. In volume qty. a system like this, modified for
production, could cost well under $100.
Best Regards,
George Holling
Normal Single Motor List Price:
SR130S $1,925
SR130M $2,225
SR130L $2,475
So basically if you buy 100 motor/controller sets ($10,000) then the price comes down to $100 each... otherwise you pay waaaaay too much for them right now. (they don't seem to want to deal with small time players very much)
Dear Safe,
For the initial testing you may wish to use some of our standard
"off-the-shelf" products. You may wish to look at the SR130S
or SR130M and the SRTD2550. I have attached our standard
price list. In volume qty. a system like this, modified for
production, could cost well under $100.
Best Regards,
George Holling
Normal Single Motor List Price:SR130S $1,925
SR130M $2,225
SR130L $2,475
So basically if you buy 100 motor/controller sets ($10,000) then the price comes down to $100 each... otherwise you pay waaaaay too much for them right now. (they don't seem to want to deal with small time players very much)
dirty_d
10 kW
ouch
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