Alternator to Motor Conversion

[fechter]

You might try checking with Rick Roller at Transmagnetics. He make some custom rotors for motors he built. Transmag rotors look like that too, but I don't know if they would fit.

 

[lawsonuw]

On the Honda Civic and Insight hybrid motors, there is a band that goes around the outside of the magnets. A band could be made of thin stainless steel or made from Kevlar or carbon fiber. You could DIY a Kevlar band by using thin Kevlar thread soaked in epoxy and carefully wrapped around the rotor.

This sounds like a very good idea Fechter. Combined with some long narrow rare earth magnets (for example This magnet) on a simple round rotor it should be pretty simple to make a strong rotor without custom magnets.

Marty

 

[lepton]

The rotors on the BMC non-hub motors are completely smooth, with the magnets inside. I'm not sure what material is used to get that surface finish, but its almost like the magnets are completely covered in an epoxy, making the entire face a continuous cylindrical wall. Something like that might allow for a smaller gap, and you could use regular rectangular magnets.The magnets don't have to be physically exposed to be magnetically exposed...

I'll try to take a closer look at it when I get home.

 

[johnrobholmes]

Good thread on RCG about this right now http://www.rcgroups.com/forums/showthread.php?t=905411

 

[fechter]

The rotors on the BMC non-hub motors are completely smooth, with the magnets inside. I'm not sure what material is used to get that surface finish, but its almost like the magnets are completely covered in an epoxy, making the entire face a continuous cylindrical wall. Something like that might allow for a smaller gap, and you could use regular rectangular magnets.The magnets don't have to be physically exposed to be magnetically exposed...

I'll try to take a closer look at it when I get home.

The magnets on a BMC motor are actually one solid ring of magnet material that has been magnetized with 8 pole pairs. They are not separate magnets. I've seen this with several types of motors. When they make the magnets, the material is pressed into shape, then zapped with a huge electromagnet to give it its magnetic field. The electromagnet in this case is shaped so it imparts the desired 8 pole configuration to the material.

You could do the same thing with an inrunner, but probably not at home. Tiling rectangular magnets is about the only way to DIY.

 

[lepton]

The magnets on a BMC motor are actually one solid ring of magnet material that has been magnetized with 8 pole pairs. They are not separate magnets

Interesting. I didn't expect that, and I wonder what the boundary between polar regions look like magnetically. Either way, here are some pictures for reference.

 

[frodus]

http://www.evalbum.com/876

Its currently in our shop

We just replaced 2 batteries (after 3 years). The thing is CRAZY fast... I've never gone full throttle. It'l do a burnout. We broke 2 front wheels doing doughnuts around the shop. Its pretty neat.... 2 AC controllers, hooked to 2 alternators (we just broke the windings out so there were 3 phases, and ran them out to the controller). The alternators have slip rings and a field coil in the armature (it doesn't commutate). The AC controller has 3 phases and an extra output to "excite" the field on the armature. Fully programmable, regen, 150A per phase, 60V.

If there's enough interest, we'll start making them again.

 

[Link]

You might try checking with Rick Roller at Transmagnetics.

OT: His...his name is...Rick...Roller? :|

:lol:

Internet GOLD, I tell you!

 

[John in CR]

http://www.evalbum.com/876

Its currently in our shop

We just replaced 2 batteries (after 3 years). The thing is CRAZY fast... I've never gone full throttle. It'l do a burnout. We broke 2 front wheels doing doughnuts around the shop. Its pretty neat.... 2 AC controllers, hooked to 2 alternators (we just broke the windings out so there were 3 phases, and ran them out to the controller). The alternators have slip rings and a field coil in the armature (it doesn't commutate). The AC controller has 3 phases and an extra output to "excite" the field on the armature. Fully programmable, regen, 150A per phase, 60V.

If there's enough interest, we'll start making them again.

Frodus,

What kind of efficiency does it run? From a cursory review of that RC thread, it seems that it's possible to have an adjustable Kv. Did you guys get into that depth with the electronics on your conversions? If reasonably good efficiency is possible, then given the high power potential of from something relatively cheap and already mass produced, I can just about guarantee interest, especially if torque and Kv are variable.

John

 

[frodus]

what RC thread?

And when you say Kv, what do you mean? Volts per RPM? Its AC, you make it go as fast as the motor will handle. I'm not exactly sure what you mean bu adjustable Kv.... if you mean variable frequency, then yes, its variable frequency, so you can control speed.

Efficiency is VERY high, we've calculated it on the larger ones to be around 97% I will ask about the EFF on the AC, its going to be pretty close to the same, since we use the same controls for it, just different voltage/current FET's.

 

[docnjoj]

Curses. Even my oh-so-gangster knowledge of etymological structure isn't enough to think of anything better than "altemotor". :?

And I guess that Randy got banned because he was a bunghole. I wasn't around then, so I can't really say how big of a bunghole he was.

I'm afraid Randy is still alive and well as "Bob Diode" in Power-Assist! See his drivel on a regular basis there! I hardly ever go there any more.
otherDoc

 

[John in CR]

what RC thread?

And when you say Kv, what do you mean? Volts per RPM? Its AC, you make it go as fast as the motor will handle. I'm not exactly sure what you mean bu adjustable Kv.... if you mean variable frequency, then yes, its variable frequency, so you can control speed.

Efficiency is VERY high, we've calculated it on the larger ones to be around 97% I will ask about the EFF on the AC, its going to be pretty close to the same, since we use the same controls for it, just different voltage/current FET's.

Yes, I'm sorry Kv as rpm per volt. It's mostly huff and puff, but there is some fairly technical discussion mixed in where Johnrob linked us http://www.rcgroups.com/forums/showthread.php?t=905411

To me good efficiency combined with the ability to vary the Kv has the makings of a perfect motor for our purposes unless I'm getting something wrong. If you can have a low Kv that increases torque that is used for take-off and hills, and then a higher rpm/volt for higher speeds, then your motor can run in an efficient range under greatly varying conditions. It can eliminate the need for variable gearing as well. To me it's the biggest knock against hub motors, because their fixed gearing makes them so wasteful at low rpms. Not only does it waste battery power in the slow range, but that inefficiency directly causes problems with heat.

John

 

[frodus]

Yes, I'm sorry Kv as rpm per volt. It's mostly huff and puff, but there is some fairly technical discussion mixed in where Johnrob linked us http://www.rcgroups.com/forums/showthread.php?t=905411

To me good efficiency combined with the ability to vary the Kv has the makings of a perfect motor for our purposes unless I'm getting something wrong. If you can have a low Kv that increases torque that is used for take-off and hills, and then a higher rpm/volt for higher speeds, then your motor can run in an efficient range under greatly varying conditions. It can eliminate the need for variable gearing as well. To me it's the biggest knock against hub motors, because their fixed gearing makes them so wasteful at low rpms. Not only does it waste battery power in the slow range, but that inefficiency directly causes problems with heat.

There is no Kv value for AC motors.... its RPM/hertz... you set the frequency, changing the RPM depending on what freq you give it.

DC motors, you give them different voltage to change RPM, changing the RPM depending on what volt you give it.

Kv is motor dependant, you can't change that on DC motors, they're built to go XXX rpm per volt input. Its fixed. With AC, you give it a waveform, at a switched frequency. All motors have a torque curve, you cannot change this ALONE with a controller. An AC gives you more flexibility to control the torque at low RPM.... and Alternators allow you to change the excitation of the armature and provide more or less torque when you need it. There are still max values of torque depending on the RPM you're going with either motor, and with AC can drop off sharply depending on the freqency input.

You're always going to have an effiiency curve though, and its not as dependant on the controller as it is on the motor. Motors efficiency is based on the RPM/current/voltage. Motors are designed with an RPM in mind... and a working RPM range. Their efficiency dropss off outside of that range.

Have you ever seen and understood a motor torque curve? Have you seen one for AC?

 

[John in CR]

Thanks Frodus,

I try to learn as little about electronics as possible. Yes, I've seen the curves for DC motors, but not true AC motors. It's that field excitation that I was referring to, so that gives you a means of varying the torque when needed? Does that also mean that a true AC motor can be more efficient at low rpms vs the horrible inefficiency of DC motors that we see even in our brushless DC motors at low rpms under load?

How about this guy who made the controller you used? I couldn't find any actual product or pricing info.

John

 

[frodus]

Thats not my cart, thats a cart that our controller is in. The company is Synkromotive, there's no pricing because its not a production product. Its a prototype. I work for Synkromotive on their controllers and charging systems. The DC controller and charging setup is in my motorcycle.

http://www.evfr.net
more pictures at:
http://pics.evfr.net

I can get you info through them.

The torque can be varied Somewhat... it won't exceed the max torque the motor is rated for, but it will allow you to maximize eff for low speed, and adjust depending on how deep you are into the throttle. It can be weakened when you let off or "hold" the speed steady. Its all done in software and is completely programmable. We would have to program some of the options in if they're not already there.

read up a little on AC motors here:
http://www.educypedia.be/index.htm
It should help you get started, and answer your questions.

 

[Miles]

I found this quite a good overview: http://www.teslamotors.com/blog4/?p=45

 

[lawsonuw]

AC motors DO have a rpm per volt value. If the input rpm and field excitation to a "altenmotor" is fixed, an AC back emf voltage of fixed value will be produced. If this back emf voltage has a rms value equal to the man power supply's voltage the motor can't be driven any faster. There are two ways around this limit; increase the supply voltage, or reduce the field excitation. (note: on an AC INDUCTION motor the field excitation is induced by the input current so it is proportional to the input current and field weakening happens automatically)

Field weakening doesn't give a motor more torque. It WILL allow a motor to produce significant power at 2 or more times the maximum speed at full field and a fixed input voltage. Nothing is free though, field weakening also reduces the efficiency and power output of a motor. Field weakening is most useful for an E-bike because it will allow a motor to be geared for good low speed performance. (say 15mph max at full field) While allowing an E-bike to reach much higher speeds. (say 30-40mph limited by wind resistance and motor power output of course)

Marty

 

[John in CR]

...Field weakening doesn't give a motor more torque. It WILL allow a motor to produce significant power at 2 or more times the maximum speed at full field and a fixed input voltage. Nothing is free though, field weakening also reduces the efficiency and power output of a motor. Field weakening is most useful for an E-bike because it will allow a motor to be geared for good low speed performance. (say 15mph max at full field) While allowing an E-bike to reach much higher speeds. (say 30-40mph limited by wind resistance and motor power output of course)

Marty

Marty,

Does "good low speed performance" mean good efficiency while providing high torque at low speeds? If so then how much efficiency is lost as the field is weakened for higher speeds? Might this kind of alignment make a lot of sense in terms of overall efficiency for someone with a lot of stop and go and/or hill climbs in their typical route, while having a great top end for the flats at reasonable efficiency. In terms of being able to dissipate heat, some sacrifice of efficiency at high speed may be worth it if there's a gain in efficiency under heavy load at lower speeds.

John

 

[lawsonuw]

Does "good low speed performance" mean good efficiency while providing high torque at low speeds?

Yep. Assuming the bike is not speed limited by air drag and the field can be weakened to half it's peak value, a bike would be able to use a gear reduction of about double what it would have with no field weakening.

If so then how much efficiency is lost as the field is weakened for higher speeds? Might this kind of alignment make a lot of sense in terms of overall efficiency for someone with a lot of stop and go and/or hill climbs in their typical route, while having a great top end for the flats at reasonable efficiency. In terms of being able to dissipate heat, some sacrifice of efficiency at high speed may be worth it if there's a gain in efficiency under heavy load at lower speeds.

John

Let's assume the field is running at half of its full current. The motor now can spin twice as fast, has half the torque per amp, but the armature still has the same resistance. Hm... time for some equations. The equations in the attached picture are for DC motors, they still apply to an alternator conversion if a box is drawn around the controller/stator and the controller is locked at 100% throttle. For now we can ignore the Inductance term, it is only significant during rapid changes in input voltage. (aka during PWM) "Wm" is the rpm of the motor expressed in [radians/second] "Tdev" is in [Newton*Meters] With a bit of algebra it can be shown that the motor's power output is ideally unaffected by weakening the field. (i.e. a reduction in "if") The loss in efficiency comes in because a lot of parasitic losses of the motor are proportional to the rpm. (or the rpm^2) Also, for a given torque output more current is needed with a weakened field so I^2*R losses go up. In an ideal world basically a 1000w motor at full field is still a 1000w motor at half field, the motor just spins 2x faster. In the real world spinning 2x faster causes parasitic losses to go up a LOT and this can kill efficiency.

I hope that was clear?
Marty

 

[Dee Jay]

GENTS....
securing the rotor magnets is a non-issue when using alternators from scooters and motorcycles! They are secured inside a rotor "drum?"
http://www.powersportrider.com/cgi-bin/zcatjpg?CATPUB=BC0108C%20&CATPAGEBOX=555

They're small and light which could mean... picture this:
-Modular Motors... rated at 35 to 40 amps, you can stack two or more, side by side, all fixed and synchronized on one shaft! As many as you need for the power you need.

The downside:
-It will require fabrication of:
*shaft adaptor welded on the drum,
*motor mounts (with bearings)
*shaft

-I'm not sure if their magnets are neodymium but I think it would be very easy to replace them with the best quality neos.

-The prices are outrageous but these are new and for custom bikes ...

And arched neos!

http://www.rare-earth-magnets.com/SearchResult-CategoryID-29.html

J

 

[John in CR]

Does "good low speed performance" mean good efficiency while providing high torque at low speeds?

Yep. Assuming the bike is not speed limited by air drag and the field can be weakened to half it's peak value, a bike would be able to use a gear reduction of about double what it would have with no field weakening.

If so then how much efficiency is lost as the field is weakened for higher speeds? Might this kind of alignment make a lot of sense in terms of overall efficiency for someone with a lot of stop and go and/or hill climbs in their typical route, while having a great top end for the flats at reasonable efficiency. In terms of being able to dissipate heat, some sacrifice of efficiency at high speed may be worth it if there's a gain in efficiency under heavy load at lower speeds.

John

Let's assume the field is running at half of its full current. The motor now can spin twice as fast, has half the torque per amp, but the armature still has the same resistance. Hm... time for some equations. The equations in the attached picture are for DC motors, they still apply to an alternator conversion if a box is drawn around the controller/stator and the controller is locked at 100% throttle. For now we can ignore the Inductance term, it is only significant during rapid changes in input voltage. (aka during PWM) "Wm" is the rpm of the motor expressed in [radians/second] "Tdev" is in [Newton*Meters] With a bit of algebra it can be shown that the motor's power output is ideally unaffected by weakening the field. (i.e. a reduction in "if") The loss in efficiency comes in because a lot of parasitic losses of the motor are proportional to the rpm. (or the rpm^2) Also, for a given torque output more current is needed with a weakened field so I^2*R losses go up. In an ideal world basically a 1000w motor at full field is still a 1000w motor at half field, the motor just spins 2x faster. In the real world spinning 2x faster causes parasitic losses to go up a LOT and this can kill efficiency.

I hope that was clear?
Marty

Thanks Marty,

The parasitic losses would be primarily what, mostly the power eaten by the internal fan blade for ventilation?

John

 

[johnrobholmes]

Drive train, bearing, and internal "wind" losses go up with increased motor speed. Gotta push more air around the armature when it spins fast.

 

[POPS]

It will be a while before I get to the project. I may have another company make the rotor for me, as my expertise is not quite enough to make a high quality one. I would want the magnets held in a frame and not just glued on.

You could use a slot that went over the edges of the magnets a bit, but this would shunt part of the magnetic field unless it was nonmagnetic stainless or something. Ideally you'd want something resembling a dovetail so about half the magnet sticks out above the rotor core. Finding magnets like this would be nearly impossible and grinding them is equally difficult.
You still need really good glue too. Something that won't fall apart at high temperatures.

On the Honda Civic and Insight hybrid motors, there is a band that goes around the outside of the magnets. A band could be made of thin stainless steel or made from Kevlar or carbon fiber. You could DIY a Kevlar band by using thin Kevlar thread soaked in epoxy and carefully wrapped around the rotor.

I don't know nothing about nothing when it comes to this stuff but I'm reading and learning. I'm an auto mechanic and sorta get it but to get to the point what about the glue Chevy used on thir trucks from 1988 and on to hold the door hinges to the body ??? That stuff works darn good !! Never saw 1 come loose ! Just a thought, trying to help...POPS

 

[paultrafalgar]

Welcome to the forum, Pops. Never be afraid to make a suggestion - the only fool is the one who pretends to know everything.
Can you find a link to the supplier of that glue? If so, post it here.
Thanks

 

[fechter]

I'd be interested in knowing more about this glue...

The rotor could get mighty hot at times, so the adhesive should be good for over 150C or so.
Any hotter than this and the magnets would be toast anyway.

Even if the glue stays stuck, above a certain rpm, the magnets will try to fly apart. Weird property of super strong magnets is if they fracture, the pieces will repel each other strongly. It's like they're trying to fly apart before you even get them spinning. Some way to add some compression stress will help hold them together.