Auto Alternators As Motors

Discussion in 'Electric Bicycles' started by safe, Aug 18, 2009.

  1. safe

    safe Active Member

    Auto Alternators As Motors

    Forget about Single Phase Induction motors like the Dishwasher motor. The preferred route is to use an Automobile Alternator since it uses a Three Phase motor design.

    This thread will deal with "Auto Alternators As Motors For Ebikes".

    A good place to start:
    Last edited: Aug 18, 2009

  2. safe

    safe Active Member

    Past Auto Alternator examples include the "somewhat infamous" Randy Draper ebike:


    ...many people seem to have problems with this guy, but he just seems really excited about what he's into. Since he was already ahead of the curve back in 2003 with Three Phase power for an ebike we ought to give him some credit for being right. (you may not like him, but give him the respect he deserves)

    Induction motors seem the better option in the "big picture" compared to permanent magnets because you can push the rotors as hard as you want to get more power. With permanent magnets you literally have a "speed limit" built into the motor. Induction motors actually work best at full throttle, so that's another plus. Permanent magnets only work well in a narrow rpm range and light loads. Because of all this it's more realistic to get better overall efficiency with the Induction motor than the Permanent Magent motor because in a racing situation you are always under load.
    Last edited: Aug 18, 2009
  3. safe

    safe Active Member

    Small, Light and Powerful

    Many Auto Alternators are heavy and have more power than is needed for an ebike. This one looks light weight and small enough for the job:


    ...a little expensive at $280. (cheap alternators are $35)

    Height - 4.5"

    Depth - 5.5"

    Weight - 6 lbs

    50 amps
    Last edited: Aug 18, 2009
  4. bluegoatwoods

    bluegoatwoods Well-Known Member

    I'm anything but an authority in this field, but if you could get an alternator to work at all, then wouldn't it require AC current to do the job?
  5. safe

    safe Active Member

    You might check the rc group thread:

    ...they came to the conclusion that you can use an RC motor ESC to achieve the control over the motor and it's a simple plug in task to complete.

    All brushless motors use AC current, so this is not really that different than using an ebike with a brushless motor. The big difference is that an Induction motor has a solid rotor that is not magnetic. You literally "induce" the magnetic field in the rotor as opposed to relying on permanent magnets to attain a magnetic field.

    The big advantage is that when it comes to the power-to-weight ratio it should be possible to get incredibly high power levels from an induction motor.

    The auto alternator is a good idea... I've basically destroyed too many brushed motors to want to bother with them much any more, so I'm looking for "big power" in a light weight package.

    The Tesla Roadster is built with an Induction motor.


    Note: Technically speaking the RC motor ESC is a PWM signal and not a true analog Sine wave. They slice up the pulses in such a way as to trick the motor into thinking that it's analog.
    Last edited: Aug 18, 2009
  6. safe

    safe Active Member

  7. Tinker1980

    Tinker1980 Guest

    Feel free to shoot me down, but don't some alternators have brushes in them?

    I'd love to see an alternator powered bike... you wouldn't even need a new alternator. The part that usually fails on them is the diode, which is something you're going to toss out if you want to make a motor.

    One should poke around a junkyard... the GMT 400 chevrolet trucks (1988-1998) typically had a 105-amp alternator. Some older caddy's had them even bigger.

  8. safe

    safe Active Member

    The really old cars used to use generators with brushes.

    I looked it up on the internet and have forgotten the link, but the date that the cars started to use true alternators was about 1970. So the pre-1970 or so alternators used brushes and then pretty much everything afterwards is without them.

    Since a brand new alternator can be purchased for $40 it seems like it would be better to go to the auto parts store and look through all your options and find the best match. However, if you already have an alternator laying around it would be worth checking it out.

    I would prefer to buy the smallest and lowest amp alternator because then when you flip it around and use it as a motor it will drain your battery more slowly.

    Remember that the MAIN reasons for switching would be:

    :D No brushes means less to fail.

    :D Induction as opposed to Permanent Magnet means that the powerband is wider and flatter. As long as you have the throttle wide open you will get good efficiency... the moment you let off the throttle is when the efficiency drops. It's better to run the motor full on or full off. (race mode)

    ...of those two the brushes part is what got me started on the idea, but it's the Inductance part that really serves the need for my Electric Bicycle Road Racer concept. The idea being that the "perfect" motor will run with maximum torque at a constant input power level. If the racing class is defined as "Formula 1K" then you are allowed a maximum of 1000 Watts of input power. A Permanent Magnet would not work as well under the fixed input power concept. (this all fits together in something much larger that I'm working on) If I ran the alternator at 48 volts and 40 amps that translates to 1920 watts of input power. (which is too much) So in order to satisfy my own "restrictor plate" racing I might go:

    48 volts * 20 amps = 960 watts (effectively 1000 watts)

    24 volts * 40 amps = 960 watts (effectively 1000 watts)

    The sort of amazing thing is that I've had it set in the back of my mind that the Induction motor would be the "Holy Grail" of racing ebikes, but I somehow "never got the memo" that auto alternators were Three Phase Induction motors. That was a "wow" realization for me. :idea:
    Last edited: Aug 18, 2009
  9. safe

    safe Active Member

    Slip Rings?

    (after returning from the Auto Parts store having discovered that auto alternators all use brushes)

    Auto Alternators have Slip Rings and Brushes that are used to charge up the field current of the stator. In a generator this is necessary because at low rpm the rotor is barely going to produce any response in the coils and no current will be produced. The addition of a small "exciter" current in the coils is a trick that creates a much bigger field on the output side.

    But when you reverse things... and create a motor out of the alternator... is there any reason to keep the Slip Rings and the Brushes?

    When you want motor power you simply send the currents through the coils as a motor and I don't see the need for the field current anymore unless you want really, really high starting torque. (like a dragster)

    Remove the Slip Rings and brushes that go with them?

    Or find a way to keep them?
    Last edited: Aug 18, 2009
  10. arceeguy

    arceeguy Active Member

    You will still need to power the rotor through the slip rings, otherwise the rotor won't turn when the stator is energized by the motor controller. I've seen some auto alternators turned into PM generators by mounting rare earth magnets in the rotor. Usually used in low RPM windmills. If you did that, then you would have a large PM brushless DC motor.

    As far as converting a car alternator to a motor, remember that a car alternator is not very efficient in converting mechanical energy into electricity - so it might stand to reason that it won't be very efficient converting electrical energy to mechanical energy without significant modification.
  11. safe

    safe Active Member

    Alternators Are Synchronous Motors

    Powering the rotor through the slip rings makes the alternator a "synchronous motor" and not an "induction motor".

    I think the thought process went like this...

    For an automobile there was a desire to improve on the brushed generators they were using and so they went with an alternator that uses the three phase design. This allowed them to remove the permanent magnets and lower the costs. The problem with a pure induction alternator is that it does not produce much power at low rpm and since there is a strong need for autos to be able to recharge while at idle they needed a way to get high charge levels at low loads. It's the low load situation that was the problem. The solution is to "cheat" a little with a slip ring and charge up the rotor so that you get more power even at lower rpm.

    Now switch back to the ebikes... what is our thought process?

    For a "racing ebike" like I'm trying to make there is no "idle" situation to deal with. You are either at 100% throttle and load or you are off the throttle and braking for a turn. (the rare exception being sweeper turns that use less than full throttle which might be 5% of the time on the road) So for the "racing ebike" we want a 100% induction design because we know that we will be at full throttle and full load most of the time.

    Getting back to the auto alternator...

    The auto alternator uses slip rings to charge up the rotor and that lowers the efficiency slightly... but the REAL REASON that alternators have low efficiency is that they run at less than full load all the time.

    What is needed is to take the auto alternator apart and remove the slip rings and brushes, then short circuit the rotor like in an induction motor so that magnet fields generated on one part of the rotor create counter fields in the other parts. An induction motor's rotor can be nothing but a chunk of iron if you want.

    The final product will be an induction motor with fairly low torque at really low rpms, (assuming that your controller limits the currents strictly) but very quickly the torque will rise. Efficiency is always the highest when the load is the largest. The final fully loaded efficiency should be above 80% because many induction motors can get up into the 90% and above when loaded.

    It's all about loading... full load good... low load bad....

    The "bottom line" is that auto alternators are synchronous and not induction motors so you cannot just plug and play with them. In order to make them of use you need to modify them. That being said, they are still a good foundation to start with and have the potential to deliver the right kind of power.

    One of these days we will be able to buy stuff for our ebikes that really deliver what we want out of the box, but it just seems like we are stuck in a loop trying to get what we want. (or figure it out)



    Induction Motors

    Class B (IEC Class N) motors are the default motor to use in most applications. With a starting torque of LRT = 150% to 170% of FLT, it can start most loads, without excessive starting current (LRT). Efficiency and power factor are high. It typically drives pumps, fans, and machine tools.

    Class A starting torque is the same as class B. Drop out torque and starting current (LRT)are higher. This motor handles transient overloads as encountered in injection molding machines.

    Class C (IEC Class H) has higher starting torque than class A and B at LRT = 200% of FLT. This motor is applied to hard-starting loads which need to be driven at constant speed like conveyors, crushers, and reciprocating pumps and compressors.

    Class D motors have the highest starting torque (LRT) coupled with low starting current due to high slip ( 5% to 13% at FLT). The high slip results in lower speed. Speed regulation is poor. However, the motor excels at driving highly variable speed loads like those requiring an energy storage flywheel. Applications include punch presses, shears, and elevators.
    Last edited: Aug 19, 2009
  12. spad4me

    spad4me Member

    Show Me one you have built and run...

    I was going to spell liar here.
    I will wait!!
    2Q + 2q = 4q english to nerd translation $#@^ you
    Last edited: Aug 19, 2009
  13. safe

    safe Active Member

    You have to understand this is an "explorative theory" thread. That means that I'm taking it under consideration the possibility of using an automobile alternator as a base for an ebike motor.

    I've also looked at Single Phase dishwasher motors as a possibility.

    There is no "there there"... so to speak... this is all theory right now.

    However, it does appear that my first assumption about auto alternators was wrong. I had hoped they were induction motors, but they are not and are actually synchronous motors with their own field currents and brushes and slip rings.

    What I'm considering is if this is a good place to start or not.

    Auto alternators might turn out to be a bad idea... or it might turn out that it's a good idea with the right modifications.

    There is no good solution for an induction motor for ebikes "to my knowledge" at this point.


    I've done a lot of work rewinding brushed motors, but they seem to fail because of the commutators, so pushing the limits with them is not looking to be such a great idea. My hope is to get a flatter, more usable, motor powerband with an induction motor... which is another big reason to explore this.


    The main realization I've had about this is that a Three Phase induction motor is pretty much identical to a DC Brushless motor like the RC motors. In fact, in that RC thread they are trying to use RC based ESC controllers to control an auto alternator. It's an interesting thought.
    Last edited: Aug 19, 2009
  14. jimraysr

    jimraysr Member

    Alternator Slip Rings And Brushes

    This information will not contribute to your discussion, but it might help to know what is happening when an alternator is operating as designed.

    Automotive alternators have brushes and a revolvig field as the field strength must be controlled to change the amount of output current required by the system. If the battery is fully charged and few accessories are on, then the field current will be very low and the three phase output to the diode bank will be low. If the opposite is true, low battery, AC, lights, etc. on then the field current through the brushed and slip rings will be close to max. All controlled by the regulator, either internal or external.

    Old generators had a stationary field and the rotor carried all the output current and as a result the brushes had to do so as well. As the generating windings were rotating through the stationary field, it produced an alternating current, but it was converted to DC by the mechanical action of the commutator. Starters are similar to the old generators. So you can motorize a generator and use a DC motor to generate.

    If the revolving field was a permanent magnet, then there would not be any control of the output.

  15. arceeguy

    arceeguy Active Member

    While it is an interesting concept, I don't think it is practical. We were all taught that any motor can be turned into a generator, and a generator can be turned into a motor. while this is true, nobody said that a motor would make a good generator or vice versa. While I'm not a motor engineer, it seems to me that an induction motor would not be practical for a motorized bike because of the narrow RPM range. Sure, you can change the speed of the rotating field with a controller, but the induced current in the rotor would be minimal until it got up to its "resonant" design speed. Just look at a washing machine or dryer motor that has a separate "start" winding and mechanical cutoff switch just to get the thing to spin.

    Also - brushes in an alternator last a long time. Partly because the maximum current carried is only a few amps to excite the rotor, and partly because it is on a continuous slip ring and not a segmented commutator. If I were to try this, I would keep the rotor energized through the brushes and not have it "self excited" through induction. And power the field with the biggest RC truck brushless controller that I could find. (and even then, it might blow!)

    I've seen some interesting electric bar stools made from 24V truck starter motors run at 12 volts. At half their rated voltage (and cooling vents added), they will run continuously without overheating. Don't know about efficiency though......
  16. safe

    safe Active Member

    This I totally disagree with...

    My "Electric Bicycle Road Racer" concept is to build a class of ebikes that work best under racing conditions. Race machines are always either at full throttle (and the rpms are changing rapidly) or they are off. I've already done about 6,500 miles on my first road racer ebike with a permanent magnet motor connected to a six speed transmission and while this works well it would be nice to be able to get what I want with a single gear.

    The induction motor has a powerband that is both flatter and wider than the typical permanent magnet motor and it's efficiency is best at full load. Permanent magnet motors have the opposite and their best efficiency is when near their no load speed. (about 90% of max)

    Trust me on this one... I'm probably one of the more experienced guys around in high speed ebikes. (my flat land speeds are 50 mph and on a downhill I've hit 60 mph with rides of 30 mph average over 10 miles)

    That being said...

    The auto alternator is NOT a true induction motor, (it's actually a synchronous motor) so all this dealing with slip rings, brushes and field currents are not helping towards the goal of finding the perfect ebike racing motor.

    :dunce: Where is there a Three Phase induction motor that could be had for little money and is designed for delivering about 1KW of power?

    Most of the industrial motors are designed to be triple the needed size because bigger motors work better than smaller ones. But for an ebike you can't sacrifice the weight. (so we need something unique)
    Last edited: Aug 20, 2009
  17. safe

    safe Active Member

    Ruling Out PMA's

    On the RC thread they stumbled upon a Permanent Magnet Alternator (PMA) that does not need slip rings, brushes or field currents:

    ...but from my perspective they've missed the point of this exercise. For me the goal is to get away from permanent magnets and go towards the induction motor design. Dishwashers use Single Phase induction motors and while that's tempting they are not self starting. Three Phase induction motors are self starting and the increased number of poles actually lowers the effective rpm. Three Phase seems the best standard to go with.

    The guys at Electric Motorsport seem to have the right idea:


    ...however, for ebikes the needs are much lower.

    A 1KW motor is enough, 14KW is complete overkill for an ebike. (18 hp)
    Last edited: Aug 20, 2009
  18. jimraysr

    jimraysr Member

    Motor Controllers

    Pardon my ignorance, but I have very little experience playing with motor controllers for bikes and scooters. (Rebuilt one for a friend that he blew some switching transistors by connecting the 24 volt battery in reverse. It is a controller made up of separate components and powering a 500 watt brush motor with a PM field.) As I recall all the motors seem to have only two leads and I guessed the controller used pulse width modulation to control speed and had a PM rotating field? That would be the motive current from the controller, not a field control. So an alternator being three phase would require a controller that energized each phase in sequence, 120 degrees apart. 3 phase can be produced with capacitors and inductors, but all would require an AC input, not DC. I am not seeing anything here that would make the alternator turn using a 24 or 48 volt battery pack? It would seem you would need a triple output motor controller which would power each phase in order, if you were to run it on DC.

    Open to input to fill in my lack of knowledge?

    Last edited: Aug 20, 2009
  19. safe

    safe Active Member

    You are correct.

    Many of the ebikes out there now are using brushed motors with PWM controllers. (myself included)

    Some are experimenting with RC motors that are brushless and have three phases much like the AC motors. The RC motors use a special controller called an ESC that does not require sensors in the motor itself and can "sense" the backEMF automatically

    Other people are riding brushless hub motors that have sensors and their own controllers.


    A full blown induction motor would require some controller much like the RC motors ESC or something homebuilt.


    From my vantage point the "ideal" ebike induction motor would have many poles so that the rpm is lower. Something like 16 poles could drive the motor efficiently at around 500 rpm.

    I'm just having a hard time finding things that are going to work.

    You almost have to build a custom motor and controller if you want to get anywhere near what the laws of physics say is possible.

    (so we're kind of wallowing in ad hoc and extreme modifications these days)

    It would be great if the "ideal" motor was out there somewhere, but the auto alternator has a pretty good chance of being modified to work well enough for a first generation of testing. They are solidly built and should be able to handle some serious stress.
    Last edited: Aug 20, 2009
  20. arceeguy

    arceeguy Active Member

    Like I said, I am not a motor engineer. I am heavily into radio controlled hobbies, and have modified conventional brushed ESC's and brushless PMDC motor controllers.

    From my understanding, and induction motor IS a synchronous motor as its RPM is synchronized with the "rotating" field. If the rotor gets out of synchronization with the field, torque and power go down the tubes. An example would be a sensorless brushless motor losing sync. and "cogging" on acceleration.

    Induction motors are designed for single speed, single frequency operation. If you take an induction motor designed for 60Hz operation and plug it into a 50hz power source, it will run slower because of the lower frequency )of course) but won't it overheat or put out less power because the inductance of the windings and the rotor are engineered for 60 cycle operation?

    Brushed PMDC motors have high starting torque, and produce a lot of power for their size whan compared to AC induction motors. Varying speed is as easy as varying voltage. Downsides are brush and commutator maintenance.