Alternator Becomes Motor for This Electric Go-kart

Growing up in the 70s and 80s, a go-kart was a quick ticket to coolness, second maybe to a mini-bike. In both cases, a welded steel tube frame and a cast-off lawnmower engine were all that stood between you and neighborhood glory. Looks like a couple of engineering students caught the retro juvenile delinquent bug and built this electric go-kart for their final project.

While the frame for [Adrian Georgescu] and [Masoud Johnson]’s build was a second-hand find, the powertrain is all custom. They targeted a power output of 3 kW but found no affordable motors in that range. So, in true hacker fashion, they rolled their own motor from a used Subaru alternator. The three-phase motor controller came from an electric scooter, three LiPo packs provide the juice, and a pair of Arduinos takes care of throttle control, speed sensing, and sending data to the virtual dashboard on an Android phone. Some lights and a snappy red and black paint job finished off the build. While the video below shows that the acceleration isn’t exactly neck-snapping in the Tesla style, the e-kart can build up to a good speed – 53 km/h. Not too shabby, and no deafening engine right behind your head.

If you’ve got the e-kart bug, best check out some of our previous posts, like this kart built from off-the-shelf components, or this four-wheel-drive mini-kart. Any way you build it, you’ll rule the cul-de-sac.

42 thoughts on “Alternator Becomes Motor for This Electric Go-kart

  1. 53 km/h is about 33mp/h
    Looks cool.
    I like the repurposed alternator as motor hack but you will have problems very soon with it; keep on the lookout for an unwanted treadmill for a more efficient motor.

      1. I could be wrong and that particular alternator might have been a good choice for this build, but alternators are not designed to output motion.

        The slip-rings (for powering rotor field coils) is what usually fails if the diodes don’t first during its intended use, but they could be replaced with a rotor that has magnets. So that isn’t the biggest issue and would be fun to fix IMO.
        The aluminum encasing the bearings tends to fracture with little warning; the cases are usually very cheaply made.

        I hope they are designed better than the old Chevrolet and Ford ones that I have disassembled. I hope I’m proven wrong because it is a great use of something that is essentially free! Slightly tempted to try it anyway now that I’ve seen a go cart powered by one.

          1. A heck of a lot of alternators still have field control! Huh?

            No cars have brushless exciters for their field current, and I haven’t come across one with permanent magnets yet…

        1. as long as you stay with the normal torque and power I don’t see why it would be any less reliable than in normal use. The diodes wouldn’t be used when re-purposed as a three phase motor

          1. Car alternators don’t normally see very heavy duty operation because they only put out a high current for a short while after starting up to top the battery. Even a big car only uses up about 500 Watts to run the ignition and lights/fans/radio.

            And more importantly, the load is steady – not jerky as it is when a person is operating the throttle pedal. The battery backs up with the high demands, and the voltage controller keeps the output steady, so even with a changing motor speed the load on the alternator remains more or less constant. In fact, the faster the belt turns, the less torque the alternator has to bear to put out the same power.

            Subjecting the alternator chassis to a lot of accelerations and decelerations and sudden shifts in torque output fatigues and cracks the case and runs down the bearings which are simply not built for the task.

      1. Because it is optimized for low impedance for efficient output.

        When the alternator is turning as a motor, the stator gets an input of AC which generates a magnetic field during the cycle. Energy is being stored in the field while the current is increasing, and it is this stored potential energy that shows up as the torque in the rotor. When the current is steady, no more energy is stored in the field, so no more energy is transferred into torque, but is only lost due to the ohmic losses of the copper wire.

        The alternator’s stator windings are designed for very low impedance at the 1000-2000 RPM running speed range, which means when you input a relatively low frequency AC current to make it spin say 500 RPM, the coils don’t hold the current back. The current rises up rapidly, then the power supply saturates and cannot give any more, and the controller spends most of the cycle simply maintaining the field and wasting power.

        Due to the low impedance, the motor would only run efficiently at very very high frequencies so the current would have just enough time to rise up to maximum before the cycle is over, and no power is wasted to maintain the field.

        That speed would be much higher than is useful or practical and the alternator would break rapidly. To work better as a motor, you’d need to re-wind the stator with a slightly thinner wire and more of it, so the impedance at the desired speed range would limit the current instead of the controller or the battery.

        In short, the alternator as a motor has parameters that would work better as a ultra-high speed motor, because in return it means that the alternator has high current capability as a generator.

        The second reason is that normally BLDC motors have permanent magnets in the rotor, whereas the alternator uses extra power to energize the field coil. The field coil can be shorted out, in which case the alternator acts as an induction motor – problem being that an induction motor has zero torque at zero speed so it’s really bad at getting off the line and stalls very easily, and again the rotor coil is not optimized for induced AC so the performance is awful.

          1. technically speaking all motors have zero torque at zero rpm… If they are not rotating they cant produce any torque. Once it starts spinning a little bit that is a different story.

          2. “technically speaking all motors have zero torque at zero rpm”

            PM motors can have torque at zero speed, because the rotor is permanently magnetized. The phase difference between the field and the rotor makes the torque.

          3. “an induction motor does not have zero torque at zero speed.”

            Yes it does. When the field is not rotating, no induction is possible. The field has to rotate to induce a current in the rotor cage – at very low speeds the induced current is small and the torque is little, and that’s why induction motors have worse properties at low RPM.

            “A Tesla uses an induction motor, it doesn’t have any problem getting off the line”

            That is accomplished at the cost of horrible efficiency when you put your foot down from a standstill. The motor controller and battery, and the motor are all screaming lord mercy trying to keep the torque up, and full power launches are not possible if the battery isn’t topped up above 95%.

            However, the motor has a fixed 9.73:1 ratio to the wheels. When the wheels are turning at 60 RPM the motor is already doing ~600 RPM. Suppose 19″ wheels – that would be 1.5 m/s or a walking speed. When the car is at highway speeds, the motor is going over 10krpm. It’s geared to run so high precisely so that the induction motor could operate at least semi-efficiently across the normal speed limits.

          4. The reason why induction motors stall easily is because of the typical shape of their torque curves:

            Basically, when the rotor is turning below the typical 80% of your drive frequency, any increase in load causes the speed of the motor to drop, which causes the torque to drop, which causes the speed to drop… etc. and the motor stalls.

            Trying to start up the motor from a standstill by applying the full drive frequency results in sluggish performance until the motor picks up speed, so you have to start the drive frequency low, but then you got no induction going in the rotor, so you also have to use massive amounts of current to get any torque out of the motor. That can’t be sustained long or the motor will simply burn.

            In contrast to that, a PM motor, much like a stepper motor, has full torque available genuinely from 0 RPM.

        1. Very educational, Dax.
          And now the practiced foot gesture of the driver, where he gives while a little push before he engages power, makes more sense: the locked rotor current on that alternator must be truly fearsome.

          1. In the case of the alternator, if the field coils are energized from the battery, it behaves much like a PM motor and has good starting torque without special considerations. The field current controls the back-EMF of the motor just as it controls the output voltage when it was a generator, so the motor can be made to “behave” by pushing more or less current in the field coil – of course at the cost of efficiency because the field coil is just a wire with DC going through it: an electric heater.

            The relationship between the back-EMF and input voltage in DC motors is often signified by a motor constant “kv”, which means roughly speaking “revolutions per minute per Volt” – assuming no load.

            When a motor is spinning, the effective voltage across the stator coils is the input voltage minus the voltage generated by the spinning rotor acting as a generator – both motor and generator action happen simultaneously – and when that voltage is zero the motor can spin no faster. Increasing the field current then has the same effect as increasing the number of turns of copper in the stator coils – it changes the motor “kv” constant.

          2. In fact, there are “universal” motors that operate on both AC and DC because the field coil is in series with the stator through a brushed commutator. The reason is to have a cheap if somewhat inefficient motor that doesn’t require permanent magnets, has high power density, and doesn’t stall without active control like an induction motor would.

            Angle grinders and vacuum cleaners are a typical application of those.

      2. It’s a three-phase AC generator that puts out a really funky waveform that is rectified into DC. Even with a VFD it won’t see the wave it would normally generate, and putting funky waves into motors does odd things.
        As a dead serious thing, pulling a DC motor from a treadmill would actually make a good drive motor, PWM or rheostat control easily enough, directly powerable from batteries and none of the funkiness of alternators to worry about, plus a broken control treadmill can be had cheaply enough and the motor has decent power and doesn’t mind running at many varying speeds as it’s designed to do that.

    1. Haha, and stalled starter motors get hot quick! ;)
      Seems everyone has access to ‘free’ 12v/18v cordless drills with dead batteries; that’s the go-to now it seems.

    2. I once used the starter motor on a ’72 VW Beetle to drive the car when I ran out of gas at an intersection. I put it in first, popped the clutch, and cranked the engine. I managed to drive at least 500 feet up a hill till I could get to a safe spot. I did it in bursts of maybe 100′ with a rest in between to cool everything down, but it worked.

      Great engineering in those German cars. Except for the gas gauge – that was crap.

  2. Guys, fantastic work. Seriously. Excellent fit and finish and great presentation of the design and process. I have been thinking about a similar project, repurposing an alternator for use in a cheap/powerful e bike. Once I get the design ironed out I would like to boil it down to a sort of kit with minimum parts and easy installation, then offer them to local folks who can’t afford cars.

    This technology has a huge potential that I can’t wait to see utilized. I was hung up on the motor controller design but you guys nailed it. Thanks so much for sharing!

    1. For an e-bike you may want to check out cordless drill motors, some are brushless especially the lithium battery powered ones.
      Good luck either way; I’m looking forward to more homemade powered vehicles! (Obviously. :D I’ve already spammed this page enough.)

    1. Civic hybrid motor! :-D … That one’s probably a bit small for that, but the offroad carts that normally have 9HP or so IC engines might be a good candidate.

      Civic hybrids seem to be getting junked in numbers high enough to make parts repurposing worth investigating.

  3. Great project and nice finish – good experimenting with building your own drive circuit even if it wasn’t 100% successful.

    Sure there are “better” motors to use but then why not just buy a complete kart – they go faster more efficient blah blah blah

    Full marks in my book for finishing the project – and it works :) and when (if) the motor fails – another one is cheap as chips

  4. Nice going.
    I too have a 1/2 built one lying in my shed.

    A field exciter coil has some advantages over permanent magnets.
    1. Temperature. Rare earth magnets tend to die at relatively low temperatures, like 90 degees C.
    2. Cost, although magnets have come down amazingly.
    3. The ability to control the field current allows one to control the amount of back EMF generated. So it can act as a type of speed control.

    One final interesting thing, (may be an issue, or a benefit, depending on how you treat it) the output of the coils are not sine waves at all, they look very triangular in shape. This may have to do with the fact that alternators are expected to generate 12V over a large range of speeds. The converse of this is to say that as a motor the current drawn at any one speed would be very triangular in shape. I suspect that would give the motor a wide range of speeds at any one throttle setting with a wide “elastic” torque range.
    May make for an interesting phase dynamic speed controller experiment.

    1. “So it can act as a type of speed control.”

      Yes, except it works backwards. Less current in means lower back-EMF which means the motor draws more current and attempts to speed up. Trying to crawl around using the field coil as your speed regulator would burn up the motor.

      1. Yes a speed control. Not a power control. Not useful for a buggy, but may be useful for applications where speed, as opposed to power may need to be increased, but where the stator field wiring’s voltage is limited.

        1. Suppose there’s a fixed resistor as a ballast for the field coil. Therefore the field current depends on the battery voltage, and the motor constant can be made to increase at the same rate as the battery voltage drops.

          If there’s very little load on the motor, it can be connected directly to the battery and it runs at a very near constant speed. That might be useful for an application like a clock or a BBQ rotisserie turning a piece of meat.

          However, since this is a three phase synchronous AC motor, none of that actually matters because the speed of the motor is decided by the VFD controller. The field current only influences how efficiently or not the motor produces torque at any given speed. Theoretically there would be an optimal field strength for each RPM if we ignore the amount of power used to drive the field current.

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