Hacky Racers – Build a Motor

A powerful ‘brushless’* motor can be built very cheaply from a car alternator. The method is widely covered on the web, so we’ll just put a quick overview here.

Building a motor like this would only be done for the fun of it, really. There are now so many affordable brushless motor options available in the E-bike / E-scooter spares market that it’s hardly worth the bother to make your own – rather inefficient – motor.

Beware that it is easily possible to overspeed an alternator-based motor causing it to fail in a catastrophic way. It could even disintegrate violently. Design your controller to prevent this.

*Although the motor is 3-phase, has no commutator, and is driven by a standard brushless controller, there are in fact brushes in this motor. Unlike a true brushless motor it has field windings instead of a permanent magnet. It therefore requires brushes and slip-rings to carry the field current.

Howto Part 1 – Build the Motor

Obtain a reasonably modern 12V alternator. Ours is from a scrap Vauxhaul and cost £13 including delivery from ebay. The unit has no visible branding or specification, other than ‘Bosch’ on the regulator. The seller claimed it to have a 100A rating. Which might even be true.

The alternator itself is the cheap part of this experiment. Because you’ll also need:

  • A brushless motor driver capable of providing a significant current at up to 28V. We like the ‘RedBrick’ 200A ESC from HobbyKing, which is intended for RC models and costs about £30. We take it’s ‘200 amp’ rating with a pinch of salt.
  • Something to control the motor driver. The easiest way to control an ESC is to connect it to a RC system or a servo-tester. Alternatively to an Arduino. We’re not going to cover the ‘how’ for that here.
  • A smoothed DC power supply capable of 18V-25V and at least 10A. A very much higher current will be required if the motor is going to be run under load. We use a moderately large bench supply when testing an unloaded motor, but something much more powerful – such as a large battery suitably fused – is required when testing a loaded motor.
  • A second, variable smoothed DC power supply capable of around 3V-6V and up to 2A. We use a bench power-supply which has both current and voltage control.

Begin by removing the rear cover and the regulator from the alternator. These parts are usually designed to be removed so simply require unscrewing from the chassis. Chances are that the brushes are part of the regulator and – since you’ll need the brushes – you’ll have to keep the regulator to modify and refit later.

Then disconnect the rectifier pack. On ours, this was external to the chassis. It was held in place by screws but also by having crimped and welded connections to the main copper coils inside the alternator. The crimps can easily be released by a small screwdriver, and the welds can be cracked using a pair of pliers.

Once the wires are disconnected, remove and discard the rectifier pack.

If the brush pack is part of the regulator, then crack off the top of the regulator, use a drill/saw/ingenuity to break the connection from the +ve brush to the regulator circuitry, and use solder/self-tapper/ingenuity to connect a suitable conductor to the +ve brush. Optionally refit the top on the regulator. Refit the brush/regulator assembly to the alternator.

Finally, wire up as shown here. Explanatory notes below.

For DELTA connection:For STAR (‘Y’) connection:


  • There are three main coils embedded in the alternator chassis. With luck, each of their ends (six wires in total) will already be brought out at the rear of the chassis ready for connection. In our case the wires were neatly arranged ( start1 / end1 / start2 / end2 / start3 / end3 ). You may not be so lucky 🙂 and might therefore need to do some work with a multimeter to figure out which wire is which. You may even need to try a few different configurations before it works. In the worst case, you may need to split open the chassis to gain access to the individual coil ends.
  • For ‘delta’ wiring, connect each motor phase from the ESC onto the ‘start’ of one coil and the ‘end’ of the adjacent coil. Use large diameter wire to connect – currents will be high.
  • For ‘star’ wiring, connect all the ‘ends’ of all three coils together. This creates the neutral point. The neutral point doesn’t require any input. Insulate it. Then connect each motor phase from the ESC onto the ‘start’ of one of the coils.
  • Connect the ESC to a powerful DC PSU at about 20V. If the option is available then limit the current to – say – 20A for initial testing.
  • Connect the +ve brush to the (+) terminal of a small, controllable PSU which can provide up to about 2A DC. Connect the (-) of that PSU to the chassis of the alternator and so to the -ve brush.
  • Clamp the alternator safely into place before powering up. Be prepared to turn off both PSUs if – at any time – the motor is out of control.
  • Apply a voltage to the field brushes to attain a current of about 1A DC. Typically this will require a supply voltage of about 3V.
  • Then power up the ESC. Go through whatever set-up procedure it requires. Any signal ‘beeps’ made by the ESC will be audible from the alternator, just as they would be from any other brushless motor.
  • Gently apply throttle to the motor to start it.



Be aware that reducing field current will reduce the motor torque but will also dramatically increase the motor’s top speed – potentially to a dangerous RPM at which it could disintegrate. So always power off the motor via the ESC before turning off the field current.


  • Our motor worked best with the main PSU set to 20V-25V. Supply voltages under 18V* or over 25V** produced poor or intermittent operation, although this may have been due to the limitations of our power supply and/or controller.
  • In delta configuration the motor ran very well. It drew about 4A when unloaded but greatly more when accelerating up to speed.
  • It worked slightly less smoothly in star configuration. In particular attempts to accelerate sharply*** resulted in stuttering and power loss. On occasions when good performance was achieved the star-wired motor ran considerably slower and drew closer to 2.5A unloaded – both of which are in agreement with motor theory.
  • Field current of about 0.8A to 1A seemed optimal. Field currents under 0.5A or over 2A significantly affected performance and behaviour.
  • Swapping any two of the three motor-controller phase wires reverses the motor.
  • Reversing the field-current doesn’t reverse the motor.

*With input of less than about 18V the motor runs at a constant low speed irrespective of throttle setting and field current. We assume this is the ESC going into open-loop ‘start’ mode after failing to detect sufficient back-emf from the motor and thus being unable to determine the timing. Some ESCs may be more sensitive and thus able to operate at lower voltages. Others may be less sensitive, perhaps to the extent that they are unable to drive a motor at all.

**With input of greater than about 25V, the ESC shuts down and the motor coasts to a stop. We assume this is because the ESC has detected that the input voltage is greater than it is designed for.

***Presumably – again – the ESC was reverting to open-loop behaviour because the motor failed to accelerate in-step with the ESC’s accelerating pulses, so timing sync was lost.

Howto Part 2 – Extract the Mechanical Power

Having a powerful motor is useful… but only if the power can be extracted via a mechanical transmission. Perhaps the best form of transmission for a Hacky-Racer is chain drive: it is extremely efficient, is easy to set up with a wide range of gearing ratios, takes up minimal space, and is widely and cheaply available. Typically a Hacky-Racer would use 8mm (TF8) chain with a motor-sprocket in the order of 10-tooth, and an axle-sprocket in the order of 50-70-tooth. But reliably fastening the 10-tooth sprocket to the alternator shaft is a bit tricky. Here’s one option.

Begin with a mild steel self-colour full nut which fits the alternator shaft. Typically a modern European alternator will use a right-hand metric-fine thread, perhaps M16 (16mm x 1.5mm). Into its very end fit a short length of steel. The steel should go no more than 2mm into the end of the nut, leaving plenty of thread clear. Suitable items to use include a short length of threaded rod (in our case we jammed in a length of regular M16 studding), a nut of a smaller diameter (say M12), or a short piece of heavy-walled tubing. Even a length of rebar can be used. The piece of steel must have a diameter no more than 16mm, otherwise it will foul the chain.

Weld the piece of steel into place.

Weld the sprocket onto the end of the piece of steel, by puddling through the central hole of the sprocket. We first dropped the nut/steel combination into a hex socket which acted as a guide for aligning the sprocket. The nut rests on the bottom of the socket and the sprocket is visually aligned with the rim, ensuring that both are central and parallel.

The end result.

Thread the nut onto the shaft. The original alternator lock-nut will help with removing the sprocket, if needs be.

The result is not perfect: there’s a long overhang which may create problematic bending forces in the shaft. We could certainly have used a shorter length of steel. But this one is good enough for first trials.

All the build ideas on this page are provided free and with no warranty whatsoever, in the hope that they may be useful. There are many undocumented hazards in our builds. So if you are attempting to emulate any of our builds do take appropriate precautions to protect against injury and loss. We have not documented those precautions.