Electrifying a Puch Maxi

I while ago I bought a Turnigy 80-100 motor to put on my bike. For several reasons, mechanical, electrical and self-preservational, I ended up putting a more suitable E-Bike hub motor on my MTB and the large Turnigy motor has been lying on my unfinished-projects-shelf since I bought it. Recently my brother told me he has an old Puch Maxi moped, where the original petrol engine were broken, that would be the perfect candidate for this motor.

The motor is just a little bit larger than a soda can and capable of producing 7 kW of power for short periods of time. It will probably be more safe to mount it on a moped instead of a bike considered the moped is built to handle a lot more power than a bike.

In Sweden, there are two classes of legal moped, where this is classified as ‘moped class II’ limiting the motor power to 1 kW and maximum speed to 25 km/h. With this motor the moped will be very illegal but I will keep it off public roads. It would however be interesting to limit the power and speed electronically and try to get every permit needed to use it in traffic. Sadly I suspect that’s impossible due to all bureaucracy involved.

The moped has a chain drive with a 415 chain, hence I needed a chainwheel matching this chain and the motors 12 mm shaft. I found the Swedish company Kedjeteknik that was very helpful and helped me custom make a 10 tooth chainwheel for 415 (1/2″ x 3/16″) chain at a reasonable price. There is a 12 mm hole for the shaft with dual stop screws. I really hope this will manage the ~5 kW of power I want to get out of the motor.

I bought the 180 rpm/V wind of this motor meaning the maximum rpm would be 180 times the battery voltage. This is quite fast with only a single reduction, especially on a bike with 26″ wheels. On the moped with 17″ wheels and a rewind of the motor this will be perfect.

The rear sprocket of this moped is 43 tooth and the wheel diameter is 17″. As a battery i will use the same type as on the MTB, 12 cells of LiPo in series resulting in a battery voltage of 44.4 V. As a rule of thumb the final velocity will be about 80% of maximal velocity. With this information the expected top speed can be calculated.

Motor speed using 44.4 V battery and 180 rpm/V motor

180 * 44.4 * \frac{2\pi}{60} = 837 rad/s

wheel speed after 10:43 chain reduction

837 * \frac{10}{43} = 195 rad/s

80% of unloaded speed with 17″ wheels

195 * \frac{17 * 0.0254}{2} * 0.8 = 33.7 m/s = 121 km/h

Which is a little too fast, half of that speed would be enough. Since the equations above are linear one way of achieving this is to rewind the motor for 90 rpm/V instead of 180 rpm/V. This motor is wound with 6 turns per phase and terminated in delta. By increasing the number of turns to 7 and terminating the motor in Y instead the resulting kV will be ~90 rpm/V. (I think I’ll write a more in-depth article about brushless motors in the future describing why)
Last wekend my brother and I made a motor mount out of 5 mm aluminum that bolts on to the original motor mount of the moped. I only have a picture from a mobile camera right now but I will update this post with better pictures in the future.

Motor mount on Puch Maxi
Motor mount on Puch Maxi

Next post will be about the motor modifications, which are extensive…

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