Electric Moped Update #3: Power Upgrade 1.0

Finally, the part I’ve been dying to do, POWA UPGRADE! Being a poor college student  (the kind who frets over buying ramen in a cup over packaged ramen because of the 20 cent difference) I had a hard time bring myself to spend the money on the parts needed to upgrade the mopeds power system. After a few weeks of riding it around campus and then pushing it up every little bitty hill, I came upon I finally caved in and ordered the parts I needed. I figured I’d be using the moped for at least the next 4 years to get around campus so the investment would be well worth it.

  • BLDC Motor (sk3 hobbyking)
  • Electric bike Speed Controller (aliexpress)
  • sprocket (banggood)
  • keyed Throttle and Voltage readout (amazon)
  • hall effect sensor (equalsZero Design)

Motor Mount

To mount the motor I designed a 3D printed part that matched the mounting holes of the original motor and placed the shaft of the new motor in the exact same place as the old one.  I added holes underneath the motor to attach the hall effect sensor for better startup torque. With the 3D printed motor mount testing I did with the Mostly Printed Electric Bike I’m sure this new mount can stand up to the same load and same motor heat.

After grinding a flat spot on the motor shaft so the sprocket grub screw would seat properly I bolted the motor to the new mount, screwed on and adjusted the sprocket, and bolted the whole assembly where the old motor used to be. The chain rolled onto the new sprocket without error and it all fit perfectly behind the plastic chain guard. I’m a little concerned about how close the motor is to the wheel, there is definitely going to be some water and dirt thrown up by the wheel onto the motor but for now, I’m going to call it good until I can make some kind of mudguard.


Now because I’m extremely impatient when it comes to getting projects done and the bike speed controller hasn’t come in yet (that’s what I get for ordering cheap from China) I’m going to jerry-rig the electronics for not just so I can have something to get me up some hills.  I have an 80 amp speed controller left over from my initial work on the Electric Bike that I’ll use to power the motor. I stopped using it because it had zero startup torque and once when it got out of phase at high speed it locked up the motor sheering every spoke on one side of the bike’s wheel.  The problems I had with this speed controller on the bike shouldn’t be a problem on the moped because of the clutch on the moped that allows it to still coast even if the motor is stopped. Plane speed controllers require a PWM Servo signal to operate so I connected the speed controller and potentiometer throttle to an Arduino and wrote a little code to convert the potentiometer value into a PWM signal the Motor Controller can read. Snagged a 5v regulator out of my parts bin to power the Arduino, ran the positive side input to the throttle switch to turn everything on, and wham bam shang a lang we are all good to go!

In Use

Well first off the power increase after the upgrade is HUGE. The thing accelerates faster than a bullet. Because of the zero startup torque getting going is a little odd but once it’s moving the throttles pretty smooth as long as you don’t jerk the throttle. Every once and a while the motor will lose phase and stop spinning if I jerk the throttle too hard, but with a little finesse, it’s not too hard to get it spinning again. The torque with this gearing is pretty awesome and doesn’t even slow down when I try to climb the steepest hills around campus. The plane motor controller can only handle up to 24V so I’m only running the motor off one of my 24V cells, at the current gearing that comes out to about a 15mph top speed. It’s not too bad but it is a little slow for my taste. I’ll be able to run the full 48V when the E-Bike speed controller comes in doubling the top speed. Pretty happy with how it is right now but it’s not quite as new user-friendly as I’d like (every person who’s tried it jerks the throttle and the motor makes a horrible suicidal noise). The bike speed controller should be a good upgrade for usability and top speed.

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