AC induction motors are everywhere, from ceiling fans to vehicles. They’re reliable, simple, and rugged — but there are some disadvantages. It’s difficult to control the speed without complex electronics, and precisely placing the shaft at a given angle is next to impossible. But the core of these common induction machines can be modified and rewired into brushless DC (BLDC) motors, provided you have a few tools on hand as [Austin] demonstrates.
To convert an AC induction motor to a brushless DC electric motor (BLDC), the stator needs to be completely rewired. It also needs a number of poles proportional to the number of phases of the BLDC controller, and in this case the 24-pole motor could accommodate the three phases. [Austin] removed the original stator windings and hand-wound his own in a 16-pole configuration. The rotor needs modification as well, so he turned the rotor on a lathe and then added a set of permanent magnets secured to the rotor with JB Weld. From there it just needs some hall effect sensors, a motor controller and power to get spinning.
At this point the motor could be used for anything a BLDC motor would be used. For this project, [Austin] is putting it on a bicycle. A 3D printed pulley mounts to the fixed gear on the rear wheel, and a motor controller, battery, and some tensioners are all that is left to get this bike under power. His tests show it comfortably drawing around 1.3 kW so you may want to limit this if you’re in Europe but other than that it works extremely well and reminds us of one of our favorite ebike conversions based on a washing machine motor instead of a drill press.
Awesome! How does free wheeling work?
(It says “fixed” gear on wheel…not sure if it’s really “fixed”.)
The bicycle chain side is definitely not fixed, it looks like a standard single-speed freewheel.
On the motor side, the motor/belt/toothed pulley are fixed, so the motor shaft is turning at all times, even when there’s no electric assist. But it’s happy to “freewheel” in the sense that the electric motor shaft spins relatively freely when the motor is off/disconnected.
Of course this adds weight/friction when coasting vs. having no motor at all. Putting a freewheel on the electric motor side would mitigate this, but it also adds complexity and makes it impossible to use regenerative braking.
A normal three phase induction motor turns freely when the stator windings are disconnected. Only rotor inertia and bearing friction applies…
ah, he modified the rotor to a magnetic one, so there’s rasonable force needed to turn the unpowered thing
Why does the stator need to be rewired?
I assume it’s to be able to use a lower voltage, and also to go from single phase -> three phase
I believe rewiring the stator simplifies the usage. An unmodified induction motor can be used as a generator with a AC motor drive and any torque beyond the synchronous speed will add current to the DC bus which can be collected.
I assume the motor was a mains voltage part, so you’d have to rewind for the battery voltage.
Changing the count of poles basically shifts the balance between speed and torque.
More poles = less speed, more torque.
I this case 16 poles might have been the sweet spot for speed/torque for the BLDC driver’s frequency range.
In a sense, in most industrial settings AC induction motors are basically used as BLDCs, as they are driven by VFDs (VFDs are like a big rectifier, followed by a BLDC driver). Even in constant speed applications they are sometimes used for soft starting drives, where you’d classically have used a star/delta setup…
Looks like” I built a brushless DC motor using the shaft, bearings and case front and ac motor”