Motor
In Jordi's original YouTube video, he stated that the motor is 15 cm (6 inches) in diameter and weighs only 4 kg (9 pounds). It makes 15 hp (11 kW) at 10,000 rpm. Impressive!
Just to provide some context for the power claim, 11 kW at 10k RPM implies a torque of 10.5 Nm, which I assume is a peak (as opposed to a continuous) rating. 11 kW from a nominally 56 volt power source implies a current draw of 196 amps at 100% efficiency.
It looks like a QSMotor design, but I could well be wrong. It uses a U-V-W position encoder.
A parts diagram shows the motor having a pinion gear with 25 teeth.
The motor uses ceramic balls in its bearings which require a break-in period, during which time more acoustic noise will be noticeable.
I measured the phase-to-phase inductance of the motor as 7.25 uH (direct axis) and 8.0 uH (quadrature axis) both at 100 kHz. The phase-to-phase DC resistance was 7.0 milliohms @ 18.5 degrees C.
I'm estimating the motor comprises 36 coils (formally know as “slots” in motor design terminology). In a 3-phase BLDC motor, the number of slots must be evenly divisible by 3.
I'm also estimating the number of magnets (formally know as “poles” in motor design terminology) at 20. The number of poles must be evenly divisible by 2 since motors always utilize magnets in pairs (a north pole and a south pole).
This motor is classified as an “outrunner” because the rotor (rotating part) is outside the stator (stationary part).
Motor windings visible after removing an unsealed access cover.
Ingress Protection Rating?
One huge disadvantage of this motor is that it's not sealed.
You can see the windings by removing an access cover under the encoder cover. There is no gasket anywhere.
The only saving grace is that the opening is quite high on the bike.
I certainly would not recommend spraying water anywhere near this area.
And this is not just a hypothetical problem, as I have heard of two drown Dragonflys during the 2024 Ute Cup in Colorado. But, apparently, they were able to continue.
The only good thing about this opening is that it allowed me to peer inside to estimate the number of coils and magnets.
Motor Position Encoder
The Dragonfly's motor encoder is a simple device compared to the one EM chose to use. EM's encoder is expensive ($440) and unusual, producing one sine wave and one cosine wave per revolution. Furthermore, the chip inside is likely the sole-sourced AM4096, manufactured by RLS in Slovenia.
The Dragonfly's encoder is marked Hohner Automation MRK-1012.D00N. Hohner is a Spanish company. Unfortunately, I could not find that part number on their website but it appears similar to their MR series. I'm betting it was a custom design because the wire colors perfectly match those in the mating SiliXcon connector. It is a U-V-W encoder.
The Dragonfly spare parts price list published in April 2024 is difficult to use - it's incomplete, there are no part numbers, not all parts have an English translation and those that do often leave me puzzled. Having said that, I think the motor encoder and magnet are called a Sensor Plate and retail for 162.50 euros.
Likely locations for magnetic sensors circled in yellow
Close-up of motor position encoder and ring magnet
Oscilloscope capture of U-V-W encoder outputs when turning the motor by hand approximately 1 revolution
Credit: Phoenixamerica.com
Radial Multipole Magnet Ring
A multipole radially magnetized ring magnet is attached to the motor shaft.
The adjacent photograph shows an example of how the poles would be arranged.
I am guessing there are 36 poles, alternating north-south (N-S-N-S-N-S) around the perimeter of the Dragonfly's encoder magnet.
Magnetic viewing film shows locations of poles in magnet ring
Magnetic pole identifier indicating a North pole.
Motor Temperature Sensor
SiliXcon recommends using the KTY81 series of thermistors with their controllers. Based on a resistance measurement of 1800 ohms at 55° F (12° C) I'm guessing the thermistor may be a KTY81/210 (tighter tolerance) or a KTY81/220 (looser tolerance). It has a positive temperature coefficient. This is an obsolete part according to NXP Semiconductors.
Close-up of motor temperature sensor
Backside of encoder with attached thermistor.
Thermistor Table
Based on very little actual information, I constructed the adjacent table for a KYT81/220 thermistor as recommended by SiliXcon. Its use in the Dragonfly is nothing more than an educated guess. The KTY81 datasheet lists resistance values at specific reference points 10 degrees C apart. Resistance of thermistors is also typically specified at 25° C. In this case, it's 2000 ohms.
Temperature is converted to degrees Fahrenheit for Americans.
The SiliXcon manual reveals the internal circuit for reading a thermistor is a voltage divider comprised of the thermistor on the bottom with an upper resistor of 10k ohms pulled up to 5 volts.
This allowed me to calculate the voltage divider's fraction and the corresponding voltage measured by the controller.
It's likely the controller will not allow the motor's internal temperature to exceed 125 degrees C, and would begin limiting power well before that.
Motor bearings suffer at high temperatures, and this may have driven the decision to use ceramic balls.