# BLDC motor information to build an E-bike

I'm planning to build a light e-bike kit and want to use a super-efficient BLDC inrunner RC motor.

Many people seems to build e-bikes around AstroFlight 3210 2kW motor, because it is the most efficient motor on the market. However I'm not in for a monster, but a smaller 250-300W motor is good for me, so I have been thinking about using Astro 819M-9T. https://www.astroflight.com/819m-9t-astro-19-brushless-motor-details.html

I will use a high reduction gearbox, so don't think about the RPM to wheel for now, I have a solution for that.

My problem is I don't now anything about RC motors or ECS and I don't even understand how to read the specification for this motor. They only specify maximum power/amps etc. and I need to now nominal values for it. Also there is no torque rating for it, only a description of what size of prop and weight of models.

Questions:

1. What will the nominal values be for this motor?

2. What torque does it have and approximately how will the torque curve look?

3. Suggestion of other motors that would be good for this?

4. Would it be better to have a BLDC with sensor?

• Remember that Torque $\infty$ Current. This means that the battery must also be able to supply the starting current. Commented Mar 29, 2020 at 19:02

## 1 Answer

They only specify maximum power/amps etc. and I need to now nominal values for it.

'Nominal' values aren't particularly useful. The important parameters are Kv (velocity constant) or Kt (torque constant), internal resistance, no-load current, maximum rpm and maximum operating temperature. The data provided covers most of this.

What torque does it have

Kt is the inverse of Kv. This is in SI units, so first we must convert rpm to rad/s (radians per second). 1375 rpm x 2π/60 = 144 rad/s. 1/144 = 0.00694 N⋅m/A. The rated maximum current is 25 A. Subtracting the no-load current of 0.45 A (which doesn't produce output torque) we get 24.55 A x 0.00694 N⋅m/A = 0.17 N⋅m (newton meters).

and approximately how will the torque curve look?

In any PM DC motor, as torque load is increased speed drops approximately linearly (due to voltage drop across the motor's internal resistance) down to zero at stall. Conversely torque increases linearly from zero at no-load to a value determined by the stall current (which Ohm's Law says is V/R). So the relationship between speed, torque, current, power output and efficiency looks like this (graph from pololu.com):-

There may be some deviation from this at high rpm due to winding inductance (which increases the apparent resistance), but the current vs torque curve is usually very linear.

Maximum power output always occurs at ~50% of maximum rpm and torque, though this may be beyond the motor's power dissipation limit and therefore higher than the rated output power (particularly in a high performance motor such as this). Note that the power rating of this motor is actually the input power. Due to internal losses the output power is lower (50% less at theoretical maximum output power).

Suggestion of other motors that would be good for this?

This is a good motor for a high power, lightweight geared application. Other manufacturers (eg. Kontronik, Mega Motor) make similar motors, but generally use Neodym magnets which have a lower temperature rating. You might also consider motors designed for 1/10th or 1/8th scale RC cars.

Would it be better to have a BLDC with sensor?

Depends on how you intend to operate your e-bike. If the motor needs to run from a standing start then sensors are better because sensorless back-emf doesn't work at very low speeds. If the motor is used to assist peddling then sensorless makes for simpler and more robust wiring.