Triggered by a number of discussions between @hummie, @devin and @brams and because of science I’m digging into this, making a fool out of myself trying to explain why a hub motor gets warmer than a satellite motor cruising at the same speed. Btw, the short answer up front: because of the lack of gearing!
Disclaimer: I’m not a professional writer nor a teacher. And English is not my first language. Just some dude trying to explain things. I will also have to simplify stuff, hopefully not oversimplifying. So please be patient with me but step in when I’m wrong.
How does a BLDC motor work? First we need to understand how a Brushless DC (BLDC) motor works. Regarding the mechanics a BLDC motor is very similar to a step motor and a synchronous motor and a brushed DC motor. I’m not going to discuss mechanics here for now.
The main difference between the mentioned motors is how they are controlled. Synchronous motors are AC motors. They ran at a frequency set by the AC current supply. Synchronous motors don’t have much with regards to control as the frequency of the input and thus their RPM is normally given. Under load they still run with the same frequency but increase to draw current. Note: most synchronous machines are used as generators. Actually, all of the synchronous motors can be used as generators.
Brushed DC motors have the “controller” “build it”. Input into the controller is a DC Voltage. The controller is a mechanical thing with brushes and contact areas. This controller converts the DC input into AC output which is then used as input for the actual motor. This is called commutation and the controller is a commutator. The commutator is a mechanical thing and is actually quiet complex. Because of the brushes and the contact areas it has mechanical wear which is a big disadvantage. So the brushed DC motor is actually an AC motor very similar to the synchronous motor. Because overall input is DC it’s called a DC motor. Speed control is done by varying the input voltage into the system, so by varying input voltage into the commutator. Higher voltage results in higher speed. Under load the motor uses more torque and draws more current. Torque and current are proportional. Btw, more current produces more loss due to heat.
Now we come to our beloved Brushless DC Motor which is similar to a brushed DC motor … without the brushes . Or better: without the physical commutator. The physical commutator is replaced with an electronic commutator which normally sits outside of the motor! Same as the physical commutator the electronic commutator (EC) is quiet complex. You guessed it, it’s the ESC. The ESC commutates the DC input into AC output which is then AC input for our motors. So our BLDC motors are actually AC motors, only the mostly external ESCs make them DC motors … And they are very similar to AC generators.
How is speed control done in the ESC for our motor? Simple answer is it’s the same as for the brushed DC motor, the ESC varies the motor input voltage. But this is oversimplified. Also as we know the voltage into the ESC is normally fixed. In our case it’s the voltage of the battery. A technique called pulse width modulation (PWM) is used to create an average voltage into the motor. The higher this average voltage the higher the RPM. Higher average voltage is achieved by wider pulses. All this is calculated by electronics and software. In the VESC we have a chip that does all of the calculations plus we have a driver chip that gets input from the calculation chip and that drives the MOSFETS. The MOSFETS open and close the gates for current and voltage. So the voltage going out of the VESC and into the motor is the average voltage produce be PWM and it’s always lower than the voltage going into the VESC. It cannot be higher (unless produced by the motor, we come to that later). (note: our RC-style remote receiver controls the throttle of the VESC also using PWM, but this has absolutely nothing to do with this, just the same technique)
Now we have to discuss Kv of a motor. Kv is one motor constant, the motor velocity constant. It is fixed and based on the physical characteristics of the specific motor. Kv is two things:
- most people will say that Kv is the RMP per Volt of an unloaded motor. As an example the OllinboardCo OM5065 is a 170Kv motor, so at 10s=37V it will have an RPM of 37V*170RPM/V = 6290 RPM (without load) – this is not the definition of Kv, but it’s almost true, it’s very close. The reason is explained below.
- the exact definition of Kv however is the following: Kv is the reciprocal value of the Ke, the back-EMF constant. Kv = 1 / Ke. The back-EMF is the voltage a motor generates when rotated. So when chakas motor is rotated with 5000 RPM it produces 5000 RPM * 1/ 170RPM/V = 29.4V . Remember? Our motors are also AC generators.
When does a BLDC motor turn / accelerate? Answer: When the average voltage from the ESC is higher than the generated voltage. Then current starts to flow through the motor. But how much current does flow? This depends a) on the mechanical power needs to be produced by the motor and b) the RPM the motor is currently running and thus the back-EMF in V. This mechanical power is produced by electrical power and is calculated voltage times current - P = U*I where U is the back-EMF generated by the motor. If a motor does not need to produce power it will spin up freely and not much current will flow and the RPM reached is very close to the theoretical maximum Kv * U .
If the motor needs to produce a lot of power, more current will flow, especially if the motor is at a low RPM (and thus low back-EMF).
This concludes the first part of my article. I’m working on the second part where I want to show some numbers and calculations and dig into the hub motor story. This needs some time, so please be a little patient.