I hope this is not the wrong subforum.

I have a simple question. (?)

From my understanding, if a brushless motor "fired" (I believe 2 phases fire at the same time.) when the pole and stator (or whatever these things are called) "line up" it would or could stall the motor. This would be 0deg timing I think. Sort of like a sensorless system firing phases "in the dark" and glitching at 0RPM.


So it is my guess that a brushless motor that reads 0deg on the external timing ring is not really at 0deg.


So if you are trying to figure out the timing that is actually in place, you need the manufacturer's built in "get it going" timing, plus the external adjustment plus whatever dynamic timing you give it from the ESC.

The max timing? It couldn't be more than 120deg, obviously. There are only 3 phases.

So, what is the unstated built-in physical timing? Do all manufacturers use the same value?

What is the maximum unstated+external+dynamic timing?

Also, given that most motors have a 30-40deg physical timing range, how much does this timing affect the motor at the extremes? Everyone says "more timing is less torque and more rpms" and less timing is "more torque and less rpms". How much of a spread is there? Would a 13.5T with 0deg external timing be compared to a 17.5T with 40deg external timing? My limited experience with brushless motors tells me that less timing is less heat, and that makes sense, I think.

If a 17.5T motor is rated at 190W at 95% efficiency then at optimal conditions it is bleeding 9.5W off as heat. Where is this optimal point? I suspect we spend very little time there.

My next goal: Understanding this:

"By Lenz's law, a running motor will create a back-EMF proportional to the RPM. Once a motor is spinning so fast that the back-EMF is equal to the battery voltage (also called DC line voltage), it is impossible for the ESCs to "speed up" that motor, even with no load."

Any resources or guidance is appreciated. I just want to understand how all this works. My Google searches so far have either been useless or have required that I go to school to be an electronic engineer.

Thanks!

First, your timing question:

When a motor is physically set to 0 degrees, that simply means that the Hall sensor output will transition from one state to the other when the back EMF for that phase changes polarity (the so-called "zero crossing point"). For an "ideal" motor and controller, this should be the best switching point for commutation ("best" being defined as making the best usage of the flux linkage between rotor and stator).

A polyphase permanent-magnet electrical machine requires no phase advance (AKA "timing") to generate torque from a dead stop; this is the primary advantage of using a three-phase design (as compared to single-phase motors such as those used in window fans, air compressors, and so on). In fact, at low speeds, phase advance beyond 0 degrees is almost always disadvantageous to the goal of making maximum torque.

We use timing for a couple of reasons. First is to counteract a variety of effects that occur in a practical system - ESC software and hardware has latency, windings have inductance, and magnetic fields distort slightly with increasing torque. The second reason (and the most noticeable to hobby users) is that advanced timing can allow the injection of more current as the BEMF increases; you'll need to read through to figure out why this is important.

Second, your question about efficiency:

Without knowing the exact design and specifications of the motor, it's pretty much impossible to determine the efficiency at any given operating point. It's really difficult to even give any general suggestions, since relatively minor changes to any component can dramatically change the area of peak efficiency. Basically, higher speeds cause greater "iron losses" (ohmic losses in the stator iron caused by eddy currents and hysteresis losses; both get worse at higher rotational frequency), and higher torque causes greater "copper losses" (ohmic losses in the winding and controller FETs due to the greater current required to generate torque). As hard as we push the motors in hobby applications, both are considerable.

Third, your question about Lenz's "law":

Any permanent-magnet electrical motor is also acting as a generator whenever it is spinning (whether by an external force acting on the shaft, or by internal magnetic forces). The voltage that is impressed upon the winding is called "back electromotive force" or "back EMF" (sometimes also abbreviated to BEMF). It is proportional to the speed of rotation, and can be characterized by a simple expression of volts per RPM for those of us antiquated enough to still use imperial units. We call this parameter "Kv". The rest of the world would prefer to use the parameter "Ke" with units of volt-seconds per radian, which seems clumsier but actually makes life a bit easier as we'll see in a moment.

This is important for a couple of reasons. As you pointed out, this BEMF opposes the supply voltage; thus, as the motor spins faster, less effective voltage is available to force current to flow through the winding, and the output torque decreases. Eventually, no additional torque is available for acceleration, and the motor speed reaches equilibrium with the load.

In an ideal system (one with no losses), the maximum unloaded speed is indeed the battery voltage multiplied by the Kv parameter. This is never the case, since we always have losses, but hopefully you get the idea that motors with a higher Kv constant can spin faster using the same supply voltage.

Next, there is another parameter called "Kt", which expresses the relationship between torque and current. In the metric system, it has the units of newton-meter per ampere, and has the same value as Ke. If you do the math, you will find that as Kv increases, Kt decreases and vise-versa. A motor with a higher Kv value can spin faster, but requires more current to create the same shaft torque (file this one under "there is no such thing as a free lunch").

Advancing the timing has the effect of applying battery voltage to the winding before the BEMF rises, so we can extend the speed-torque envelope. The consequence is that we do not make best use of the flux linkage in the motor by applying commutation that is out of phase with the BEMF, so we are not as efficient in producing torque. Thus, more current is required, and we lose more power in the winding, and the motor runs hotter.

This all starts describes the relationship between the critical inputs and outputs of the motor - namely voltage, current, shaft speed, and shaft torque. Hopefully this is reasonably easy to understand; if not, shoot out some more questions and I'll at least attempt to answer them.

What you wrote above is going to take a few days for me to figure out.



I'm sure I'll have a question or 3 later on.

If you have anything else to write about feel free!

Thanks a bunch!

Don't be afraid to ask questions - it's the way I learned (and am still learning) about this stuff. Frankly, like any other complex subject, it's one of those situations where it's easy to feel like the more you learn, the less you know

It'd be nice to find someone that writes in a less dense manner than I do, so that we could create some good stickies on motor-related topics.