Punx0r wrote: ↑
Feb 15, 2018 5:45 pm
Buk___ wrote: ↑
Jan 23, 2018 12:44 pm
Part 1: Output power is torque * rpm. At pull away, rpm is 0, so power is 0, until ...?
As you say, BEMF of a stalled motor is zero and the resistance seen by the controller is indeed only the (very low) winding resistance. If you connect a controller without current-limiting and apply full throttle, phase current will indeed be I = V/R = lots. If the motor doesn't start turning and generate BEMF the current will remain high and if the controller cannot handle this stall current it will crap out. This is traditionally what happened when cheap RC controllers, which had no current limiting, were used to drive a large vehicle.
An appropriate controller will limit this initial surge AFAIK with a throttle ramp that prevents 100% throttle being applied instantly and allows the current sensor time to detect the over-current and roll back the throttle/PWM.
As you say, into a stalled motor the phase voltage may only be a handful of volts. Conversely, the phase current is very high but battery current is very low (power in to the controller = power out [minus small efficiency loss]).
If you keep demanding 100% throttle, as the motor spins up BEMF rises, and at some point, the phase voltage is no longer sufficient to push maximum (limited) current any more, the current limiter disengages and phase voltage = battery voltage & phase current = battery votlage. Phase current and torque starts to reduce. This is the gradual tapering off of acceleration you feel as your vehicle gets closer and closer to its top speed. Eventually the torque exactly balances the load (friction and air drag) and acceleration reaches zero. This is top speed.
All of that makes sense. I mean to me
; rather than the rest of the world which you already knew
But ... (Ya knew it was coming ...
there is still a gap in my intuition about what happens right at the very start. The bit emboldened below
Punx0r wrote: ↑
Feb 15, 2018 5:45 pm
True. The power output of a stalled motor (which is what it is at start-up) is zero and efficiency is 0%. What the motor is producing though is torque, it is applying a static force even though it is doing no work. If that force is greater than the forces holding the motor still, slight rotation will occur
. Now you have (very low) velocity and so (also very low) power. Things ramp up from there as long as torque remains greater than the load.
There are many things that can break the status quo. The rider presses on a pedal. Or has a slight tendency to lean forward. Or the bike is on a slight decline. Or the air pressure from a passing bus nudges the bike ever so slightly....
Or, more likely, the net difference of the forces attracting the magnets, and those repelling the magnets -- due to the exact rotational relationship of the rotor and stator at the moment power is applied -- is such that it pulls a tad more than it pushes.
Any of these can start the snowball rolling and physics takes over.
Where I get really vague -- despite looking hard -- is how does the controller ensure that the motor takes off forward?
Let's stick with a sensorless controller for now, and I can see that a sensored controller can interpret the 3 hall signals and determine the current -- stationary -- phase alignment of the rotor. I'm not sure that they do; but they could.
But in a sensorless controller, there is no BEMF until the (snow)ball is rolling, so how do they ensure that the motor starts up the right way?
I can see that they could just apply power, run until they detect BEMF, and then reverse things if it was going the wrong way, but it seems to me that it would require at least 120° (electrical) and possibly more to determine the direction of rotation; and on any motor with a low pole count, that -- 1/3rd or 1/2 turn the wrong way -- would be noticeable, but it does not appear to be so, even on 4-pole fan motors.
In short, how does a sensorless controller chose which phase to power first to ensure that the motor takes off in the right direction?