Thanks for taking the time to go through that in detail. It really is helpful.
I personally struggle when people tell me to think in terms of power. Power is a useful fiction. It’s useful for magically producing values of voltage/current (that must be true because of conservation laws) but leaves me totally clueless as to what real, physical effects are causing them.
In this case, voltages seem to be what I ought to think in, because they’re the causal root of everything else. By which I mean, the ODrive can’t set power or current any other way than by manipulating voltage levels via PWM signals.
- the current on the motor side comes from the difference between those two voltages * the internal resistance of the winding. no. the current on the motor side is controlled by a closed-loop system. It is always proportional to the torque demand that you set.
So in this case, the current is (hopefully) “set” to be whatever value gives you the torque you want. But what’s really setting it is the voltage the ODrive applies to the windings, and nothing else.
So if I were to rephrase it as
"the current on the motor side comes from the difference between those two voltages (when summed as spinning vectors, not flat values) * the internal resistance of the winding, plus a larger component that grows towards V/R with time constant L/R because of the motor's inductance" - would it then be closer to the truth?
And then all those sums coincidentally wind up at the exact current value we want, because the ODrive has very cleverly chosen the voltage to make sure that’s the case.
The DC Bus current is the motor current * the PWM duty cycle %
Just one question: which motor current? There are three windings, so three currents. If you can’t just add them up (even as spinning vectors) because they sum to zero, what do you do?
The DC bus current bit otherwise makes sense to me. The motor is causing the current but the DC bus is only connected to it x% of the time, so it only sees x% of the current (unless you zoom in very close to the PWM cycle speed.)
This also plays out so that the DC bus has the “full” voltage and the motor gets x% percent of it, while the motor has the “full” current and the DC bus gets x% of it - and thus power is conserved.
…consider the motor as just a coil of wire around an iron core, ie, an inductor…
The modelling the motor as an resistor+inductor bit was very useful, makes me feel like that was the missing link to understanding all of this stuff all along.
So thanks for that, I will go back and work through it in detail.
The voltage is “faded in”, the resistance is constant.
Here you’ve got me. Do you mean, the voltage drop across the resistor is faded in?
My thinking was that when the FET is off, the resistance of the resistor is
resistor+open circuit = infinity, and when the FET is on the resistance is just
So I saw that as a “resistance” fading in and out, and can happily understand it in terms of “voltage drop over said resistance” if taking about fading resistances is somehow a faux pas. But if instead something else is going on then I’ve misunderstood.
the back-EMF is always lower than the supply voltage, that’s what limits the max speed for a given voltage.
Now I’m confused again.
This is an oversimplification.
I don’t understand how the back-emf can always be lower than the supply voltage - I thought the whole concept of braking was that that’s when the back-emf is higher than the supply voltage.
At which point
- current is flowing the other way,
- power is coming out rather than in, and
- the motor is trying to slow down because that’s how Physics is making you pay for that power you’re getting out.
Hence, regenerative braking. Also, note, magic anthropomorphic Physics being invoked the minute I had to talk about “power”.
I might read up on inductors a bit and see if that helps clarify things.
yes, only 5.1A will flow through the resistor at 24V
Getting back to the immediate situation, I’ve got two of the little black 50W 2 Ohm resistors that come with the ODrive. Presumably I can attach those in parallel to get an equivalent 100W, 1 Ohm resistor.
That will, by my calculations, let me dump 24^2/1 = 576W through it, 476W of which is beyond the rating. Is that going to be safe (even just for testing)? I don’t particularly mind if the resistors burst into flames, provided nothing scary is going to happen to the rest of the board.