Mike is right, and I say that as someone who has a board each with eLofty original 58 KV DDs and a dual 6374 belt drive.
ELofty/cheaper DD drives:
My eLofty drives have been great for me, and up until recently, itās been the board I grabbed first when going for a fun ride. But my cheap DD drives have been the exception rather than the rule. Iām on borrowed time. Also, Iām a bit bored with the fixed top speed on TB 110S.
Pros:
Easy to build with. Plug and play. Work well with a number of ESCs, but best with vesc-based unit. Cheap.
Cons:
Harder than you think to find a new pair without manufacturing flaws. 1:1 limits your performance/flexibility. Adapters arenāt as prevalent/easy to find as wheel pulleys. If a motor goes bad, finding/sourcing a matching motor may be difficult with so many variations out there.
Belt drive:
The dual belts took longer to get right, but the rewards more than make up for the extra work. Offers far more flexibility. I can dial up stronger acceleration or top speed with pulley swaps. Plus, I have extra components for everything on the shelf if I need them. This board is now my favourite.
Pros:
Performance, flexibility and ease of repair. Belts are a long-time proven performer and thereās a ton of options out there. You learn a bunch of things in the process of putting it together. Properly juiced dual 6374 power (or dual 6355 in your case-not a huge difference) is great.
Cons:
Takes more time to tweak and get right. You need to source and acquire a bunch of separate pieces that come together as a whole. More expensive.
Having 12s instead of 10s indeed draws less amps from the battery for the same constant speed with no other changes, but since the vesc does a voltage change, the current in the motor is still the same. Since losses in the battery wires are negligible compared to losses in the motor, the increased efficiency caused by the reduced heating in the battery wires is negligible, because the same amount of losses will be happening in the motor. But if you donāt change the current settings, 12S enables more motor current at speeds when you are hitting the battery limit, because the same number of battery amps is more powerful at higher voltage. That increased motor current reduces the efficiency (but improves the acceleration) when you are hitting the same battery current limit on 12s compared to 10s. 12s also allows you to ride at a higher top speed, which in practive means youāll be drawing more battery current on average with 12s to achieve those higher constant speeds than you could otherwise do with 10s, which also reduces the efficiency since wind drag forces increase proportionally to the square of the velocity.
If youāre drawing the same number of watts, then a 20% larger battery will mean 20% less load on a given cell. Thatās not āefficiencyā, thatās overengineering, or overbuilding, or simply leaving yourself headroom.
For a given vehicle, with the same motors, controllers, gearing, blah blah blah, 12s vs 10s gives you 20% more voltage and thus 20% higher top speed. Aerodynamic drag is a function of speed cubed, so a 20% increase in speed increases your top-speed power consumption by 73%. Thatās not more efficient.
If you have a bigger (higher voltage) battery, you will have more energy to play with. But if you donāt adjust your setup to take the increased voltage into account, you will end up with worse efficiency overall, just like your numbers show. Your run at 12s used more energy per mile, than your 10s run did. It was less efficient.
The vesc indeed does change the voltage- the current in the battery wires is pulsed but the current in the motor (during a single commutation in bldc mode) is nearly constantā¦
for example suppose in your log you see 30a battery current and 60a motor current (50% duty cycle)ā¦ it means 60a is drawn from the battery half the time (30a), and 60a is going through the motor the whole time (at half the battery voltage):
^itās a hard concept to understand but the on-off pulses from the battery are so quick that the inductive property of the motor, combined with the fact that the motor is shorted to itself during the battery off times, keeps the current flowing through the motor the entire time during the battery off times (there isnāt enough time between pulses for the motor current to change significantly), and if the duty cycle is 50% then the voltage of that motor current is indeed half the battery voltage.
the vesc really is lowering the voltage, and that motor current at lowerered voltage isnāt pulsedā¦ itās constant. itās the battery current which is pulsed.
in reality during the on times, the motor current may be increasing from 59.9a to 60.1a, and during the off times, the motor current is decreasing from 60.1a to 59.9a. the inductance on the motor side (which is the property that keeps the current going without applied voltage, so to speak) is a result of the presence of the iron in the stator, and the coiled shape of the windings. the battery side has very little inductance because thereās no iron and the battery wires are relatively straight, so the current on the battery side can ramp up and down much more quickly than in the motor.
another way to think about the inductance is that there is actually energy stored in the magnetic field of the stator, and the collapse of the magnetic field when you remove the outside votage, āpowersā the inductance, which keeps the current flowing for a short, extra amount of time.
i disagreed with you because the vesc changes the voltage. suppose your battery is 50v, and your winding resistance is 0.05ohm, and with a sensored motor you command 60a motor current at standstill (60a motor current limit at full throttle)ā¦ the vesc will lower the voltage to 3vā¦
^3v is 6% of 50v, so that is 6% duty cycle, and 60amps is drawn from the battery, 6% of the time, which is 3.6a from the battery. 3.6a * 50v is 180w. 60a goes through the motor the whole time, and since the 60a has a power of 180w, itās only 3v. the heating the motor sees is 60a^2 * 0.05ohm which is also 180w.
that is correct. and whether itās a 10s or 12s battery, the power and heating is the same in the motor for 60a motor current @ 0rpm, itās just a different duty cycle.
The VESC doesnāt change the battery voltage. It pulses the phases on and off as you said. The BEMF combined with the driving pulses acts as a lower voltage, as you said.
But saying the VESC does a voltage change is a little misleading. Itās not changing any voltage, itās just pulsing it on and off.
and if a 190kv motor is turning at 190rpm, then itās producing 1v back emf voltage, so now the vesc has to supply 4v for the same 60a motor current instead of 3v at standstill, which takes a higher duty cycle (8%). & 4v * 60a motor current is 240w. so now 4.8a is coming from the battery. 4.8a * 50v = 240w
nope, i said the motor wasnāt spinning so there was no bemf. so in the case i described, the lowering of the 50v to 3v had nothing to do with the bemf. the bemf is caused by the spinny magnets.
(you could also call the inductance of the stator winding a kind of back emf, but that bemf is separate from the bemf caused by the spinning rotorā¦ usually in a motor context back emf refers to the back emf caused by the spinning rotor, not the back emf caused by the winding inductanceā¦ so thereās actually 2 different bemfās in play)
There are two different kinds of BEMF, one caused by a current flowing in an inductor, and another separate one caused by a magnetic field (permanent magnet) passing across an inductor. Iām referring to the first one.
well in one part of the vesc, the current is pulsing on and off, but in a different part of the vesc, the current flows continuously at lower voltage, so it does both
Duty cycle is pulsing on and off but if you add some capacitance the average voltage ends up being adjusted, as far as I gather capacitors help to level out voltage whereas inductors store/level out current. Think itās okay to say average voltage is adjusted or something but it is still a pulsing out of the ESC even if that gets smoothed on the way into the motor or opposed by current generated from the motor acting as a generator, itās splitting hairs a bit I think but if you measure output of the ESC will see it as PWM of the full voltage on and off so think I agree with @b264 here.
Iām pretty sure will see full voltage Iāll test this out for myself though.
^bldc uses āsquareā current plots in 2 of 3 phases while foc uses sine current plots in all 3 at the same time
like i said during the battery off times (in bldc), the motor current might be falling from 60.1a to 59.9a, and during the on times itās rising from 59.9a to 60.1a
Iām not disagreeing with how the ESC works. This is the part you donāt understand.
I think saying the VESC is changing the battery voltage is misleading. Because itās not.
Is it massaging the coils in such a way to have an even current flow, as-if youāre using a lower votlage battery at that point in time? Yes. But itās not doing a voltage change.
