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Basically the motor torque is directly proportional to the amount of motor current, and if you set the motor current limit and battery current limits to the same value, then you can reach the motor current limit at nearly every speed your motor can turn (except when you are very close to the no load speed). At lower speeds the same motor current and torque requires less battery current than at higher speeds.
On the other hand if you set the battery current below the motor current limit, then at full throttle you will only be able to hit the motor current limit at low speeds, and as the speed increases, the motor current will gradually decrease with increasing speed until at a particular speed close to your maximum speed the motor current will be approximately the same as the battery current limit. In other words when the battery current limit is set below the motor current limit, then at most higher speeds you actually won’t be able to hit the motor current limit, because doing so would exceed the battery current limit. So setting the battery current limit setting below the motor current limit setting causes max available torque to decrease with increasing speed.
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Thank you!
I think I understood it correctly then.
Yeah, the actual limits are set in software, but they should respect the hardware’s capabilities.
So, for the practical design of a board, I am trying to understand what is the ideal battery to match with a given pair of motors or the other way round.
E.g. for a board limited to a 12s4p battery (made of 25R), we should keep our battery amps below 80A.
If we were to combine this with the TB DD, then the motors can do 160A cont, combined. if we set motor amps to 320A combined, then the battery will be the limiting factor pretty much after we reach 1/4 duty cycle. And above 1/2 duty cycle we are not using the full continuous torque potential of the motors.
If we were to double our battery to 12s8p, with 160A cont, with the same motor settings, we would reach the battery limit only at 1/2 cycle, and use the full potential (continuous amps) of our motors even at full speed. However, we would be ‘over clocking’ our motors throughout, likely destroying them.
But as most VESCs anyways can’t safely do more than 80A per motor, that is likely anyways the actual limiting part. (up to half cycle for the 80A battery)
Or is the amps rating of VESCs related to battery amps?
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my understanding is the original vescs were limited by firmware to 60a battery and 120a motor per motor
@PrivacyDoctor hopefully this will be a decent illustration… it shows full throttle acceleration while increasing the motor current limit setting in 10a steps in a particular setup (all with 60a battery current limit setting)… the duty cycle is the purple line which is the percentage of time that current is being drawn from the battery
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this is the very same setup, but no software current limits & 100% duty cycle the whole way across…
(the battery current is underneath the blue motor current line because they have the same value at 100% duty)
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The first graph makes sense, the battery is only the limiting factor after motor-amps reach motor amps at 60, but also only in the upper dust cycle area.
Re: the second graph I am not sure what the message is here.
Something like: If you go to the absolute maximum of motor amps, the conversion efficiency is ridiculously low? Why actually?
Btw. Do you mind sharing the spreadsheet/code? I would love to play with it to get better intuition.
at 0rpm the current is limited by ohms law determined by the resistance of the winding and the battery voltage, in this case 50v and 0.05ohm… once the motor turns the back emf voltage is subtracted from the battery voltage which linearly decreases current with rpm (at 100% duty)
50v / 0.05ohm = 1000a
(50v - 1v bemf) / 0.05ohm = 980a
the vesc takes this a step further with duty cycle:
((50v * 10% duty) - 1v bemf) / 0.05ohm = 80a motor current
a 190kv motor turning 190rpm generates 1v bemf
&
motor current * duty cycle % = battery current
you can see with this setup as long as the effective voltage (battery voltage * duty cycle%) stays 4v above the bemf v as the motor accelerates, then 80a will be supplied to the motor.
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2 questions:
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So the highest current a motor will ever see is being stalled at full throttle?
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I don’t understand how ohms law applies to a 3-phase motor. What is considered the motor resistance when you have 3 winding? Do you just measure the 3 combinations and add them up (for delta config)?
^for the purposes of calculating the motor current as described above in BLDC, you connect points c2, a2 and b2 in the diagram, then check the resistance between points c1 and a1
on the other hand when the vesc measures the resistance it only displays the resistance between a1 and a2, because it uses the one phase resistance for foc calculations, which shows half the resistance of the above method
in other words you double the resistance value detected by a vesc, then when the motor is stalled you divide the effective voltage (battery voltage * duty cycle %) by the 2 phase resistance to get the motor current.
as back emf v is introduced from a spinning rotor, you increase the effective voltage such that the effective value has a certain value above the increasing back emf for constant motor current, which requires increasing the duty cycle as the rotor accelerates.
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What our Prof said. And to help explain in simple words:
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With the duty cycle, Using PWM or similar, the controller limits how much current is seen by the motor at full throttle based on your settings (which should prevent frying the ESC and motor).
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Current is being alternated on the 3 phases to alternate the magnetic fields, rotating the motor. Thus, just one pathway through the stator is relevant at a time.
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yes, in the scenario (50v battery, 0.05ohm motor), if you simply connect the battery leads directly to 2 of the motor leads, you’ll get ~1000a (do not try)
the vesc uses duty cycle / pwm to lower this to something reasonable, say 60a
without the pwm, the motor becomes more akin to something like a lightbulb or resistive heater.
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Booya! new questions.
So the main (noticeable) thing motor amps effect is the ‘torque’ of the motors? So if I ran 10a to the motors at 12s they would theoretically still be able to achieve the same speeds at the same duty cycles as another man riding 100a 12s? The only difference being their acceleration and hill climbing ability? Similarly, the graph shows an increase in motor amps leads to the decrease in mechanical efficiencies. Is this simply due to heat loss? And what would be the simplest and safest way for a simpleton to setup his motor amps?- …just start with the motors amps slightly above that of the batteries cont. amps you have established and set and then increase by 5-10a increments to find a balanced traditional power setup for your motors?? I am not one to instantly compute seemingly scary and vauge equations that have been posted here… omg so much
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so sorry that devin isn’t here to help. i certainly wish you had someone more knowledgeable than me to answer. in the meantime…
energy is needed to get work done. work is needed to accelerate your board, overcome drivetrain resistance, overcome road resistance, overcome wind drag, gain potential energy (going uphill) and probably other stuff i don’t yet understand
sure, in a vacuum on an idealized board that has no drivetrain resistance, no wheel/road resistance, on level ground.
energy is needed to get work done
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Yeah I guess I never really recognized how amps are one in their own compared to volts. Yes, it’s a good point you make there that you need some baseline capital amps to overcome frictions. I never really thought volts solely determine the top speeds. Thought the combo of da amps and volts made you go faster at top end. Makes things a bit easier to decipher. I’m just tryin to find a good way to setup my motors to keep their cheap Chinese ebay pot metal lookin asses from exploding past 120a. Hopefully the grand master returns soon
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He burned the bridge, scattered the ashes, and peed all over the remnants. Don’t hold your breath there.
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yeah, fukk him!
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Does more amps make more quicker?
Too many explanations with words that mean nothing to my brain.
But seriously…
If I currently have my each motor set to 30A (from the FOC wizard) and each VESC battery setting to 18A, when I go to 30A motor and 30A battery, it’s gonna be way quicker right!?