Where did you pull this value from? I mentioned no battery limit setting in my case.
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Merely to illustrate how a battery current limit affects the motor current limit.
If there was no battery current limit, the motor current will be a straight line across and the battery current will be a flat ramp that almost meets up with the motor current line at 95% duty cycle.
Another interesting part of the graph is where the red line crosses above the yellow line in the top left chart. Thats no load speed where the back emf voltage produced by the motor starts to exceed the max effective voltage at 95% duty cycle. The motor can’t go any faster than this rpm under its own power.
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Ok thank you for clarifying. At a 10s5p 30q pack, I have my my max battery current set to 95A but have always worried about motor max constraints
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In some versions of vesc the firmware limits you to 120a motor and 60a battery per motor, even if you input higher values.
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Im glad there are more peeps out there who have benefited from this conversation. Its a question i’ve wanted to ask for ages.
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@Pedrodemio Do you mind validating the definition i used on the Glossary of Terms page?I cited this thread post but want to ensure its accurate.
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Sure, The duty cycle one right?
that definition is not correct. in bldc the duty cycle is the percentage of time that current is taken from the battery
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for example with a 50v battery, 60a motor current, 30a battery current at 50% duty cycle, it means 60a @ 50v is being taken from the battery half the time, which averages to 30a @ 50v, while the motor sees 60a @ 25v the whole time, assuming constant speed from the load. the motor is shorted to itself during the times the battery is not supplying current, and due to the motor’s inductance and short time scales of the switching cycles, the current in the motor changes very little during the on-off current cycles the battery sees. very small changes in duty cycle result in very large changes in motor current.
https://en.m.wikipedia.org/wiki/File:PWM_duty_cycle_with_label.gif
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suppose a 190kv motor is turning 2000rpm with a 0.05ohm winding
the battery is 50v
2000rpm / 190kv = 10.5263v bemf
(11.5263v pwm voltage - 10.5263v bemf) / 0.05ohm = 20a motor current
50v battery * 23.05% duty = 11.5263v pwm voltage
20a motor current * 11.5263v pwm = 230.526w
230.526w / 50v battery = 4.61a battery current
4.61a battery / 20a motor = 23.05% duty
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Although I have read everything I’m still hugely lost, I’ve been trying to understand this stuff for a long time :p… I will read again I guess.
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Thank you. corrected the definition and quoted your example as a detail.
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I would like to add with PWM the voltage is quoted are averaged there still the full battery voltage there at the on time of the ripple
What I don’t understand is how the VESC limits the ampage. As a total guess I presume it adds resitanse sone how but I’m not sure
if the motor is clamped (no rotation and therefore no bemf voltage), the current is given simply by:
pwm_voltage / ohms_resistance = motor current
or:
1v / 0.05ohm = 20a motor current
1v pwm voltage * 20a motor current = 20w
if the battery is 50v, then:
20w / 50v battery = 0.4a battery current
^so for 60a motor current, the pwm voltage has to stay 3v above the bemf voltage as the motor spins faster, which requires increasing the duty cycle as the motor spins faster to maintain the same constant motor current
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@professor_shartsis in the build I’m planning I’m doing 12s4p 30Q. While 30Q can do 20A I’ll be limited to 60 battery amps due to planned discharge BMS.
With this split across 2 motors for 30 battery amp per battery. With the voltage and gearing, the expected speed is 40km/h. But with a goal to in most cases run 20km/h that would mean that I can run it at 50% duty cycle and therefore the motor amps are 60Ax2.
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If this is correct reasoning, wouldn’t it mean that even with only 60A it would be possible to get accross most places at 20km/h without issue?
Not really, the motor is in part a inductor, so it prevents the voltage from rising fast, since the switching frequency is so high, it never gets to the battery voltage, only when running at full duty cycle
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it’s better to think in terms of motor current than duty cycle. (throttle controls motor current, with 100% throttle equating to the motor current limit unless that would exceed the battery current limit)
constant motor current requires a different duty cycle at every different speed.
if your goal is constant 20kmh, i’d imagine it will only take less than 10a motor current to sustain, so having 60a motor current available is more about giving you the acceleration you want. also on hills it will take much more motor current to sustain 20kmh than on flat ground.
basically you use much less motor current maintaining a constant speed than you use while accelerating.
but if you want to think in terms of duty cycle, then very small changes in duty cycle at any given speed result in quite large changes in motor current.
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Thank you!
Do you by any chance have a formula for theoretical calculation of the amount of amps needed for a hill climb at different grades?
it depends, what’s your battery voltage, battery current limit, motor current limit, kv, # of motors, tire diameter and gear ratio and desired hill climbing mph
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50.4v max and 43.2v nominal, 30A battery current limit per motor, something like 60-80A motor current limit per motor, 150kv, 2 motors, 145mm tire diameter and 4:1 ratio. Total weight with rider + board: 83kg.
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