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.
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
@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
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.
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
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.
well to begin with, this is what you’ll get full throttle with 60a motor and 30a battery per motor limits (44v battery, 150kv, 145mm tire, 4:1 gearing, 200lbs, 0.05ohm winding, 0.6m^2 frontal area, 0.75 drag coefficient):
I assume the place where the purple line crosses on the 2nd image is the limits for speed at the chosen slope and wind drag. So like you said, around 20mph is the top speed for a 31 degree slope.
Seems like it will fit my requirements quite fine!
if you put three dots on a sheet of graphing paper (2 of them horizontal to each other, and one above another - forming a right triangle), the slope in % is rise / run
https://en.m.wikipedia.org/wiki/File:PWM_duty_cycle_with_label.gif
