Do bigger motors deliver more torque at the same motor current?

How-Tos & Esk8 mechanics Esk8 Mechanics
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#219

hmm. I’m almost there…

#220

the last equation is wrong. He corrected it later and you divide it by the radius.

#221

It might be… but regardless if you lower only the resistance with your calc, you get more torque, and the reason you are surprised by the amount is you are calculating for accelerating from a stop sign, where the effect would be hidden by the motor current limit. The additional thrust afforded by lower resistance decreases with additional speed, so by the time you get to 15mph where you are limited by the battery current setting and can feel the effect, it is a bit diminished from the case of accelerating from standstill.

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#222

How did you do your 15mph calculation earlier? I would be fascinated to know the difference between 46 vs 24 mOhm

#223

how do you calculate motor current at a given duty cycle?

I was using max_battery_amps / duty_cycle but that doesn’t factor in winding resistance.

#224

motor_rpm/kv=bemf_v

((bat_v * duty%) - bemf_v) / winding_resistance = motor_current

(bat_v * duty%) * motor_current = wattage_electrical

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Calculating batt current?
#225

@Gamer43 @ross_forp

First attempt. How does it look?

Let me tell you what I have done and I have a question at the end.

All I have done is taken Gamers formulas

  • Max power = battery voltage * battery current
  • Max motor amps = sqrt(Max power / winding resistance)
  • Max torque at motor shaft output = max motor amps * motor torque constant
  • motor torque constant = 60 / (2 π * Kv)
  • Max torque at wheels = gear reduction * max torque at shaft output / wheel radius (IN METERS)

except in step 2 for Max motor amps, I have taken the lowest of:

  • Max motor amps = sqrt(Max power / winding resistance) OR
  • motor amps in vesc settings OR
  • available motor amps based on duty cycle

to get max Torque at wheels. My question at the end is related to this.

Then, I get the acceleration for any given mass by

  • acceleration = force/mass

I assume force = max torque at wheels we just calculated. Can you confirm?

I already know the top speed of the boards using motorkv/gearing etc etc, so I am calculating time T to go from N to N+1 mph, starting at 0, using

  • Time = ((N+1) -N)/acceleration = 1/acceleration

and plotting the chart.

My question is, do I need to factor in motor IR for the last two options here, where I am calculating the max motor amps available?

Lowest of
- Max motor amps = sqrt(Max power / winding resistance) OR
- motor amps in vesc settings OR
- available motor amps based on duty cycle

Thanks :+1:

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#226
  • Torque = Force * Length of lever arm (which is I think the wheel radius here)

so

  • Force = Torque at the wheel / Wheel radius

That is if I am not mistaken.

Keep these all in SI, so use meters for the wheel radius.

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#227

you haven’t factored the battery current limit. you’ll hit that at a certain speed and then youll have a different motor current, which is less than the motor current limit, at every speed. see my previous post.

#228

I have.

batteryamps/dutycycle

I want to know how to factor in IR for the other two options I mentioned above. Any ideas?

#229

how did you calculate the duty cycle?

#230

dutycycle = speed/maxspeed

So, if max speed is 50mph, and current speed is 25mph, duty cycle is .5

I should probably multiply it by 0.95 but I will deal with those minor details later. The IR is throwing me off.

#231

Sorry but that’s completely wrong, you’ll have to do some algebra to rearrange the middle equation:

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#232

I saw this earlier. Isn’t

motor_rpm = motor voltage x motor kv

so

motor_rpm/kv=bemf_v

is

motor voltage x motor kv/motor kv = bemf_v

so

motor voltage = bemf_v

Is this correct?

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#233

3rd paragraph:

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#234

I understand it conceptually. How do I get the motor_rpm for this equation?

#235

It’s the speed the motor is turning at the ground speed considered, factoring the wheel size and gear ratio.

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#236

:joy: More confusion.Ok, I will think more about this later.

Is the way I have calculated acceleration and speed correct?

#237

first you have to calculate the duty cycle for the desired motor current at the desired speed, then you have to make sure the motor current at the speed you’ve calculated hasn’t exceeded the battery current limit, because if it has, you’ll have to calculated everything again to find out what the motor current is at the battery current limit at that speed.

#238

Reposting because I still feel dumb for not understanding this :sweat_smile: But the data clearly backs up that bigger → lower resistance

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