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

[mutantbass]

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

@ross_forp

…if you switch to a bigger, same kv motor and the resistance drops from 70mOhm to 20mOhm giving 10-12lbs additional thrust at 15mph w/ 30a bat / 80a mot limits per motor (2 motors, 12S, geared for 35-40mph)…

[fessyfoo]

@ross_forp

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

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.

[ross_forp]

motor_rpm/kv=bemf_v

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

(bat_v * duty%) * motor_current = wattage_electrical

[mutantbass]

@Gamer43 @ross_forp

First attempt. How does it look?

ESK8 Acceleration Comparison

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

[Dinnye]

@mutantbass

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

• 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.

[ross_forp]

@mutantbass

except in step 2

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.

[mutantbass]

I have.

@ross_forp

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.

@mutantbass

- available motor amps based on duty cycle

batteryamps/dutycycle

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

[ross_forp]

how did you calculate the duty cycle?

[mutantbass]

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.

[ross_forp]

@mutantbass

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.

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

@ross_forp

motor_rpm/kv=bemf_v

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

(bat_v * duty%) * motor_current = wattage_electrical

[mutantbass]

@ross_forp

motor_rpm/kv=bemf_v

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?

[ross_forp]

3rd paragraph:

Counter-electromotive force

[mutantbass]

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

@ross_forp

motor_rpm/kv=bemf_v

[ross_forp]

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

[mutantbass]

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

Is the way I have calculated acceleration and speed correct?

[ross_forp]

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.

[rusins]

@rusins

how certain are you that a larger motor can have both the same kv, and a lower internal resistance? To me that seems a bit unintuitive – surely as we’d increase the motor size (in length), the winding length would increase, and resistance would go up, no?

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

[ross_forp]

You can have a motor that’s 50% filled with copper and use thicker wire with the same # of turns and you have the same kv with lower resistance.

[mutantbass]

@ross_forp

first you have to calculate the duty cycle, then you have to calculate the motor current, 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.

I have done this. I calculated the max speed beforehand. Then I am looping from 0 to top speed mph in 1 mph increments and recalculating max motor amps at every n+1 mph. So for a 39mph top speed, it is calculating max wheel torque 39 times.

@mutantbass

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

This code is being run 39 times. I am just not sure about these max motor amps

[ross_forp]

Start by showing that you can calculate the motor current at a speed when you’re hitting the battery current limit because that’s harder than calculating the battery current at a particular speed at a particular motor current.