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

How-Tos & Esk8 mechanics Esk8 Mechanics
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#282

how would voltage be added to the equation to figure the foot lbs of torque out of the motor?

i do
kv/1 x amps to get newton meters but voltage isnt invovled.

trying to figure the peak motor torque at 180 amps. the motor is130kv using 58v and will do 7500rpm at no load.

1/130 x 180 is 1.38 newton meters

coincidentally im looking for torque right now…trying to figure what they mean on page T71 with “lbf in” oo figured it.

Technical-Section.pdf (sdp-si.com)
Brushless Motor Kv Constant Explained • LearningRC
Tire Size, RPM, Speed, and Differential Ratio Calculator (advanced-ev.com)

#283

The torque per amp is the inverse of the kv when the kv is stated in radians per second per volt not rpm per volt.

So when the kv is stated in rpm per volt you use:

kt = 60 / (2 * pi * kv)

where kt is Nm/A or newton meters per
motor amp.

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#284

well what makes sense to me is most good ebikes, onewheels and super high performance boards are going 70-80v+. So we will all be able to use similar chargers if most are 75ish

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#285

i assumed a 60v esc limit and surely youre both talking about different escs that can do more.

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#286

I just swapped from belt to gear drive and on one super hard brake test I had a overvoltage, so… yes.

Screenshot_20210823-151751
Screenshot_20210823-151751720×1560 160 KB

16S ubox 140KV 6384 on bn-at 5.25:1

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#287

i get a very high torque if i do it like that.

kt = 60/ (2 x 3.14 x 130kv)
kt = 13.6

? that seems very high for newton meters per amp no?

#288

You you have to do what’s inside the parentheses first. See PEMDAS.

You’ll get 0.07349

#289

i did. i know that much
but i get the same result …almost …converting from rpm to radians a second.

13.61 radians per second is 130 rpm.

so then just 1/13.6 x 180 amps for peak torque
13.222 newton meters …an amp…

#290

What value did you calculate for what’s inside the parentheses and what did you get when you divide 60 by that?

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#291

816.4 / 60…
13.6

#292

no 60 / 816.4 = X

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#293

oo du.
so .073 is the kt.
times 180 amp current and get 13.2 newton meters peak.

#294

i converted 13.2 newton meters and get 8.85 lbf in
Technical-Section.pdf (sdp-si.com)
page T71 what you think they mean for “number of grooves”? just the amount of grooves in the smallest pulley? theyre closed or id call.

#295

So my thought is, just on the pure mechanical side of things, if the same force is being applied at a further away point from the rotation axis, you get more torque.

But its just a lever advantage tradeoff.

Does that still work when you’re actually applying to a motor?

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#296

To get the torque in newton meters per amp of motor current, the only piece of information you need is the kv. To put it another way, all motors that have the same kv have the same torque per motor amp, but not necessarily the same torque per battery amp. Lower resistance same kv motors get more torque per battery amp.

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#297

wheres the voltage in the torque equation?

i always see it as just kt x amps.

#298

Voltage is not in the motor torque equation. Voltage is solely related to top speed, and once you know your top speed then gear ratio and KV is what hooks us back to torque.

Torque at motor shaft = Current * 60 / (KV * 2 * Pi)
Torque at wheel = Current * 60 / (KV * 2 * Pi) * Gear Ratio

The gear ratio and KV (KT) are the key here. A BLDC motor produces the same torque at every RPM (so also at every voltage). Therefore (unless you are battery current limited) your torque also should be the same at every duty cycle %. When designing a board you choose the desired top speed, and as Top speed is inversely related to torque, the higher the top speed is, the lower your torque at the wheel will be for a certain amount of power.

This is just to show the inverse relation of these variables:
Current * Voltage = Power
Torque * Speed = Power

#299

I remember adding voltage and getting more torque.

#300

Where is your trash can?

1 Like
#301

When you added voltage, the same battery current limit as before has increased electrical power. At higher speeds when you were limited by the battery current limit (and not the motor current limit), that increased electrical power at the same battery current as before resulted in more motor current than before (still less than the motor current limit at higher speeds), and the increased motor current was felt as more torque.