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

How-Tos & Esk8 mechanics Esk8 Mechanics
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#121

If you go full throttle from a fullstop, both boards have motor amps at 10000000 amps. What will happen? Which board will accelerate faster?

#122

The firmware will limit the motor current to 120a, and the battery current will be far less. Battery current less on the 18S setup than the 12S.

#123

You didn’t answer the question :slight_smile: Which board will get to the topspeed first?

#124

If they are geared for the same top speed, the 18S will accelerate faster, otherwise they will accelerate at the same speed.

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#125

How much faster?

What do you mean?

#126

The 18s will have a 50% higher gear ratio to get the same top speed, the motor torque will be the same for both, but the wheel torque will be 50% higher on the 18S on account of the different gearing.

If there isn’t different gearing, the 18S will go 50% faster.

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#127

The 12s has more motor torque.

#128

Another thought experiment. Put both boards up against a wall and go full throttle. Assume wheels grip and don’t slip. Which board is putting more force into the wall? What are the vesc battery current and motor current readings?

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#129

That’s easy

One don’t use vesc

two, multiply motor amps by Kt by gear ratio

Kt = 60 / (2 π * Kv)

If they both have same motor amps, then the 18s pack will have a lower battery current.

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#130

They won’t though. 12s can do more motor amps.

#131

They do the same.

the ESC acts as a buck converter.

4 Likes
#132

My bad. I misread your initial post.

12s3p set to 60 battery amps, 18s2p set to 40 battery amps. Motor amps on both set to 10000000 amps. Which board is putting more force into the wall?

If you dont like the wall experiment, lets say both are pulling a 1 ton truck so they get moving very slowly. Which accelerates faster? Setup for same top speed by changing gearing/motor kv.

#133

Same kv?? same gearing???

#134

We’ve been waiting for years for you to show us a better alternative, and yet, here we are.

Maybe vesc isn’t so bad

3 Likes
#135

Read my edit. Thanks :slight_smile:

#136

Assuming the motors are the same kv and the only thing you changed is the gearing…

The 18s with the greater gear reduction will accelerate faster.

Both packs are capable of spitting out the same motor amps at stall.

The numbers get fuzzy when you’re at or near top speed because the motor on the 18s system spins faster.

The reason for this is because the motor operates in constant torque mode and requires x amount of watts to generate y amount of shaft torque, and doesn’t give two shits what the actual battery terminal voltage is. If you use a greater reduction, you get more torque at the output for the same amount of watts put in.

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#137

Why? the 12s system has more motor amps available at any given speed.

I understand this. I am struggling to understand where this extra power/torque is coming from when a 12s3p and a 18s2p battery has the exact same maximum output wattage.

#138

No, they have the same amount available because the ESC acts as a buck converter.
If for a given motor Kv, the motor requires 1s worth of voltage to reach saturation point, lets say 720 amps,
then the 12s pack will deliver 12 * 60 = 720 amps
and the 18s pack will deliver 18 * 40 = 720 amps
into the motor.

There is no mechanical power being generated, you’re at stall condition. You’re dumping the same amount of power into the motor in both cases and getting zero mechanical power out of it; it’s all getting burned up in the winding resistance.

You’re looking at the wrong part of the motor performance graph.

Now let’s say the load IS moving. Why do we get more mechanical power out of the 18s system? Well, the motor is spinning 50% faster! It’s at a more efficient operating point!

You’re not getting more than 2400W mechanical out of either system, but the 18S system will be more efficient over the operating range since the motor spins faster. This extra efficiency translates to additional mechanical power.

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#139

Two setups geared for the same topspeed. 39mph

18s setup

18s2p samsung 30q
140kv motor
100mm wheels
15/36 gearing

battery amps = 40a total. motor amps are setup to 10000000amps.

12s setup

12s3p samsung 30q
190kv motor
100mm wheels
15/32 gearing

battery amps = 60a total. motor amps are setup to 10000000amps.

You are at a standstill and floor your throttle. All else being equal, which will accelerate faster and get to the topspeed first?

#140

What is the winding resistance of the motors?

You’re changing two independent variables here, both Kv and gearing, so the systems are difficult to fairly compare against each other.

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