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

[mutantbass]

Tbh my understanding of electricity is pretty basic. I do Know one rule of life though. There is no free lunch.

[rusins]

That one’s also complicated

No free lunch theorem

[mutantbass]

I am a simple man. Leave me alone

[Madmiah]

Pardon my naivety I’m a car and … guy before I found eskate.

Wouldn’t adding stickers that say things like GT or DRI etc etc add more Nm of torque at the wheels?

[Dinnye]

@rusins

The main reason TO go high voltage is if your ESC is the bottleneck.

An easy fix would be better cooling. The problem with higher voltage ESCs, is the higher RDSon of the mosfets - so unless the ESC has more mosfets to distribute the current draw, the power actually doesn’t really increase, or even if it does, not by the amount you expected it to! Remember…

@mutantbass

There is no free lunch.

[fessyfoo]

that moment i understood the dodging ESC/Motor current limits with higher voltage.

Battery amps vs motor amps vs voltage

I haven’t been able to stop thinking about this. pretty excited to understand it better. I did another version of the power curve comparator on desmos. parameterized it in terms of NsMp cell counts. and added a “gearing” component. In the default comparison, 10s6p (blue) and 12s5p (red) the power curves are the same. Red has more top speed but the torque/amps continues to drop as the rpm/volts (dutycycle * volts) goes down. keeping max power the same. (!!!) bingo light going offf for…

[mutantbass]

Any sticker in Red will do.

@Madmiah

Wouldn’t adding stickers that say things like GT or DRI etc etc add more Nm of torque at the wheels?

[ross_forp]

@mutantbass

Whats the difference between these two systems?

18s

40 battery amps total.
20 motor amps each

12s
60 battery amps total
30 motor amps each

Assume everything the same except gearing is 50% higher on 12s.

The thing with both of these setups is, performance wise, they will be the same. Each set of motors is putting out the same mechanical power either way for any set of riding conditions, whether under acceleration or constant speed. But the 18s set of motors with less motor current will have lower I^2R losses for any identical performance condition. When you add up the I^2R losses and the mechanical output, it approximates the battery wattage. So essentially with the 18S option here, the same mechanical power is coming out of the motor, but less electrical power is going into the motor, because less I^2R losses are coming out of the motor in the form of heat.

So essentially these 2 setups give the same performance, but not the same electrical wattage for identical performance. The 18S draws less wattage from the battery for identical performance.

But while we’re on the subject, there’s no good reason to lower the motor current limit on the 18S option, so essentially, lowering the motor current limit is pointless, and when you don’t do that, the 18S option gives you 50% more torque if you gear for the same top speed.

Also I think you meant gear ratio is 50% higher on the 18S (not the 12S) for same top speed.

[Titoxd1000]

I don’t think anyone is running 190kv at 18s and spinning up the full RPM allowed. Usually people are going down in the 120-140kv range. Lower KV motors handle less amps.

[mutantbass]

@ross_forp

But the 18s set of motors with less motor current will have lower I^2R losses for any identical performance condition. When you add up the I^2R losses and the mechanical output, it approximates the battery wattage. So essentially with the 18S option here, the same mechanical power is coming out of the motor, but less electrical power is going into the motor, because less I^2R losses are coming out of the motor in the form of heat.

We talked about this earlier. The higher mosfet RDSon and lower KV motors having higher internal resistance would offset the 18s advantage.

@ross_forp

The 18S draws less wattage from the battery for identical performance.

This is just a claim that I have yet to see justified.

@ross_forp

But while we’re on the subject, there’s no good reason to lower the motor current limit on the 18S option, so essentially, lowering the motor current limit is pointless, and when you don’t do that, the 18S option gives you 50% more torque if you gear for the same top speed.

Umm there is. Those extra amps do not exist on the 18s setup. Your P count is lower.

@ross_forp

Also I think you meant gear ratio is 50% higher on the 18S (not the 12S) for same top speed.

@mutantbass

Assume everything the same except gearing is 50% higher on 12s.

I said gearing higher, not higher gearing ratio. Gearing higher on 12s is saying the same thing as higher gear ratio on 18s. lol

[Gamer43]

@mutantbass

We talked about this earlier. The higher mosfet RDSon and lower KV motors having higher internal resistance would offset the 18s advantage.

Resistance in the motor goes up by a ratio of 18/12 = 3/2 if we adjust the Kv. (same thing applies to the battery)
Current goes down by a ratio of 12/18 = 2/3

(2/3) ^ 2 * 3/2 = 2/3.

Actually, if the Kv is the same. The resistance stays the same so in reality it’s 4/9 if you just change the gearing.

The problem is you need a multistage reduction at that point.

[mutantbass]

@Gamer43

The problem is you need a multistage reduction at that point.

Bingo. That is going to introduce it’s own inefficiencies.

I have gone from 6s to 10s to 12s. Zero difference when setup the same way. Only reason I run 12s is that my board is a travel lipo setup and 12 goes very nicely into 2/3/4/6.

[rusins]

@ross_forp

But while we’re on the subject, there’s no good reason to lower the motor current limit on the 18S option, so essentially, lowering the motor current limit is pointless, and when you don’t do that, the 18S option gives you 50% more torque if you gear for the same top speed.

Did you see the beautiful graphs @fessyfoo shared? They are excellent at highlighting how any extra speed is with less torque at the high end, or any extra torque on the low end ends sooner than usual.

And I will argue that you can achieve the same effects remaining on 12s by going higher kv, or higher kv with a larger gear ratio, as long as you can still increase motor current in your ESC.

[ross_forp]

@mutantbass

Umm there is. Those extra amps do not exist on the 18s setup. Your P count is lower.

This is where your misunderstanding is. You can still get 30a motor amps through both motors at the same time that you are not exceeding 20a battery amps per motor (40a battery combined.)

@mutantbass

Whats the difference between these two systems?

18s

40 battery amps total.
20 motor amps each

12s
60 battery amps total
30 motor amps each

Assume everything the same except gearing is 50% higher on 12s.

[Gamer43]

additional reduction stages don’t really reduce efficiency as much as 2/3, but they don’t fit on skateboards

Man, it hurts seeing all the wrong shit FRC teaches kids about motors make a simple question turn into this 100 reply long thread.

[mutantbass]

@ross_forp

This is where your misunderstanding is. You can still get 30a motor amps through both motors at the same time that you are not exceeding 20a battery amps per motor (40a battery combined.)

But on a 12s setup, you will always have 50% more amps available. I don’t see what your point is. Its like saying you are faster than a ferrari as long as no one is driving the ferrari. LOL

[mutantbass]

@Gamer43

additional reduction stages don’t really reduce efficiency as much as 2/3, but they don’t fit on skateboards

I think the broader point is that saying 18s > 12s isn’t really true. We can talk about a little give and take here and there but for esk8 atleast, 10s,12s,18s doesn’t really matter as long as you know what you are doing.

[ross_forp]

@mutantbass

Whats the difference between these two systems?

18s

40 battery amps total.
20 motor amps each

12s
60 battery amps total
30 motor amps each

Assume everything the same except gearing is 50% higher on 12s.

There is no good reason to lower the motor amps to 20a motor amps each on the 18s setup. You can draw 40a battery amps total (20a battery per motor) while at the same time putting 30a motor current through each of the 2 motors.

So please explain any rationale for reducing the motor current limit from 30a per motor to 20a per motor on the 18S setup. You’re giving up torque by doing it, but for no rational reason.

[mutantbass]

@ross_forp

There is no good reason to lower the motor amps to 20a motor amps each on the 18s setup. You can draw 40a battery amps total (20a battery per motor) while at the same time putting 30a motor current through each of the 2 motors.

So please explain any rational for reducing the motor current limit from 30a per motor to 20a per motor on the 18S setup. You’re giving up torque by doing it, but for no rational reason.

What I am saying is that you have 50% extra amps on the battery side with a 12s battery.

lets says 36 samsung 30q cells

12s3p = 60 amps battery max
18s2p = 40 amps battery max

Lets assume 100% efficiency. You can only draw 20a per motor at max speed on the 18s setup. On the 12s setup, you can draw 30a per motor at max speed.

My point is, you will always have 50% extra amps available through your system on a 12s setup at any given speed/throttle position.

@ross_forp

There is no good reason to lower the motor amps to 20a motor amps each on the 18s setup.

There is also no reason to not use the 50% extra amps you have available on the 12 setup.

[ross_forp]

Yes, I understand your rationale for lowering the battery amp limit on the 18 S set up, but there is no logical reason to lower the motor current limit.