The relevance of motor kv with gear ratios taken into consideration?

[Evwan]

the watt rating is a recommendation of how much power you should run through the motor, but that means nothing. 3000w at low RPM is insane, while 3000w at high rpm is just fine.

You arent running 3000w at 1rpm, and if you were, that would be impossible is my point. Factors like acceleration depend on your phase current, gear ratio, wheel size, and motor kv. not the rated wattage of the motors.

I’m not good at explaining this stuff. Maybe someone else can do a better job.

[MysticalDork]

@Audiobahn1000

wouldent a motor with a higher watt rating offer superior acceleration and hill climbing ability throughout its speed range than a lower watt rated motor on the same board?

This is true only if you have a battery and ESC cant can support the larger motor. Putting a giant motor on a board with an anemic battery or low-spec ESC will get you exactly zero benefit.

Also, as Evwan said above, the watt rating is a product of the current rating, not the other way around. Both the current and wattage ratings are more like suggestions, and should be taken with a grain of salt. Depending on your cooling and application, you may or may not be able to run the motors to those ratings, or beyond.

[b264]

@Audiobahn1000

Say we have two identical builds differing only by motor and gearing, one with a 140kv motor and one with a 190. Say that we changed the gearing on one of the builds so that both builds have a top speed of 30 MPH. What would be the characteristic differences between the two at that a point? Which would be more electrically efficient? Which would have more torque through the acceleration band, ect? Since we can chose what top speed we want just by gearing, what does it really matter what the kv value is of the motor?

The 190Kv is going to be better. It will burn less energy off as heat in the stator because it’s got a lower DC resistance.

It will also have better sensorless startup because it will be geared higher.

[Audiobahn1000]

Blockquote

Ok so say we DO have an ESC and a battery that can handle any motor we throw on the board. Then what would be the advantages/ disadvantages of using a high power motor over a lower one? Like, would a board have better range if it had dual 2000w motors vs dual 5000w motors or is there no disadvantage at all besides price and weight to using larger motors?

[tech.shit]

bigger motors have more torque and can make use of more current without overheating, if the current is there to make use of

[b264]

@tech.shit

bigger motors have more torque

While I have experienced this in person, this is a controversial topic which has its own thread even.

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

Today It was suggested to me that people who upgrade just their motors to larger motors get more torque with the same motor amp settings. backed up with experiences of both bikes and esk8s doing this and feeling more torque. My understanding is that motor torque is proportional to motor amps x Kt. (torque/amp) and that Kt is inverse of Kv. (it’s 1/Kv) so a larger motor of the same kv maybe able to handle more motor amps, but with the same motor current it’s torque should be the same. A…

It’s fair to say not everyone agrees with that.

[MysticalDork]

Generally, going for a really massively overkill motor on a lower-average-power build (like dual 80100s on a street board, for example) will lose you some efficiency, because of the extra friction, eddy, and hysteresis losses. It’s best to match the motor more closely with the intended application.

Not to mention higher cost and weight.

I’m not using wattage numbers, because those are irrelevant. But bigger, beefier motors, that can handle higher current, are worse at low load.

[Evwan]

@b264

While I have experienced this in person, this is a controversial topic which has its own thread even.

I really wanna make a scientific test with this. Maybe a motor dyno or something

@Audiobahn1000

Ok so say we DO have an ESC and a battery that can handle any motor we throw on the board. Then what would be the advantages/ disadvantages of using a high power motor over a lower one? Like, would a board have better range if it had dual 2000w motors vs dual 5000w motors or is there no disadvantage at all besides price and weight to using larger motors?

Once again, that wattage number means nothing. If you’re running a motor on 20s, it’s going to have twice the wattage of a 10s setup at the same duty cycle % and same current. If you have a setup that can dump a lot of current into a motor, the larger motor will be better as it should have less resistance and it would have more thermal mass to deal with the extra heat of increasing current.

[poastoast]

@ShutterShock come do your dyno thing

[ShutterShock]

Ughhh

It’s on a list somewhere haha

Also there already is a motor dyno thread

[Evwan]

I just dont see why a larger motor would have more torque. If it’s more efficient, the actual number would just be negligible and it shouldn’t be more noticeable.

[MysticalDork]

@Evwan

I just dont see why a larger motor would have more torque.

For the same motor current, at the same KV, it wouldn’t.

A larger motor will have more torque capability, because you could push it harder before it saturates or otherwise reaches its limit though.

[ross_forp]

Up to somewhere between 6-10k rpm, higher kv, same size, same no load speed factoring gearing will run cooler on account of the lower copper losses but at some point spinning the motor faster and faster will cause the iron losses to exceed the copper losses, defeating the efficiency advantage of spinning faster, but you’ll have to measure it experimentally to be sure what that point is.

[Evwan]

Well, yeah, that’s what I’m saying. FWIW, in most cases a decent 6374 motor will max out the current capabilities any esc, especially if the rider isn’t a fatass.

[ross_forp]

@Evwan

I just dont see why a larger motor would have more torque. If it’s more efficient, the actual number would just be negligible and it shouldn’t be more noticeable.

It’s covered in a different thread, but the lower resistance same kv motor has the same torque for the same motor current, but the lower resistance one has more motor current for the same battery current, so the larger motor gets more torque at the same speed for the same battery current.

[Evwan]

Isn’t the difference in resistance almost negligible unless you’re going from something small to something really big?

#derail

[MysticalDork]

Exactly. I wouldn’t expect the difference between a 6355 and a 6374 to be more than a handful of percent under most conditions.

I could totally be talking out my ass though, it happens.

[ShutterShock]

@MysticalDork

Exactly. I wouldn’t expect the difference between a 6355 and a 6374 to be more than a handful of percent under most conditions.

However another consideration is that larger motors take longer to get heat soaked - not a problem for some people, but definitely a problem for larger riders or hilly areas

With motors overheating, braking gets reduced too.

[ross_forp]

@Evwan

Isn’t the difference in resistance almost negligible unless you’re going from something small to something really big?

#derail

I think it could be noticeable…

@ross_forp

But to summarize the answer to the OP’s question, 10-12lbs additional thrust @ 15mph is plausible by dropping the motor resistance from 70mOhm to 20mOhm with a larger, same kv motor and changing nothing else. I assumed 2 motors, 12s, geared for about 40mph, with 30a bat and 80a motor limits per motor. At 15mph, they’d be hitting the battery current limit and not the motor current limit, and essentially the lower resistance motor has more motor current at that speed than the higher resistance one on account of lower I^2R losses (both have less than 80a motor current available at that speed on account of the 30a battery limit, but the lower resistance motor’s motor current is higher).

[Evwan]

70mohm to 20? That’s a huge difference.