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

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?

1 Like

In general, for the same motor construction/shape/size/whatever, to change the KV, the number of windings (turns of wire) around each stator tooth must be changed. More turns lowers the KV.

Since there’s only a finite amount of space in the stator for wire, adding turns usually means that each turn has to be thinner, and more turns means more length as well. Since longer, thinner wires have more resistance than short, thick ones, generally the lower the KV of the motor, the more electrical resistance it will have.

Higher resistance means more ohmic losses, so more heat generated for a given drive current.

The other side of things is eddy current and hysteresis losses, caused by interactions of the magnetic fields in the motor with the motor internals. In general, these get worse the faster the motor spins.

This is one of the reasons we don’t just shoot for ridiculously high KV motors - that and our ESCs can only keep up so fast before they begin to lose sync or get damaged. Also trying to gear a 15K RPM motor down to something appropriate for a skateboard wheel has its own set of challenges and efficiency loss.

tldr, higher KV will generally be better for ohmic (copper) losses, but lower KV will generally have lower magnetic (iron) losses. It shouldn’t make a huge difference between reasonably similar motors, but it may be noticeable.

2 Likes

you beat me to it, but this is a much more detailed response :rofl:

There’s certainly a sweetspot, and outside of the electrical aspects, running 300kv at high voltage will destroy your eardrums.

I’m gonna run 190kv motors on 16s. This will be loud, but since those motors will have less resistance, I expect it to be more efficient than a similarly geared 140kv setup (I’m going to be using 6.5:1 gear drives)

1 Like

You didn’t see anything <.<

1 Like

yeah I have no clue what you’re talking about. what’s there to see?

1 Like

it’s loud? :eyes:

The faster it spins, the louder it’ll be in general. Especially if it isn’t perfectly balanced.

good to know :eyes::eyes:

to continue derailing this, @jack.luis rocinante on 20s and 140kv motors is deafening.

i send you message :wink:

1 Like

What about motor watt ratings? Money and weight aside, should one always just use 5000W motors on every build regardless of intent to use that much power or is there a distinct advantage in using smaller motors?

motor watt raitings mean nothing

if I shove 3000w through a 6384 100kv motor at like 100rpm, it’s going to fucking explode. That’s 3000 amps if my monkey math is right

Rated current is a better metric to go by. That’s what’s going to generate the heat.

2 Likes

up to a point* i think

i dont know math, but 3000A doesn’t sound right

100rpm on a 100kv motor should mean 1 volt? or is my monkey math wrong

Well as the W rating goes up so does the A rating. So isint the watt rating of a motor correspond to how much HP it can produce? All things equal, 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?

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.

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.

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.

1 Like

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?

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