Brushless Motor questions

[SharkTeethWizard]

Sorry if this has been asked but my search function isn't working

I'm familiar with brushed, and fuel fed motors/engines, but I've only recently been looking into brushless upgrades for my older brushed RC car.

So KV, would a 3200kv Motor be faster than a 5800kv motor, but have less torque? (Or something like that)

And what does "4-pole" mean?

Help is very appreciated,
Joe

 

[rockyboy]

The kV rating is a measure of RPM per volt supplied to the motor - so if you're using the same battery, a 5800kv motor will be significantly faster than a 3200kv. Torque isn't really addressed by kV - or really in any of the standard measurements and ratings with brushless motors typically used in planes or multirotors. A bigger stator size generally aligns with a higher torque rating, which is often also found with lower kV but that's a correlation, not a causation.

For the pole question, I'm used to seeing the number of poles frequently relating to the kV rating, especially in the same brand/size of motor, but I'm not sure if it really has any importance for the modeler to have more or fewer poles.

 

[SharkTeethWizard]

So a bigger kv Motor is faster. Got it.
I appreciate your help.

 

[Konrad]

So a bigger kv Motor is faster. Got it.
I appreciate your help.

Nope. The larger the Kv number the more "RPM" the shaft will turn per volt.

This is a lot different from speed or velocity, Kv pertains only to RPM per volt.

This might help.
https://en.wikipedia.org/wiki/Motor_constants

The pole count helps describe what the torque curve looks like. It is also an important number in determining how often the ESC must perform the commutation function. All ESC have this "electrical rpm" limit.

 

[rockyboy]

From a basic user's perspective, when using the same battery, bigger kV is going to be a faster motor.

The nuance Konrad is pointing out (I think) is a lower kV motor on a 4 cell battery could still be faster than a higher kV motor on a 2 cell battery.

 

[Konrad]

From a basic user's perspective, when using the same battery, bigger kV is going to be a faster motor.
...

Faster as in moving the vehicle (Car)? No, that is making the assumption that there is enough torque Kt to move the vehicle.

Kt is the amount of torque a motor has per amp.

All things being equal in a motor, the Kv and Kt are in an inverse relationship to each other. So for a motor of the same size and construction the one with twice the Kv will have half the Kt.

Or in practical terms the high Kv motor will sit on the start line burning up in the stalled condition!

All the best,
Konrad

 

[SharkTeethWizard]

The car weighs a hair under a pound, with some extra stuff to be removed, any recommendations?
I was looking at a 7200kv Motor, on 2s 1500mah LiPo. No clue on gearing yet.
I'm not looking to win races (yet) I just want to go to the local track and troll the new traxxas vehicles with my 10 year old RC car (lol)
I would still like enough torque (or just right gearing) to rocket-ship off the line, but not wheelie.

Also another question, if you have a LiPo and a NiMH that are physically the same size, would the LiPo be lighter?

Thanks for the help so far,
Joe

 

[Konrad]

If weight is part of your size criteria then no. But for the same energy output/storage Lipo generally have 3 times the energy density.

Remember that to feed a more powerful set up requirers a larger energy source, measured in watt hours, well if you want to finish the race.

As I don't know what a traxxas is I can't help your there.

All the best,
Konrad

 

[SharkTeethWizard]

Just ordered a 7200kv Motor/ESC combo and a 2s 2500mah LiPo.
Took off unecessary parts and lightened it a bit, now the roller weighs 367 grams.
In a top speed calculator with my Motor and gearing, it says I should be able to get up to 63.7 MPH... all in a 1/18 car. Lol
I'll be able to dominate the novice class at my local track.

 

[ElectriSean]

Just ordered a 7200kv Motor/ESC combo and a 2s 2500mah LiPo.
Took off unecessary parts and lightened it a bit, now the roller weighs 367 grams.
In a top speed calculator with my Motor and gearing, it says I should be able to get up to 63.7 MPH... all in a 1/18 car. Lol
I'll be able to dominate the novice class at my local track.

Assuming you can keep the shiny side up

 

[SharkTeethWizard]

Hahahahaha. Good point!

 

[Hai-Lee]

Just something to consider! Brushed motors provide a great deal or torque at zero motor speed but a brushless motor with an ESC will stall, or stutter, if the motor load is too great at zero RPM.

For best performance I would suggest a clutch mechanism to allow the motor to develop some rotational speed before the motor is heavily loaded perhaps a centrifugal type clutch which are available for the IC RC racing cars.

Have fun!