KV…. It just doesn’t make sense! 😅

[Robii]

Hi Guys!

Ok so this is a question I’ve had for a while.

I’ve owned a typhon 3s and a 6s, the 6s (2050kv) had a lower KV than the 3 (3200kv) but everyone tells me the higher the KV the more power you get, obviously not the case between these cars.

I’m sure there’s a ratio you can follow somewhere that makes it all make a lot more sense but I haven’t found one yet - can anyone help!?

Also I’ve got my 3s running on 4s and I’m loving it, might go for 6s but will I need a new motor!? (added a pic of my current motor just incase)

 

[icephase]

3200KV on 6s = about 70.000 rpm... that's way too much for that motor!

 

[Robii]

Thanks @icephase - but how did you know that and how do you know that 70.000rpm is to much for the motor?

Is there a chart I can follow or something, that’s what I’m looking for really!

 

[Yonic]

Power is deducted not really by the KV, but by the size (or weight) of the motor (assuming you compare motors who built the same).

For a specific motor size/design, higher KV will draw more power. consider KV as an internal motor "gearing"

Thanks @icephase - but how did you know that and how do you know that 70.000rpm is to much for the motor?

Motors are generally built for 50,000rpm max. so 4s on that motor (3200 * 16) is pushing it

 

[aahwhatever2]

The KV of a motor is the revolutions at a certain input power. As you noticed, the higher the voltage, the lower the kv is used.

Let’s just say you have a 3200KV motor that you use 3S on…. yes, you can get away with running 4S, but you will be pushing higher speeds and higher speeds mean higher temps… So it is easier to over heat a system if you just increase voltage. So, to upgrade, you would want a lower kv motor and in turn can run higher voltage without over heating the system….

Now, there is WAY more to it then just that. You also have to consider your gear ratio, your ESC (if it can handle the higher voltage) and what batteries you use! If you want a better ESC/MOTOR, you will have to pair it with a battery that can deliver the amps that this new system will require!! And don’t forget, you have to chose the right gear ratio to match what you want to put down AND all the drive components from the motor to the wheel!!!

It is one of the BIGGEST rabbit holes you can go down and the amount of money…..shoot, sometimes it is cheaper to buy another RC that is setup for what you want to do with it. This way you are getting the WHOLE RC setup properly to run the electronics you are wanting. This is why Arrma made the EXB roller…then all you do is pick the electronics that you want to run on it!!

 

[icephase]

You DO know that KV is the rpm of the motor per Volt right? The higher the KV, the lower the torque. Less torque = hotter engine when you gear up. There is no chart, it depends on the size of the motor, the size and weight of the car and of course the gearing what works and doesn't work.

 

[Robii]

Ok @Yonic that makes a lot of sense than just: ‘more KV = more power’

I still think there must be some sort of graph out there though. Maybe one that includes esc’s to…. Even if it’s one per manufacturer Spectrum/Castle etc

 

[icephase]

This is as close as you'll get to a graph: https://www.sodialed.com/utilities/rc-gear-ratio-calculator
You can calculate the theoretical speed with selected gears, kv, cells etc. but it won't predict the motor temps. That is simple impossible to predict.

 

[GraniteFox]

3200 motor on 6s =

 

[rotauq]

Ok @Yonic that makes a lot of sense than just: ‘more KV = more power’

I still think there must be some sort of graph out there though. Maybe one that includes esc’s to…. Even if it’s one per manufacturer Spectrum/Castle etc

A chart would also have to include vehicle and motor weight/scale. A big 56mm motor can have the same kv as a smaller 36mm motor.

50k rpm no load as the goal so: 50000/intended voltage= kv you want to use (leaving most gearing the same)

Aside from kv, motors have a power rating, expressed in Watts. Generally the bigger the more power. Your esc should be able to deal with the amps a motor specwise os going to pull:
peak power rating of the motor/intended voltage you wanna use.

 

[#ARRMArcfanboy4life]

3200 motor on 6s =

So that is where my dumpster went

 

[JABzNY1]

Higher torque gives a higher ability to overcome resistance in a system( mass and friction are the main ones we deal with) a higher KV motor could only withstand a minimal amount of resistance in a system before is it overcome with heat. A high torque motor, while having a much lower KV and overall rotational speed makes up for it by have a much greater ability to overcome restrictions in a system because of the higher rotational force. This is generally why the larger the vehicle the lower the KV because the torque is needed more to move the weight than the general rotational speed.

 

[LibertyMKiii]

High KV works really good on very light weight cars. This generally means 2s packs (which are lighter) and mostly plastic chassis cars.
As soon as the car gets heavy with aluminum upgrades, or runs into air resistance at higher speeds, or tall grass running resistance... that high kv motor will overheat dealing with the torque demands of the situation.

This is why the little light cars come with 3500kv motors and the big fast heavy cars come with 1400-2000 kv motors. The lower kv motors provide more torque and can cope with the higher torque demands.

 

[jondilly1974]

To make this an easier discussion you need to forget what you’ve heard before. Higher KV does NOT mean more power. It is what determines how FAST the rotor spins (RPM). It has nothing to do with HP, torque, or watts.

The reason why your 2050kv motor has more power output than the 3200kv is the size of the magnets inside the can are larger.

Now where things get more complicated is how much voltage you apply determines the RPMs like I said above. You can always apply more volts than rated, but generally this will severely reduce the lifespan of the motor due to additional heat. This will demagnetize the magnets inside. It can even short out the wiring if the voltage is high enough. The added heat is from excessive RPMs.

These motors are both probably rated for the same safe RPMs of around 50,000 RPM. Anything above that and you run the chance of cooking it. Here is the math:

V x KV = RPM
6S (25.2V) x 2050kv = 51,660 RPM (Safe)
6S (25.2V) x 3200kv = 80,640 RPM (Unsafe)
3S (12.6V) x 2050kv = 25,830 RPM (Safe)
3S (12.6V) x 3200kv = 40,320 RPM (Safe)
4S (16.8V) x 3200kv = 53,760 RPM (Safe)

Most motors are safe up to 50,000 RPM, but others are only rated to 25,000 or up to 80,000. It all depends on the design.

 

[SrC]

Thanks @icephase - but how did you know that and how do you know that 70.000rpm is to much for the motor?

Is there a chart I can follow or something, that’s what I’m looking for really!

Most motors are generally rated up to 50k or less RPMS. As a very general rule. Due to BB contraints and armature durability reason. etc.
35k- 50k rpms is a decent range depending on the type of rig and its purpose. Bashing or Speed running and everthing in between, adjusted through final gearing adjustments.

6s/ 22.2 volts X 3200Kv = 71,040 RPMs ( NO GOOD)

You don't need a graph or chart.
Just this simple formula above.
Volts times Kv equals Rpms.
KV stands for Constant Velocity per one measured volt.

 

[Robii]

@jondilly1974 and @SrC - your legends, you guys put that in a way even I can get.

To summarise super rough guid:

6S (25.2V)
3S (12.6V)
4S (16.8V)

volts X Kv = RPMs (try and keep it under 50k)

Then take in to consideration gearing, weight of rig, surface etc

It might seem simple to most of you guys but to be honest that’s super helpful to someone who likes to tinker but doesn’t really have a clue!

 

[slick2500]

Most motors are generally rated up to 50k or less RPMS. As a very general rule. Due to BB contraints and armature durability reason. etc.
35k- 50k rpms is a decent range depending on the type of rig and its purpose. Bashing or Speed running and everthing in between, adjusted through final gearing adjustments.

6s/ 22.2 volts X 3200Kv = 71,040 RPMs ( NO GOOD)

You don't need a graph or chart.
Just this simple formula above.
Volts times Kv equals Rpms.
KV stands for Constant Velocity per one measured volt.

Castle motors are rated for 60,000 rpms.

 

[SrC]

@Robii

When you calculate RPM's, always use the Nominal Volt value of a pack.. This is the industry standard reference used by Motor Mfr's.
6s-22.2v
3s-11.1v
4s-14.8v

Castle motors are rated for 60,000 rpms.

I agree that 50k rpms is a general rule. Some motors are far higher spec'ed. And some motor brands offer no rpm specs at all.

Edit:
Nominal volts per cell = 3.7v

 

[jondilly1974]

Castle motors are rated for 60,000 rpms.

Some are. The 2028-1100 for instance is rated to 45,000 RPM, the 1717-1650 is rated to 56,000 RPM, and the 1515-2200 is rated for 60,000 RPM

 

[LibertyMKiii]

Some are. The 2028-1100 for instance is rated to 45,000 RPM, the 1717-1650 is rated to 56,000 RPM, and the 1515-2200 is rated for 60,000 RPM

Castle motors in my mind are to blame for so much of the motor choice KV confusion.

Each KV option from Castle is also a different motor size. (exception of the 2028 has 1100kv and 800kv) You are never accurately comparing one KV to another KV in that scenario as the can is longer or larger diameter etc....

Take TP Power for example they offer something like 12 KV options for 1 motor can size. Then they have the CM and SCM rotors that are good for 75k+ rpms. I love TP Power