Replacing your RC car’s motor can be expensive and confusing, with many different ratings and specs. This article will examine one more confusing aspect: turns vs. Kv.
An RC motor’s turns refer to the number of times the copper wire is wrapped around the armature, and lower turns in an RC car’s motor mean a higher RPM. The Kv (not kV, as in kilovolt) refers to the actual RPM value while the motor runs without a load on one volt of electricity. Higher Kv means higher RPM.
Most new drivers think they should buy a motor with a higher Kv and lower turns because that equates to higher RPMs.
But that’s not always the case because motor RPMs don’t always mean higher torque.
At the end of this article, you’ll have a broad overview of what turns and Kv are and how they affect your RC car’s motor.
Let’s get started.
What Do Turns Mean on RC Brushless Motor?
The turns rating refers to the number of times the copper wire is turned (looped) around the motor’s armature.
Most RC brushless motors range between ten and twenty-seven copper windings, or motor turns.
The number of turns affects the motor’s electrical resistance. Higher turn ratings mean higher resistance.
In practical terms, the turn rating impacts the motor in two ways:
- Lower turns mean more rotations per minute, so higher RPM motors will have lower turn ratings.
- Higher turn ratings typically indicate lower RPMs but higher torque.
Though a higher turn rating motor won’t run as fast as a lower turn rating motor, the high turn motor will have more torque. This is helpful in heavier RC cars or crawlers going over rough terrain.
Minipro.com created a chart showing the average RPM generated by different turn ratings. Here’s a brief summary:
- 3.5 turns: ± 10,500 RPM
- 8.5 turns: ± 5,000 RPM
- 21.5 turns: ± 1,800 RPM
Remember that these are only estimates and averages. Additional factors can also affect the actual RPMs you get from your RC motor.
Understanding Motor Kv for Brushless Motors
Before we start, we need to note the difference between motor Kv, which we discuss here, and kV, which means kilovolts. You may see these two concepts written the same, but they are entirely different.
If you didn’t sleep through High School Physics (like I did), you’ll remember that a motor’s Kv rating refers to the motor’s constant (K) velocity (v).
In plain English, Kv refers to the number of rotations an unloaded motor would do per minute while using one volt of power.
For example, a 1,000 Kv motor that’s unloaded (not powering anything else) would turn 1000 times per minute when connected to a 1-volt power source.
You can multiply that information by the power you provide to get the actual RPM.
Keeping with the previous example, that same unloaded motor would turn at 9,000 RPM when powered by a higher voltage 9V battery.
It’s worth noting that Kv are typically discussed only for a brushless motor. Brushed motors still have a Kv, but it usually has to be calculated.
Turns Vs. Kv Rating: How They Tie Together
The one factor that ties turns and Kv together is the motor RPM.
We’ve already seen that a brushless motor with fewer turns will have more RPM. This means that your motor’s turns and Kv rating are negatively correlated.
Thus, a motor with high turns will have a low Kv rating, while a motor with a higher Kv rating will have fewer turns.
In practical terms, if you want maximum speed for your drag car, opt for a high Kv motor. If you’re building a crawler and need more torque, choose a lower Kv motor with more turns.
Beginners tend to choose a sweet spot somewhere in the middle. It won’t win you any races, but you will have the best of both worlds while you’re learning the hobby.
What Else Affects The Kv Rating?
The turn rating is a significant factor affecting a motor’s overall Kv, but it’s not the only factor.
Here’s a summary of some other factors, apart from turns, that can affect a brushless RC motor’s total Kv:
Physical Size Of The Motor
Larger objects generally take longer to complete a rotation than smaller objects when the same amount of voltage is applied.
So a larger motor typically runs at a lower RPM than a smaller motor. This leads to a lower Kv rating in a physically bigger motor.
The Number Of Magnetic Poles
Brushless RC motors work on the principles of electromagnetism, so the number of magnetic poles inside the motor also affects engine speed.
All things being equal, a motor with a higher pole count will have a lower Kv value.
For example, a 2-pole motor’s Kv would be considerably higher than a 4-pole motor with the same turn count with the same voltage.
Inrunner Vs. Outrunner Motors
Inrunner motors have a rotor spinning inside the motor’s case, while with outrunners, the motor’s casing itself rotates.
Due to the motor’s casing size, outrunner motors tend to run slower than inrunner motors.
In other words, an inrunner motor would have a higher Kv rating than an outrunner motor if all the other factors were equal.
Motor Quality
This should be logical, but a higher-quality motor usually performs better than a lower-quality one.
Motors made from high-quality materials allow for better electrical conductivity and better magnetic properties. This leads to improved performance and longer life for your motor.
This usually means higher-quality RC motors have a higher Kv rating than lower-quality motors. Similarly, higher-quality motors with high turn ratings have better torque than lower-quality motors with the same turn ratings.
Motor Winding Termination
When we talk about windings, we are still talking about the motor turns.
Winding termination deals with how the copper wires used for the turns are terminated or connected to the power supply.
There are two termination methods: the WYE or STAR wind and the DELTA wind.
WYE wind termination leads to lower Kv ratings. In contrast, using DELTA wind termination in a motor gives it a higher Kv value.
Wrapping It Up: Is Kv or Turns More Important?
If you’re trying to make your RC car go faster, a higher Kv is generally preferable.
Remember, though, that Kv is based on an unloaded motor. Once there’s a load on the motor, you won’t get the same RPM as an unloaded motor. Still, on average, higher Kv means more speed.
If speed is less important than the force applied, focus on a higher-turn motor. Higher turn counts in a motor mean it can pull more weight, move larger cars, and traverse rough terrain more easily.