In the world of RC, motor Kv is one of the most well known specification for those who build. Chances are, if you’re building an RC vehicle, you’re going to come across this term at some point. It is also safe to assume that most of us know that more power will equate to more speed. This is true, however does higher Kv produce more power? To better understand further about our topic today, we have to dive in to some technical details. Let’s start with a simple kv definition.
For more details or a video version on this topic, view the video below.
Understanding the Term Kv and Kt
Kv is a term that stands for the Velocity Constant of a brushless motor. As such, we can determine the amount of rotational speed that we get from our motor for each volt that we put in. If we know the voltage input we can calculate the unloaded RPM output of a brushless motor. Let’s take an example. Suppose we have a 2000Kv motor with a 11V Battery. The expected unloaded RPM output would be 2000 x 11 = 22,000RPM. Nevertheless, this does not tell us the power output. Consider the mechanical equation for power output.
For those of you familiar with horsepower and torque, the formula for HP is: HP = RPM x Torque / 5252 using imperial units. Whereas for metric units, Power in Watts is simply equal to Rotational Speed in rad/s multiplied by Torque in Nm. For instance, using our Kv specification and Voltage, we can determine RPM but we don’t know the torque value. Interestingly enough, Kv is very much related to Kt. Kt is another motor constant relating to torque. The torque constant can be determined by Kv using the equation Kt = 1/Kv. If you are going to try and use this formula, just make certain to convert RPM to rad/s. What does this now mean for us?
We can Now Get Torque Output!
We can now get the torque output of our Brushless motor. The only missing link is we need to know the amount of current that the motor will be drawing from the battery. Similar to how Kv multiplied by voltage gets us the total RPM, Kt multiplied by current provides us with the torque output.
To illustrate what we have so far covered, RPM = Kv x Voltage and Torque = Kt x Current where Kt = 1/Kv.
Substituting Kv Back in to Mechanical Power Equation
To determine if Kv is going to tell us how much power we get out of our motor, let’s substitute the value in our power equation. We will run through the equation below:
Power (Watts) = Rotational Speed x Torque
-Substitute for Kv and Kt
Power (Watts) = Kv x Voltage x Kt * Current
-Here we know Kt is equal to 1/Kv
Power (Watts) = Kv x Voltage x 1/Kv * Current
-Kv * 1/ Kv is equal to Kv/Kv = 1, therefore we can eliminate Kv from the above equation and rewrite below as
Power (Watts) = Voltage * Current
We just derived a different formula from our original mechanical formula.
Higher Kv equates to more power conclusion
As a result of the the substitution that we have done, there are two takeaways. The first takeaway is that we have been able to eliminate Kv from the equation. Secondly, we have a new power equation. The new equation is the electrical formula for power measured in watts, Power = Voltage x Current. We have been able to show that Kv can be eliminated from the formula quite easily. The truth is, Kv alone does not tell us much about the motors true power output. To really understand power output, we need to know Voltage and Current. Or we can use mechanical characteristics such as RPM and output torque. Even after this determination, let’s not stop here and check in with a typical Motor Manufactures Specification sheet.
Comparing Motor Manufactures Specifications Power vs Kv
Firstly, when two brushless motors are compared, one item to keep constant is the size of the motor. To the left is a chart from a manufacture that displays five different Kv values for one specific motor model/size.
Kv vs Power – From the Specs Point of View
The specification in the chart that stays constant is the max wattage for each Kv option. Each motor of a different wind, has a maximum power output of 2400W. One thing to note is that as the maximum current increases, the maximum voltage decreases. It is this relationship that demonstrates to us that a motors Kv value really does not tell us much about the power potential of the motor.