Introduction to measuring the electric brushless outrunner motor constants Rm, I0, and Kv.

#### Summary

Accurate motor constants are needed to analyze the performance of our electric motors. You need to know them in order to use my free electric power system calculator, for example.

It is a sad fact, but more often than not the constants supplied by the motor manufacturers are wrong. Some of this is just due to the natural variability in manufacturing each motor. But other times numbers are published to make a motor look better or to match a competitor’s numbers. The only way to find out the truth is to measure the constants yourself.

I wish I could tell you that I have come up with a very accurate and very easy method for measuring the constants in our motors. I did not, because it cannot be done. But I have done my best to break down the process into easy to follow steps.

This series of videos will only cover brushless outrunner electric motors. Brushed motors are a little different and I will not be discussing the differences.

Because of the challenges involved, there are many different ways of measuring the motor constants. They will all give you slightly different results. Each method involves trade-offs in accuracy, safety, and convenience. Safety is very important to me. I will explain first the method that I use myself. It is very safe, reasonably accurate, and easy to do.

The tools that you need for the different methods also vary. I will devote a video just to talk about the different tools you might already have or need to buy.

There are three key constants. First is the no load current (I0), which is the minimum amount of electric current needed to turn the motor. Second is the winding resistance (Rm), which is the electrical resistance of one loop in the motor’s copper windings. These two constants are critical to measuring the efficiency of the motor.

Finally we have the voltage constant (Kv), which relates the input voltage to the motor’s RPM. This is the constant which you are more likely to be familiar with, since it also helps determine what size propeller you need to use with the motor.

#### Articles in Series

## Measuring Motor Constants: Introduction

## Measuring Motor Constants: Tools

## Measuring Motor Constants: Power Factor

## Measuring Motor Constants: No Load Current (I0)

## Measuring Motor Constants: Winding Resistance (Rm)

## Measuring Motor Constants: Voltage Constant (Kv)

## Measuring Motor Constants: Rm and Kv Method 2

Are you sure the no load current is not at full revs and measured at a voltage similar to that used in practice?? The program I have need the parameter measured that way not the turn over amps as you detail.

Traditionally the no load current is measured at 8 volts. I do not know why that number is used. I can say that the current stabilizes when the voltage is about five volts or higher.

I devote an entire video just to this measurement. The process might be clearer after watching that video.

Hi Carlos

I assume the Rm specified by manufacturers is winding resistance of one phase only.

However, in a Delta type brushless motor (for example) the resistance one would measure across the terminals in any measurement method will be only 2/3 of that. I have not seen anyone referring to that. Please comment . Appreciate your input.

Hi. Yes, the idea is to measure the resistance in one phase. You are right, a delta wound motor actually measures across all three loops.

I had the same question as yours while I was doing my research for the videos. I did not come across any references to the motor winding when it came to measuring the constants. My conclusion is that the goal is to measure the electrical resistance across two of the motor wires, regardless of how the motor was wound.

[…] Measuring Motor Constants: Introduction […]

[…] Measuring Motor Constants: Introduction (web) Share this: Recommend on Facebook Share on Google+ Tweet about it Tell a friend Reprint for free! This article by http://www.RCadvisor.com is licensed under a Creative Commons Attribution-NoDerivatives 3.0 Unported License. googletag.cmd.push(function() { googletag.display('div-gpt-ad-1340857160574-0'); }); Related Posts:Measuring Motor Constants: Rm and Kv Method 2Measuring Motor Constants: Voltage Constant (Kv)Measuring Motor Constants: IntroductionSimplified Motor and Propeller Selection MethodInrunner vs. Outrunner MotorsMeasuring Motor Constants: No Load Current (I0)Measuring Motor Constants: Power FactorMeasuring Motor Constants: ToolsMotor Voltage Constant KvCarlos’ Power Rule ExplainedMatching a Propeller to a MotorMeasuring Motor Constants: Winding Resistance (Rm)Understanding Model Airplane DownthrustModel Airplane Propeller Downthrust AngleUpgrading to a Bigger RC Battery PackGetting Started with Electric RC: Speed ControlsMotor I0 and Rm ConstantsReal Review: Hobby King Turnigy Park 480 MotorFluid Dynamics of Drag Video SeriesUAVs For Search and Rescue Operations […]

NICE MOTOR I LIKE IT