Don’t panic! Using my special trick, measuring this one is dead simple.

#### Recap

I have already talked about how to measure the no load current (I0) and winding resistance (Rm) of a brushless outrunner electric motor. The procedures I described were quick and easy. The hardest part was just hooking up all the pieces together.

#### Wrong Method

You might think that you can measure the voltage constant by just running the motor, measuring the voltage and RPM, and then dividing the RPM by the voltage. Unfortunately, once again, the simplest and most obvious method just does not work.

The problem is that electric motors experience internal power losses. The RPM will always be lower than predicted by the voltage constant. Unless we take these losses into account, our calculations will be wrong.

#### Easiest Method

While working on this project I discovered what has to be the safest and easiest method for measuring the motor’s voltage constant (Kv).

The key to the method is using my Eagle Tree eLogger to electronically measure the motor’s RPM. The eLogger works by counting the voltage pulses going into the motor. Hook up the logger’s two wires onto any two of the three motor’s input wires.

Since the RPM is being measured electronically, you do not need to attach a propeller to the motor. This means that you can take an RPM measurement when you are measuring the no load current. Those are then all the numbers that you need.

The formula is not too hard: (voltage constant) = (I0 RPM)/((I0 voltage) – (I0 current)*(winding resistance)). Current is in amps and winding resistance in ohms.

There is only one complication with this method. You have to tell the eLogger how many poles the motor has. By poles they mean the number of magnets attached to the rotating can of the motor. If you carefully peek inside the motor, you can count them without having to take the motor apart.

All the outrunners that I have seem to use 14 magnets. I would count them anyway, just to be sure.

#### Alternative Method

What if you do not have an eLogger or another way of electronically measuring the RPM? Then you can use a variation of the method I just described.

First, measure the no load current using the method described in the earlier article. Then mount a propeller and take RPM, voltage, and current measurements.

The formula is almost the same as above: (voltage constant) = (RPM)/((voltage) – (current)*(winding resistance)). Again, current is in amps and winding resistance in ohms.

In the next article of this series, I will describe a very different method of measuring the winding resistance and voltage constant.

#### 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