This is an alternate method that is easier but less safe.

#### Same No Load Current

Regardless of how you choose to measure the motor constants, the first step is always to measure the no load current (I0). This is easy to do using a wattmeter, though you will get more accurate results using multimeters. Remember to take the readings at 100% throttle. Use a power source of about 10 volts.

#### Method 2 Procedure

For this method, you are going to attach two different propellers to the motor. You need to run the motor at 100% throttle to take measurements. Ideally, you want one propeller to be close to the maximum current rating of the motor and the other to be at about 50% of the current rating.

With each propeller at full power, write down the voltage, current, and RPM. You can use a standard optical tachometer to take the RPM readings. You will end up with two sets of numbers. The numbers can be plugged into the “Method 2“ spreadsheet to compute the motor’s winding resistance and voltage constant. That’s it.

#### Comparison to Method 1

Method 1 is more complicated, since it involves a separate step to measure the winding resistance. It also requires purchasing a power resistor.

But method 1 is far safer, since you never have to run the motor with a propeller attached. This is a huge plus to the method, which is why it is the method that I use.

### Summary of all Methods

Here is a summary of the two methods I have described in this series of articles. If you do not have an electronic tachometer, but still want to follow method 1, I describe the alternate procedure as a separate method. This alternate procedure is still safer than method 2, but not by much.

#### Method 1 – Electronic Tachometer

- Measure the no load current using a wattmeter or multimeters. This is done without a propeller attached to the motor. Run the motor at full power.
- Electronically measure the RPM while measuring the no load current. This can be done using an electronic tachometer like the Eagle Tree eLogger.
- Measure the winding resistance. Use a power resistor for this.
- Use the spreadsheet “Method 1“ to calculate the voltage constant.

#### Method 1 – Optical Tachometer

- Measure the no load current using a wattmeter or multimeters. This is done without a propeller attached to the motor.
- Measure the winding resistance. Use a power resistor for this.
- Attach a propeller to the motor. Run the motor at full power with the propeller attached. Measure the voltage, current, and RPM. You can use a standard optical tachometer to measure the RPM.
- Use the spreadsheet “Method 1“ to calculate the voltage constant from the readings you just took with the propeller attached.

#### Method 2 – Two Propellers

- Measure the no load current using a wattmeter or multimeters. This is done without a propeller attached to the motor.
- Attach a propeller to the motor. Try to get close to the maximum current rating of the motor. Measure the current, voltage, and RPM (I1, V1, RPM1). You can use a standard optical tachometer to measure the RPM.
- Attach a different smaller propeller to the motor. Try to get close to 50% of the current rating of the motor. Again, this has to be done at 100% throttle. Measure the current, voltage, and RPM (I2, V2, RPM2). You can use a standard optical tachometer to measure the RPM.
- Use the spreadsheet “Method 2“ to calculate the winding resistance and voltage constant from the readings you took.

#### Conclusion

This ends my seven part series on measuring outrunner motor constants. I hope you found it useful. As always, if there are topics that you would like to see me cover, please do not hesitate to contact me.

Spreadsheet for Methods 1 and 2 (xls).

#### 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

Two comments: 1. It is good practice to measure the RM for all motor wire pairs. Recently I measured the RM on a motor for all pairs and found that two of the pairs had more than double the resistance of the other pair, indicating that one of the windings had some sort of problem. That motor went into the experimental bin.

2. I made a 0.25 amp constant current portable source using three 1.5 V alkaline batteries with a switch, an IC voltage regulator configured as a constant current source, and banana jacks with alligator clips housed in a plastic box. The whole thing cost less than $10 and works great. Just measure voltage at the motor terminals with a good multimeter using the millivolt scale. Rm = mV/.25A = milliohms.

1. I won’t disagree on that one. It is good practice to measure the winding resistance (Rm) on all three motor loops.

2. That’s clever. I like using my converted PC power supply because I can also use it for the no load current (I0) measurement.

Hi Carlos,

Can you please give a brief rationale for the calculations of Kv and Rm (Method 2) using your spreadsheet?

Cheers,

Stephen.

No. Seriously – I started to write an explanation. Then I realized that there is nothing that I can write that is both short and clear. All the key formulas are in my first book (Model Airplane Design Made Easy). All of the calculations derive from them. The spreadsheet applies the formulas and solves for the required variables.