Friday, August 3rd, 2012 08:32 am GMT -7 Friday, August 3rd, 2012 08:32 am GMT -7Friday, August 3rd, 2012 08:32 am GMT -7
Very large electric brushless outrunner motor

The simplest motor constant to measure, I only recommend one method for doing this.





The no load current is abbreviated I0. It is the amount of electrical current that the motor needs just to turn itself without a load such as a propeller. Current is normally measured in amps. Note that I’m not just talking about moving but actually spinning continuously. This is obviously energy that is not going into any useful work. The I0 value is needed in order to compute the efficiency of the motor.

Traditionally the no load current is measured at 8 volts input. I do not know the history of why this specific value was chosen. I do know that, for the motors that I have tested, the no load current stabilizes when the input voltage is about 5 volts or higher. Seems like the value of 8 was chosen with this behavior in mind.

There’s nothing that says that the measurement has to be taken at 8 volts. The better power system calculators, including mine, let you enter the actual voltage at which the reading was taken.

Outrunner motors do not make it easy to measure the current that is flowing through them. I only recommend measuring the motor current and voltage at the connection between the power source and the speed control.

Doing this will affect the accuracy of the results, but not by much. Modern speed controls are very efficient. If you are paranoid about this, take measurements with different speed controls and use the one that appears to show the smallest power loss.

For the most accurate results, the timing of the speed control needs to be set to neutral. This is changed through the programming interface to the speed control. Every brand is different, but they may call this “low” timing.

Don’t put anything on the motor shaft. A prop adapter will make little difference in the results, but it is best to leave that off, too.

Since you are measuring the voltage and current before the speed control, the only reliable reading is at 100% throttle. Otherwise, you will not know what voltage the motor is actually seeing. As I explained before, a voltage rating of 9 to 12 volts is probably ideal for your power source.

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


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