Brushless Motor Kv – Measure a Motor’s Kv

What is a Brushless Motor Kv value?

The Brushless Motor Kv value is one of the three important electric motor constants. This motor constant is important as it is one that is used very frequently. When looking for a motor for an RC vehicle, the Kv value is what you will pay attention to. Choosing a motor with the wrong Kv value can ultimately destroy your entire power system, including batteries, ESC and the motor itself. Before motors started using Kv values as a standard, it was typical of motors to show turn values. A turn represent the copper wire making one complete wind within the motors armature. A 2 turn motor would have been a very high strung wind. However it does not tell you anything about the motor, that we would care about in terms of performance.

The definition of the Kv value is how many RPM a motor outputs per every volt. This directly translates in to unloaded RPM for any specific application when the Battery Voltage is known. It is common practice to use Nominal voltage values for the unloaded RPM. This represents the average unloaded RPM throughout a full battery discharge.

For example;

  • 1000 Kv Motor with 14.8v LiPo battery provides 1000 x 14.8 = 14800 Unloaded RPM
  • 2200 Kv Motor with 11.1v LiPo battery provides 2200 x 11.1 = 24420 Unloaded RPM
  •  220 Kv Motor with 37v LiPo Battery provides 220 x 37 =  8140 Unloaded RPM

The key to this motor constant is to know that the Kv value represents an unloaded condition where the value can vary under load. Unfortunately, the Kv values provided by manufactures vary within a slight range with varying production tolerances and in some cases can be incorrect. For the most accurate real value, it is recommended to measure the Brushless Motor Kv value.

Measuring the Brushless Motor Kv Value

There are a few different ways that the motor Kv can be measured in your own home with common tools. Both methods provide an accurate result.

Data Logger Method – Brushless Motor Kv

This method is rather easy to perform. Hardest part is obtaining the Rm value for the motor from the specification sheet or measuring it yourself.
The tools required in order to use the data logger method are:

  • Data Logger such as an ESC that has RPM monitoring or an eagle tree system with RPM monitoring.
  • Electronic Speed Control / ESC
  • Radio To wire to ESC
  • LiPo battery to spin motor up to approximately 60% of its rated speed
  • Formula below / or Calculator tool at bottom of page

This method includes the ESC effects, however with modern ESC these days the results should not be much effected at low current values.

Procedure:

  1. Connect the logging device up according to its instruction manual.
  2. Connect motor and battery to ESC
  3. Power up the system and run the motor up to 100% throttle for a brief moment of time.
  4. Review and Record the following values: RPM, Voltage, current, amperage. The 4th value required is the motors internal resistance in one phase. This can be measured as shown in the Measuring Rm page or taken directly from the manufactures provided specifications.
  5. Plug these values into the formula below or use the

Kv = RPM / ( Voltage – ( Current * Rm ) )

 

Drill spinning up Motor to measure voltage

 

Data Logger Method Brushless Motor Kv Calculator

 

 

Drill Method – Brushless Motor Kv

This method is a little more difficult to perform depending on the type of drill used.
|The tools required in order to use the drill method are:

Procedure:

  1. Connect the drill to the shaft on the Brushless Motor
  2. Spin up the drill and use the tachometer to measure the Exact RPM of the drill while spinning the Brushless motor.
  3. Connect 2 of the leads from the Brushless motor to the multimeter For example Leads 1 and 2.
  4. Spin up the drill and take a reading of the voltage produced at the leads. Record this value.
    Optional Steps:
  5. Swap the leads from 1 and 2 to 2 and 3 and retake the voltage reading with the drill spin up.
  6. Swap the leads one last time to leads 1 and 3 and spin the motor up taking the voltage reading.
  7. Take the average voltage reading and plug that in to the formula below or use the Calculator tool below.

Kv = RPM / ( Voltage x 1.414 x 0.95 )

 

Drill Method Brushless Motor Kv Calculator