How to Measure Motor KV Rating with SimpleFOC

The KV rating of a motor (in rpm/V) tells you how fast the motor spins per volt applied, under no load. Knowing your motor’s KV is important for accurate torque control and current estimation in FOC applications.

What is KV Rating?

KV (“velocity constant”) is the number of revolutions per minute (rpm) the motor will turn per volt applied.
Example: A 100 KV motor spins at 100 rpm for every 1V applied (no load).

Why Measure KV Yourself?

Datasheet values are often inaccurate or missing.
Real-world values can differ due to winding, magnets, or assembly.
Accurate KV is needed for advanced torque/current control modes in SimpleFOC.

How to Measure KV with SimpleFOC

Wire up your motor and sensor (encoder, magnetic, or Hall sensor) and driver as usual.
Set up SimpleFOC in voltage torque mode:
- Set the control mode to torque: motor.controller = MotionControlType::torque;
- Set the torque control mode to voltage motor.torque_controller = TorqueControlType::voltage;
- Set a target voltage (typically 1V, but you can try other values for confirmation).
Run the motor and read the velocity:
- Let the motor spin up to steady speed.
- Read the measured velocity (in rad/s) from the serial monitor or via code.
Convert velocity to KV:
- Use the formula:
  \(\text{KV} = \text{velocity (rad/s)} \times \frac{30}{\pi}\)
- Example: If you measure 20 rad/s at 1V, then KV = 20 × 30/π ≈ 191 rpm/V.

Tip: The KV value should not change much with voltage. Try 1V, 2V, etc. and average the results.

Example: Serial Command to Calculate KV

SimpleFOC provides example sketches that let you set the voltage and calculate KV directly from the serial terminal. See examples/utils/calibration/find_kv_rating/ in the Arduino-FOC repo.

Example Code for BLDC, Stepper, and Hybrid Stepper

BLDC motors Stepper motors Hybrid Stepper motors

Below are minimal examples for each motor type. Replace the sensor and driver with your hardware as needed.

#include <SimpleFOC.h>

// BLDC motor & driver instance
BLDCMotor motor = BLDCMotor(11); // 11 pole pairs example
BLDCDriver3PWM driver = BLDCDriver3PWM(9, 5, 6, 8);
// Sensor instance (e.g., encoder)
Encoder sensor = Encoder(2, 3, 500);
void doA() { sensor.handleA(); }
void doB() { sensor.handleB(); }

float target_voltage = 1; // 1V for KV measurement
Commander command = Commander(Serial);
void doTarget(char* cmd) { command.scalar(&target_voltage, cmd); }
void calcKV(char* cmd) {
  // Print KV: velocity (rad/s) / voltage (V) * 30/pi
  Serial.print("KV = ");
  Serial.println(motor.shaft_velocity / target_voltage * 30.0f / PI);
}

void setup() {
  Serial.begin(115200);
  sensor.init();
  sensor.enableInterrupts(doA, doB);
  motor.linkSensor(&sensor);
  driver.voltage_power_supply = 12;
  driver.init();
  motor.linkDriver(&driver);
  motor.controller = MotionControlType::torque;
  motor.init();
  motor.initFOC();
  command.add('T', doTarget, "target voltage");
  command.add('K', calcKV, "calculate KV rating");
  Serial.println(F("Set voltage: T [value], Calculate KV: K"));
}
void loop() {
  motor.loopFOC();
  motor.move(target_voltage);
  command.run();
}

#include <SimpleFOC.h>

// Stepper motor & driver instance
StepperMotor motor = StepperMotor(50); // 50 pole pairs example
StepperDriver4PWM driver = StepperDriver4PWM(9, 5, 10, 6, 8);
// Sensor instance (e.g., encoder)
Encoder sensor = Encoder(2, 3, 8192);
void doA() { sensor.handleA(); }
void doB() { sensor.handleB(); }

float target_voltage = 1;
Commander command = Commander(Serial);
void doTarget(char* cmd) { command.scalar(&target_voltage, cmd); }
void calcKV(char* cmd) {
  Serial.print("KV = ");
  Serial.println(motor.shaft_velocity / target_voltage * 30.0f / PI);
}

void setup() {
  Serial.begin(115200);
  sensor.init();
  sensor.enableInterrupts(doA, doB);
  motor.linkSensor(&sensor);
  driver.voltage_power_supply = 12;
  driver.init();
  motor.linkDriver(&driver);
  motor.controller = MotionControlType::torque;
  motor.init();
  motor.initFOC();
  command.add('T', doTarget, "target voltage");
  command.add('K', calcKV, "calculate KV rating");
  Serial.println(F("Set voltage: T [value], Calculate KV: K"));
}
void loop() {
  motor.loopFOC();
  motor.move(target_voltage);
  command.run();
}

#include <SimpleFOC.h>

// Hybrid Stepper motor & driver instance
HybridStepperMotor motor = HybridStepperMotor(50); // 50 pole pairs example
BLDCDriver3PWM driver = BLDCDriver3PWM(9, 5, 6, 8);
// Sensor instance (e.g., encoder)
Encoder sensor = Encoder(2, 3, 8192);
void doA() { sensor.handleA(); }
void doB() { sensor.handleB(); }

float target_voltage = 1;
Commander command = Commander(Serial);
void doTarget(char* cmd) { command.scalar(&target_voltage, cmd); }
void calcKV(char* cmd) {
  Serial.print("KV = ");
  Serial.println(motor.shaft_velocity / target_voltage * 30.0f / PI);
}

void setup() {
  Serial.begin(115200);
  sensor.init();
  sensor.enableInterrupts(doA, doB);
  motor.linkSensor(&sensor);
  driver.voltage_power_supply = 12;
  driver.init();
  motor.linkDriver(&driver);
  motor.controller = MotionControlType::torque;
  motor.init();
  motor.initFOC();
  command.add('T', doTarget, "target voltage");
  command.add('K', calcKV, "calculate KV rating");
  Serial.println(F("Set voltage: T [value], Calculate KV: K"));
}
void loop() {
  motor.loopFOC();
  motor.move(target_voltage);
  command.run();
}

Practical Notes

The measured KV should be stable across voltages (try 1V, 2V, 3V).
If the value changes a lot, check your wiring, sensor, and supply voltage.
For best results, run the test with no load on the motor.

For more details, see the SimpleFOC documentation and the example sketches in the Arduino-FOC library.