When a BLDC motor turns, each winding develops a back electromotive force voltage (BEMF) which, according to Lenz's law, is in the opposite direction to the main voltage supplied to the windings. The polarity of this back EMF is opposite to the excitation voltage. Therefore, the voltage drop across the winding can be calculated by subtracting the back EMF value from the supply voltage. The design method using the back EMF constant is as follows: When the motor is running at the rated speed, the potential difference between the back EMF and the supply voltage is sufficient for the motor to consume the rated current and provide the rated torque. If the motor speed exceeds the rated speed, the back EMF will increase significantly, thereby reducing the voltage drop across the winding, and reducing the current, resulting in a drop in the torque curve. Since the motor work is mainly reflected in the RMS voltage, the back EMF test is mainly to test the RMS value of the back EMF.

Back EMF mainly depends on three factors:

1) The rotor angular velocity...for the test equipment is the servo drag speed.

2)The magnetic field generated by the rotor... This is a constant when the product is fixed.

3)The number of turns of the stator winding...this is constant when the product is fixed.

Back EMF Constant: Ke     Unit: V/rpm or V/Krpm or V/rad/s;    therefore, it is recommended to test the rotation speed at 1000rpm.

Formula: BEMF=(E)∝NLRBω

N: The number of turns of each phase winding.

L: The length of the rotor (magnet part).

R: The inner diameter of the rotor.

B: Rotor magnetic field density.

ω: angular velocity of the motor.

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