Brushless motor: How to change rotation speed


Up to now we power the motor always with maximal voltage, so we get the maximal speed depending to the mechanical load. If the mechanical load changes than the speed changes.

At the moment we do not have a closed loop control for the speed. To make a speed control we first must have  possibility to change the speed. The torque of the motor is direct proportional to the current of the motor. If change the current than we change the torque of the motor and following we change the speed.


To change the motor current, we change simply the motor voltage.

We can do it with a D/A-converter but in the practice we take a PWM modulation and a simply digital output.

(*) Spnnung = voltage (*) Strom = current (*) Spule = coil (*) Magnetisches Feld = magnetic field (*) Drehmoment = torque (*) Drehzahl = speed

With PWM Duty-Cycle we can control the DC-voltage on the motor. Condition to get a DC-voltage from a PWM-signal is to have a system connected to the digital output that has a low-pass characteristic. A DC-motor shows exactly this characteristic.