The China high speed dc motor manufacturers tells you pulse width, speed control.
As we have said before, the speed of a DC motor is proportional to the average(average) voltage value on its terminals, and the greater the value, the faster the motor rotates until the maximum allowable motor voltage. In other words, more voltage is faster. By changing the ratio between the "ON" time and the "OFF" duration, which is called the "charge/space ratio", "Mark/Space ratio" or "duty ratio", the average motor voltage can therefore change its speed. By changing the pulse width, the voltage of the motor can be controlled and therefore the power applied to the motor can be controlled, and this type of control is called pulse width modulation.
Another way to control the speed of the motor is to change the frequency(and therefore the time period for controlling the voltage), while "ON" and "OFF" remain constant. This type of control is called pulse frequency modulation.
Using pulse frequency modulation, the motor voltage is controlled by applying a pulse of a variable frequency. For example, the average voltage voltage applied to the motor at a low frequency or at a very small pulse is low, so the motor speed is slow. At higher frequencies or with many pulses, the average motor terminal voltage increases and the motor speed also increases.
Then, the transistor can be used to control the amount of power applied to the DC motor, and its operating mode is "linear"(change the motor voltage). "Pulse width modulation"(change pulse width) or "pulse frequency" modulation "(change pulse frequency).
Reverse direction of DC motor
Although controlling the speed of a DC motor with a single transistor has many advantages, it also has a major disadvantage. The rotation direction is always the same. It is a "one-way" circuit. In many applications, we need to operate the motor in both front and rear directions. To control the direction of the DC motor, the polarity of the DC power supply applied to the motor must be reversed so that it rotates in the opposite direction.
