The principle of using PWM output to drive a brushed DC motor

In addition to being used for motor driving, PWM (Pulse Width Modulation) is also widely used in power conversion of power supplies such as DC/DC converters and AC/DC converters, and is widely used in many fields. In principle, PWM is a method of sending the required power by turning pulses on and off. The size and period of the pulse are constant, and the power to be sent is controlled by adjusting the pulse width (time) when it is turned on. The output voltage is the average size corresponding to the ratio of the time of conduction and pulse period = duty cycle.

Below is a schematic diagram of the PWM drive principle of a brushed DC motor.

The motor is represented by a motor symbol and resistance R + inductance L + induced voltage Ec. When voltage is applied, the motor voltage Ea is applied to both ends of the motor and the motor current Ia flows. When OFF, both ends of the motor pin will be short-circuited. In this state, current regeneration will occur. Since the inductance of the coil plays a role in maintaining the current, when the voltage is repeatedly applied in a cycle that is short enough relative to the time constant of the inductance L and the resistance R and the motor ends are repeatedly short-circuited, a constant current will flow.

The process is represented by an H-bridge as follows:

When voltage is applied, SW1 and SW4 are turned on, SW2 and SW3 are turned off, and the direction of motor current Ia is shown by the gray dotted line. When turned off = short-circuited across the motor, SW2 and SW4 are turned on, and SW1 and SW3 are turned off.

The voltage and current waveforms in this case are as follows:

The voltage waveform shows that the motor voltage Ea is applied during the on period with a constant repetitive cycle, but not during the off period (regeneration due to short circuiting of both ends of the motor). The current enters a stable state by repeatedly performing the "cycle of increasing when on and decreasing when off". The average current Iave at this time is the value obtained by multiplying the applied voltage Ea by the duty cycle m and dividing by the resistance R. As a simple example, assuming Ea = 12V and m is 0.5 (duty cycle 50%), 6V is applied to the motor. Because current is supplied from the power supply only when voltage is applied, the power consumption of the power supply will be reduced.

In the previous article, we discussed the off state (current regeneration) by short-circuiting the motor terminals. In fact, there are other current regeneration methods, each with different considerations. The next article will introduce other current regeneration methods in PWM drive of brushed DC motors.