Driving Brushed DC Motors Using PWM Output: Principles of PWM Driving

From this article, we explain PWM driving of brushed DC motors. PWM driving of DC motors makes possible reduction of the power from the driver power supply, and so adoption of this method of driving has been increasing of late.

Driving Brushed DC Motors Using PWM Output: Principles of PWM Driving

PWM (pulse width modulation) is widely used not only in motor driving, but in many other applications as well, and is also widely used for power supply power conversion, in DC-DC converters, AC-DC converters, and the like. In essence, through on/off pulse switching, the required power is transmitted. The pulse height and frequency are constant, but the on-pulse width (time) is adjusted to control the power that is transmitted. The output voltage is an averaged value corresponding to the ratio of the on period to the period of a pulse signal, that is, the duty cycle.

Below are schematic diagrams illustrating the principles of PWM driving of a brushed DC motor.

The motor is represented by the motor symbol and by the resistance R, the inductance L, and the induced voltage Ec. When a voltage is applied, the motor voltage Ea is applied across the motor terminals, and a motor current Ia flows. On the other hand, the motor is shut off by shorting the motor terminals. In this state, current regeneration occurs. Because the coil inductance acts so as to preserve the current, if a voltage is applied and the motor terminals are shorted repeatedly in cycles that are sufficiently short with respect to the time constant of the inductor L and the resistor R, a constant current flows.

This is shown using an H bridge as follows.

When a voltage is applied, SW1 and SW4 are on, SW2 and SW3 are off, and the motor current Ia flows as indicated by the gray dashed line. When the motor is turned off with the motor terminals shorted, SW2 and SW4 are on, and SW1 and SW3 are off.

The voltage and current waveforms in this case are as shown below.

The voltage waveform indicates that the voltage Ea is applied to the motor during on periods that are repeated at a constant frequency, and is not applied during the off periods (in which current regeneration occurs due to shorted motor terminals). The current increases during on period and decreases during off period repeatedly, and enters a steady state. The average current Iave at this time is the result of multiplying the applied voltage Ea by the duty cycle m, and dividing by the resistance R. In a simple example, if Ea=12 V and m is 0.5 (duty ratio 50%), then 6 V is applied to the motor. A current is supplied from the power supply only when a voltage is applied, so that the power consumption of the power supply is reduced.

Up to this point, we have discussed methods in which the two motor terminals are shorted together in the off state (current regeneration). In fact, there are other methods of current regeneration as well, with different matters requiring study in each case. The next article will explain other current regeneration methods.