The Mechanics
2. From Pulses to Motion
Now that we know what PWM is, let's delve into how it makes a motor spin faster or slower. It's not just about flipping a switch rapidly; there's some clever electrical engineering involved. When a PWM signal is applied to a motor, the motor's internal components, particularly its inductance (think of it as electrical inertia), react to the rapid switching. The motor doesn't instantly start and stop with each pulse. Instead, it integrates, or averages, the pulsed power over time.
Think of it like pushing a swing. You don't need to push constantly; you just give it a push at regular intervals. The swing's momentum carries it through the rest of the cycle. Similarly, the motor's inductance smooths out the pulsed power, resulting in a more continuous motion. The higher the duty cycle (the longer the "on" time), the more average voltage the motor receives, and the faster it spins. Conversely, a lower duty cycle means less average voltage and a slower speed. The PWM frequency (how often the pulses occur) is also crucial. If the frequency is too low, the motor might jerk or vibrate due to the noticeable on/off cycles. A higher frequency ensures smoother operation by further averaging the power delivered to the motor.
Consider a DC motor. When a PWM signal with a high duty cycle is applied, the motor sees a relatively high average voltage, causing it to draw more current and spin faster. If we drop the duty cycle to almost zero, the average voltage drops accordingly, and the motor receives hardly any current, causing it to either stop or spin very slowly. It's all about that carefully controlled balance of "on" and "off" times.
It's important to choose an appropriate PWM frequency. Too low, and you'll hear (and feel) the motor "cogging," a jerky, uneven motion. Too high, and the switching losses in the control circuitry can become significant, reducing efficiency. There's usually a sweet spot that depends on the motor type and application.
