Issue link: https://resources.mouser.com/i/1525523
| 16 divider can change the speed because speed is proportional to voltage in DC motors. However, a voltage divider will still draw the same amount of current, making this technique inefficient. A better solution for a brushed DC motor that needs to rotate only in a single direction is a pulse width modulator (PWM), which controls the average voltage to the motor through its duty cycle. The PWM circuit needs only a transistor to switch at a frequency sent from the control signal and a diode to provide a route to dissipate back- EMF (Figure 1). If the direction of the motor needs to be reversed, an H-bridge circuit can be used. The H-bridge design uses four transistors, each with a flyback diode. The actual controller for this type of circuit would normally be a simple microcontroller with an integrated PWM controller. Brushless DC Brushless DC (BLDC) motors operate similarly to AC motors, with the stator creating a rotating magnetic field that causes the permanent magnet rotor to rotate. Voltage is used to energize the electromagnets in the windings of the stator in a specific order to create the rotating magnetic field. A BLDC motor can have as little as three stator coils or, more typically, six or more. A three-coil version will have windings 120 degrees apart. At any one time, one coil will be positive, one will be ground, and the third will be open. If the stator coils are continually switched in order, the rotor will "chase" the flux and rotate. Using this type of control alone can cause a jerky kind of motion as the magnet on the rotor jumps from one pole to the next, which can cause vibration and noise in the system. For smoother rotation, sinusoidal control can provide continuous current change. This type of control usually has an inverter adjusting the voltage and current into each stator winding. A PWM produces the sinusoidal effect as the duty cycle is lengthened or sequenced. To the winding, a high- duty cycle will look like the voltage is being increased, prompting more current flow, and a low-duty cycle will reduce the current. Sinusoidal control is far more complex than the simple PWM control required for a brushed DC motor, requiring three voltages to be generated and coordinated. As shown in Figure 2, sensors feed a decoder with positional data from the rotor, which is then used to calculate two sinusoidal waves that are phase-shifted 120 degrees from each other. Those signals are then multiplied by a calculated value to provide the desired torque. The resulting signals are input to two P-I controllers, which regulate current in the stator windings through a PWM and output bridge. The third winding is the negative total of the two calculated currents and is not separately controlled. A microcontroller with the right peripherals can carry out most of the functions found in Figure 2; for example, devices from the Renesas RX family integrate input capture and a PWM timer for three-phase motor control. While sinusoidal control is ideal for providing a smooth, noiseless output, trapezoidal control (Figure 3) maximizes the motor's torque. In this technique, only two windings of equal magnitude are energized at a time, while the third Figure 1: Brushed DC motor control when the motor rotates in a single direction. (Source: Author)