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Consider the Three Basic Motor Topologies The three commonly-used motor congurations for low-power DC motors are brushed, brushless DC (BLDC), and stepper. Each uses interaction between the currents in coils (or windings), permanent magnets (in most designs), and the resultant magnetic eld attraction/repulsion to instigate motion. They have some similarities but differ in how they control necessary switching of current ow through the rotor and stator windings. They also differ in what they can do, how well they do it, and the exibility they have in their controllability. In brief: • The brushed motor is the oldest DC motor design. As the rotor turns, contact brushes (which are actually made as solid contacts, usually of graphite) touch corresponding areas on the rotor (Figure 1). As the rotor turns, the change in brush contact points causes reversal of the current ow direction and, thus, the magnetic eld. Then, the magnetic eld interaction between the rotor and stator reverses, and the rotor is urged to keep moving. This mechanical commutation is conceptually simple. However, the down side is that the brushes wear and need replacement, implementing smart control is harder because the switching action is hard on and hard off, and the brushes generate electromagnetic interference (EMI), also known as radio- frequency interference (RFI). In its simplest form, the brushed motor requires no electronics for control; it simply free runs as a function of current and mechanical load. In other cases, the motor power rail is turned on and off via a simple transistor circuit to activate the motor, so there is some basic control. The use of a driver IC can improve performance and allow Due to electronic control, the on/ off current ow does not have to be hard-switched on and off. Instead, the switching can be shaped at the gate driver with controlled rise and fall times to reduce EMI/RFI. However, the tradeoff is that the softer switching also results in power loss and reduced efciency, which the designer must balance. Some newer gate drivers use a variety of sophisticated and subtle tricks to make this tradeoff less harsh. • The stepper motor takes the concept of the BLDC motor further, by using a large number of coils (or poles) positioned around the motor periphery (Figure 3). By sequencing the turning on and off of these poles, the rotor is induced to step and rotate precisely, and it can be directed to do so in both forward and reverse directions. The number of poles can be as few as 16 or as high as 128 (or more, in some cases) with rotational precision on the order of a degree (depending on the number of poles). Stepper motors are available in unipolar two-phase and bipolar two-phase, three-phase, and ve-phase congurations. The bipolar two-phase is the most common one. The stepper motor is well suited for rapid stop/start motions, positioning, and back/forth motions but not for some amount of control over speed and torque. • The BLDC motor replaces mechanical commutation with an electrically switched commutation of the coils, with the current switching via transistors—most likely Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) at these power levels, preceded by gate drivers (some designs use Insulated Gate Bipolar Transistors (IGBTs)). A separate controller turns the coil- control switches on and off at the precise instant necessary to keep the motor turning at the desired speed (Figure 2). (Note: BLDC motors are sometimes called electronically commuted (EC) motors, which is an accurate description.) 15 Figure 1. In the brushed DC motors, the changing direction of the current (commutation) reverses the direction of the motor. It can be done entirely by mechanical means, using "brushes" that make contact with the rotor body. (Source: STMicroelectronics) Figure 2. In the brushless DC motor, the rotor's magnetic eld is always present and generated by a permanent magnet. When current is directed from one motor phase to another, the magnetic elds are combined, thus, generating the changing stator eld. (Source: STMicroelectronics) Figure 3. In the stepper motor, the rotor's magnetic eld is generated by a permanent magnet, while the stator magnetic eld is generated by forcing a current in one phase. As a result, the rotor will align with the stator's magnetic eld so the targeted step position is reached. (Source: STMicroelectronics)

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