Electronic Design

Motor Terms

Technically, the term electric motor includes all motors, but it’s typically used to refer to rotational motors. In linear electric motors, movement is linear instead of rotational. The stator is the fixed part of the motor. The rotor, which is the part of the motor that moves, typically rotates. The armature is the rotor coil.

The commutator connects brushes on the stator to the coils on the rotor. It’s used to reverse the current flow in the windings. It also includes a minimum of three segments to prevent dead spots. More segments can be used for better, more efficient operation. In continuous motors, the rotor moves continuously. In stepper motors, the rotor moves in increments. The servo provides positional feedback.

There are two types of ac motors: synchronous and induction/asynchronous. In synchronous motors, speed is independent of load. Synchronous motors also will hold position if the dc current is applied to the rotor and stator. They provide accurate speed and position controls using an open-loop system. And, they include active rotors or permanent magnet rotors. Induction/asynchronous motors include squirrel cage, slip ring, or solid-core rotors. They can include one, two, or three phases.

Brushed dc motors depend on how the coils are wound and connected. Windings can be separately excited, as each stator/rotor gets its own power source. With a shunt setup, the field and armature windings are connected in parallel, just like separately excited windings but with the same power source. Or, the field and armature windings can be connected in series for high current. Compound windings mix shunt and series arrangements. Meanwhile, coreless dc motors use a special case with only windings. Optimized for rapid acceleration, they’re used in high-performance servos.

There are two types of brushless dc (BLDC) motors: servos and reluctance/ continuous. The reluctance/continuous variety provides high power density and low cost. BLDC motors require a permanent magnet, usually in the rotor. They also offer low electromagnetic interference (EMI), and they don’t need a commutator, brushes, or sparking. However, they require feedback motor control. Sensor feedback involves the Hall effect or rotary encoders, while sensorless feedback uses electrical feedback.

Universal motors can run on ac or dc, though they typically use ac. They’re often used in locomotives that can use ac or thirdrail dc. With their high-speed, high-torque, and compact design, they’re typically used in blenders and drills. They also use a simple thyristor motor control.

Linear motors can be induction or stepper motors. There are ac and dc versions as well as brushed dc and BLDC versions. Finally, piezoelectric motors use crystals that change shape, making rotational and linear motors possible. They can range in size from small to extremely small.

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