The Basics of a Small DC Motor
Small DC motor are compact powerhouses that drive innovation across a variety of industries. These tiny electro-mechanical devices transform direct current (DC) electrical energy into mechanical motion using the electromagnetic interaction between magnetic fields and current-carrying conductors.
They offer speed control through voltage regulation and can be quick to start and stop. They also deliver great reliability.
Stator
DC motors are used for many different applications, including robotics, automation systems, and electric vehicles. Their small size and efficiency make them ideal for applications where space is limited. These motors are also very easy to integrate into projects, thanks to their simple design and compact size.
The stationary component of a DC motor is called the stator. It houses the field windings that generate magnetic fields to drive the rotor. These coils are known as armature coils. Each armature coil has a different polarity, which allows it to interact with the magnets in a particular way. The commutator, which is attached to the stator, continuously flips the direction of Small DC motor current into each coil, so that every other armature coil has the opposite polarity. The fixed brushes supply electrical energy to the commutator, thereby enabling it to flip the coil polarities.
A low-cost brushless motor, such as the one from a computer fan, typically has three leads for the wye-connected rotor windings and two for the Hall sensors (which probably function like the ones in a Microchip TC4469). It is not clear from the advertising whether it has a control module that accepts commands from a radio controlled model or sensorless control, using the back emf, which requires a more sophisticated processor.
Rotor
The rotor contains a set of electromagnets or coils that are wound onto a frame known as the armature. There will also be an axle that extends from the armature and this can have components like drill bits or fans connected to it (see fig below). The coils are usually coiled around a central iron core with two to eight pairs of north and south poles. Each of the coils is energised to create a magnetic field that interacts with the permanent magnets in the stator to generate torque.
To continue this interaction between the magnets and the armature coils the commutator must switch current in and out of the armature coils. This is achieved by using carbon brushes that make contact with the commutator in a series of switching pairs. When the switches are operated in the correct combinations the motor continues to operate. However, the arcing of the brushes produces considerable electrical noise that can cause damage to other circuits in the motor.
The commutator is designed to allow the brushes to make contact with the armature coils and reverse the direction of current in each pair at exactly the right moment to continue the interaction between the permanent magnets and the coils. This type of commutation is called trapezoidal and it allows the motor to be controlled without the use of any additional control electronics. Other forms of commutation, such as sine commutation, require more complex control systems.
Commutator
The commutator of a Small DC motor is the sliding electrical switch that reverses current between the rotor and brushes. When DC is applied to the commutator, it causes different sets of rotor coils to be alternately attracted and repelled by the fixed magnets in the stator, causing the rotor to rotate.
The current is switched between the rotor and brushes via the commutator’s segments, which are insulated from each other. During normal operation, the brushes contact multiple commutator segments and this small 12v motor creates sparks between them. To prevent excessive sparking, the commutator segments are designed to withstand high mechanical wear.
In applications where only unidirectional rotation is needed and no speed or torque control is required, the commutator can be eliminated. This simplifies the motor construction, reduces cost and improves reliability, especially when dirt and humidity are present. In these cases, a simple combination of Hall sensors is used to monitor the back electromotive force (EMF) of an inactive phase to determine the rotor’s position relative to the stator. More advanced sensorless methods like sine commutation require more complex motor control electronics and are generally more expensive than trapezoidal commutation.
Brushes are made of various materials, and the type used depends on the motor’s application. For example, copper brushes are ideal for low-speed applications with high durability. The brush contacts the commutator at several angles, and the material from which it is made determines how quickly it can spin without wearing down.
Windings
DC motors are very efficient at producing a lot of torque with minimal current, and they can also rotate much faster than AC motors. They are commonly used in small blowers and ducted fans to generate a rotating force that can move the air. They are also used in radio controlled cars and airplanes as well as in hobby servos.
The rotor of a Small DC motor has windings that interact with the permanent magnet to create the magnetic field. The windings are connected to a commutator and brushes that allow the rotational movement of the rotor to be translated into electrical energy. The rotor also has a shaft that supports it.
The brush commutator of a Small DC motor is usually a triangle (delta) arrangement, with two opposite polarity brushes touching each other and energizes three sections of the commutator at the same time, giving the force to rotate in one direction. When the commutator is spun back and forth, these three windings are energized in different phases creating a different force moving the rotor in another direction.
The speed of a Small DC motor can be varied by varying the voltage supplied to the commutator, or by adjusting the strength of the current through its field windings using Pulse Width Modulation. Typically, commercial off the shelf decoder IC’s like the TC4469 from Microchip are used to provide these functions.
