Electric Motor Types: DC, AC, and Stepper Explained
Understanding DC Electric Motors
The simplest Direct Current (DC) electric motors, often utilized in battery-operated toys, feature a stator consisting of a cylindrical permanent magnet. Inside this is the rotor, which is constituted by electromagnets (coils surrounding a magnetic core).
The positive and negative poles supply current to the coil through brushes (wipers) and a commutator located at one end of the rotor. These commutator segments are connected to the coil threads, thereby receiving the current needed to produce the magnetism required to rotate the rotor.
In these motors, the core of the electromagnets is not a solid iron mass but is formed by thin, insulated, and highly compressed plates of magnetic iron. This ensures better overall performance because less heat is generated compared to a solid core.
Besides these simple motors, others are commonly used that do not operate with permanent magnets. Instead, the inductor, located in the stator, has electromagnets, which significantly improves motor performance. The rotor often has several electromagnets, typically in an odd number, which makes the engine work more regularly.
AC Electric Motor Variations
There is a variety of Alternating Current (AC) electric motors. Most AC motors receive power through collectors or brushes, much like DC motors do. These motors have a very regular speed because it depends on the frequency of the alternating current. If the frequency varies, which is easily achieved with modern electronic circuits, the speed can be changed.
Other AC motors, called induction motors, work without brushes or collectors. The oscillating nature of the alternating current itself creates a rotating magnetic field that causes the rotor to turn. These motors offer several advantages:
- Excellent power output
- Resistant to overloads
- Easy to start
- Can be stopped abruptly
Due to these qualities, they are widely used in machine tools, pumps, conveyors, elevators, etc.
Stepper Motor Operation
Generally, electric motors rotate continuously. At most, you can make them spin faster or slower, or change the direction of rotation.
However, some types of motors operate with non-constant motion. Based on electrical impulses, they turn in discrete steps of variable angles and can move forward or backward as needed. This type of motor, called a step-by-step or stepper motor, is especially useful for operating robots, particularly when pivoting arms or joints to a specific, determined angle.
Stepper motors have stators with several electromagnets located typically at 90° or 45° intervals. By controlling the polarity of these electromagnets, very small rotational increments can be achieved, which, with specially designed rotors, can be as small as 1.8 degrees.
While this type of motor has been known for years, its application wasn’t widespread until electronic systems, especially digital ones capable of creating pulses quickly and easily, were readily produced. In robotics, these motors are typically directed by computers.
Controlling DC Motor Speed
In DC electric motors, controlling the direction and speed of rotation is relatively straightforward:
- To change the direction of rotation: Simply reverse the polarity of the current supply.
- To change the rotation speed: Keep in mind that speed is proportional to the voltage supplied to the motor. Therefore, an input variable acts on the current collector (or power supply). As this voltage increases, the motor’s speed increases, reaching maximum speed only when working at full voltage/load.
Given these characteristics, DC motors are used in specific machines. They were especially indicated, for example, for electric railways and trams, although many of these now operate with alternating current.