An engine or motor is a machine designed to convert energy into useful mechanical motion.Wikipedia
Typically used in large machinery and appliances, such as washers, dryers, machine tools, etc, AC motors are considered to be more challenging to use than DC motors. They are not the focus of this document.
Commonly used in smaller applications, such as toys, robotics, and generally favored by hobbyists, designers, artists, and the like.
How a DC motor works
The movement of electric current through a magnetic field creates a physical force, known as the Lorentz force. The more current, the more force. In a simplified DC motor, a wire is coiled around an armature. This armature freely rotates on bearings between two magnets. When current flows down the wire, the Lorentz force causes movement in the wire. Because the wire is looped around a free-standing armature, the entire armature begins to rotate due to this force. This rotation movement is then transfered mechanically to the motor shaft, which spins with the wires.
There are many types of DC motors. Below are a few common varieties.
Basic DC motors
The most basic types of motor simply have two leads for electric connection, and a coil of wire that spins on an armature in relation to a stationary magnet.
Basic DC motors generally have high speed and low torque. This makes the usage of gears advantageous, since gears can be used to increase torque and decrease speed of the output shaft of the motor.
Servo motors contain a feedback circuit keeps track of how much the motor has rotated. Thus, if the motor comes against resistance or difficulty as it tries to rotate to its destination, it will continue pushing until it reaches its destination. Because of this feedback control system, servos are very reliable.
A third wire that connects to the motor can be used to instruct the motor (via pulse width modulation) exactly how much it should rotate. So, rather than spin freely, servos are usually used in situations where a precise amount of rotation is desired, and servos usually are generally designed to be incapable of rotating a full 360 degrees.
Servos enclosures contain the motor, the feedback circuit, and a set of gears to increase the amount of torque the motor can apply. The gears allow the motor to apply more torque (and thus can move heavier objects) than a basic DC motor without gears is capable of. The amount of torque a servo supplies is generally consistent, regardless of how fast it is rotating.
With some hacking skill, servos can be made to rotate freely, like basic DC motors. In these cases, servos can be used anywhere a regular DC motor can be used, but apply more torque because of their gears.
One of the other advantages of servo motors is that they are cheap. Since they are used commonly in toy airplanes and other small devices, they are produced in large quantities and can are a good solution for situations where a motor with a gearbox is desired for cheap.
Stepper motors, like servo motors can be used to perform a precise amount of rotation. Stepper motors contain multiple magnets that can be activated in sequence to move the rotating wire-coil armature in discrete steps. Often, the stepper motor will have separate sets of wires for each magnet, and thus require a motor controller chip or other motor-controlling circuit dedicated to activating the multitude of wires becomes preferable.
Unlike servo motors, stepper motors operate in an open loop configuration, meaning they do not have a feedback circuit that keeps track of exactly how much they have rotated.
Movement of stepper motors at low speeds tends to be choppy, since they move in discrete steps rather than continuously. But, stepper motors are able to sit in a stationary, non-moving, position while maintaining a high level of torque due to the nature of the exact magnetic control of their position. Because they actively hold stationary positions, they consume current even when they are not moving.
Stepper motors, like servos, can often be found at inexpensive prices.
Vibrators are often just basic DC motors with a weight offset to the rotating shaft. When the shaft rotates, the uneven distribution of the attached weight causes the whole device to wobble.
Shape memory alloy, a.k.a. muscle wire, is a material which “remembers” its original shape. If you heat the alloy, it will return to the original shape in which it was formed. Simply supplying a current to the alloy is often enough to heat it and have it return to ots original shape.