From DT Online
Electric motors are devices for changing electrical energy into rotational movement. The turning power or torque of a motor can vary greatly. Generally speaking small motors have low torque whereas large motors usually have a high torque rating.
Features and Applications
Motors come in alternating current (AC) and direct current (DC) versions, DC types of between 3 - 12 volts being most suitable for general use. The current direction controls the direction of rotation for DC motors.
It is usually necessary to ‘gear down’ a motor as their speed of rotation (typically 2400rpm) is too great. This can be achieved either mechanically or electronically.
DC Motors have the advantage of being relatively inexpensive and can be obtained in high torque versions which are most suitable for driving models. The motor's speed can be controlled over a wide range (e.g. using a variable supply voltage)
Stepper Motors respond to electrical pulses sent from an external Driver Circuit or via computer or micro-processor control. Each pulse causes the motor to rotate by a single increment (or Step) and then lock in that position. The process can be repeated and a continuous stream of pulses will result in full rotations but the motors are used mainly to turn to a precise angle or through a set number of rotations.
Model DC Motors in Transistor Circuit
DC motors are available in a range of voltages and power outputs. Some are also available with an integral earbox to reduce the output shaft speed. Small DC motors are designed for light duty work such as model systems, rotating signs etc.
Reversing the polarity of the supply to the motor causes it to rotate in the opposite direction. Stopping the motor by removing the supply and then connecting the two motor terminals together causes the motor to be braked rapidly.