Operating Principles of DC Motors

Torque

• Torque refers to the force that causes the motor to turn; in DC motors ts operation is based on the attractive and repulsive properties of magnets. When the opposite poles of the two permanent magnets inside the motor repel and attract one another, a force is generated that makes the motor turn; the flow of current as the motor turns is then controlled by the electromagnet in the form of an electric coil located at the center of the motor.

Commutator

• A commutator is a switch responsible for reversing the direction of the electric current; it consists of bars of copper connected to the coils of the motor (armature). The commutator is stationary; it moves only when electric current is passed through the motor. Its operation is based on the principle that as electric current flows through and the armature, a force is generated that turns the motor. Additionally, current is switched to the adjacent bars of copper; this switching action transfers the electrical energy to the moving part that in turn maintains the motor's motion.

Permanent Magnet (PM) Motors

• Permanent magnet motors (PM) are probably the most commonly used DC motors with built-in commutation (mechanical brushes); they are also referred to as "DC brush motors." Such motors operate on the principle that when current is passed through the coils of the motor, a magnetic field results which rotates the motor, causing mechanical forces to move the coils on the rotor.

Brushless Motors

• Brushless motors have no commutator; however, the mechanical switching function performed by the brush and commutator in a brush-type DC motor is replaced by electronic switching. In this regard, the motion of the armature must be controlled by turning the electric energy on and off. Consequently, the torque is generated using a number of electromagnets that must be switched "on" and "off" by the use of electronic switches.

Motor Speed Control

• To manage the rotation speed of a DC motor, voltage must be controlled. This is accomplished using the principle that the higher the voltage is, the higher the speed of the motor will be. Since regulation of voltage can cause power loss on control circuit, a such motors use the pulse width modulation (PWM) method, in which the power that operates the motor is turned on and off to alter the current that flows to the motor.