A direct current or DC motor, converts electrical energy into mechanical energy. It is one of two basic types of motors: the other type is the alternating current or AC motor. Among DC motors, there are shunt-wound, series-wound, compound-wound and permanent magnet motors.
A DC motor consists of an stator, an armature, a rotor and a commutator with brushes. Opposite polarity between the two magnetic fields inside the motor cause it to turn. DC motors are the simplest type of motor and are used in household appliances, such as electric razors, and in electric windows in cars.
Basic DC Motor Operation
A DC motor is equipped with magnets, either permanent magnets or electromagnetic windings, that produce a magnetic field. When current passes through the armature, also known as the coil or wire, placed between the north and south poles of the magnet, the field generated by the armature interacts with the field from the magnet and applies torque. In a DC motor, the magnet forms the stator, the armature is placed on the rotor and a commutator switches the current flow from one coil to the other. The commutator connects the stationary power source to the armature through the use of brushes or conductive rods. Furthermore, DC motors operate at a fixed speed for a fixed voltage and there is no slip.
DC Motor Types
The field coils and the armature in a shunt-wound motor are connected in parallel, also known as shunt, formation, causing the field current to be proportional to the load on the motor.
In series-wound motors, the field coils and armature are connected in a series and the current flows through the field coils only.
A compound-wound motor is hybrid of both the shunt-wound and series-wound types and features both configurations. The field in a permanent magnet motor is created by permanent magnets as the name allows.
The Commutator Problem
The function of the commutator is to transfer current from a stationary point to the rotating shaft. Brushes, electric contacts on the commutator, connect the power source to the armature. At certain times during the rotation of a DC motor, the commutator must reverse the current. This can lead to decreased motor life because of the wear and tear that occurs through friction and arcing.
Brushless DC Motors
Brushless DC motors alleviate the wear and tear on the commutator in conventional DC motors. In these types, the permanent magnet is located in the rotor and the coils are located in the stator. The coil windings create a rotating magnetic field because they are separate from each other electrically, which allows them to be turned on and off. The commutator in this motor does not carry the current to the rotor. Instead, the rotor's permanent magnet field chases the rotating stator field, making the rotor field. One other key component of the brushless DC motor is the use of electronic circuitry and sensors with the commutator to excite the motor to produce torque.
- Electric Motors and Drives; Austin Hughes; 2006