A direct current – DC motor is an electric machine that converts electrical energy to mechanical energy. DC motors are one of two general types of motors, the other being the alternating current – ac motor. DC motors have a few basic components including the commutator, armature, a stator and a rotor.
All dc motors generate a magnetic field, either via electromagnetic windings or permanent magnets. An armature, which is often a coil of wires, is placed between the north and south poles of a magnet. When current flows through the armature, the field produced by the armature interacts with the magnetic field from the magnets and eventually generates a torque and thereby motion. Part of the simplicity and attractiveness of dc motors is that they operate at a constant speed for a fixed voltage.
DC motors are available in a few basic configurations; the basic brushed dc motor, brushless motor, permanent magnet motor and wound-field motors.
In a brushed dc motor, the magnet acts as the stator. The armature is integrated onto the rotor and a commutator switches the current flow. The commutator’s function is to transfer current from a fixed point to the rotating shaft. Brushed dc motors generate torque straight from the dc power supplied to the motor by using internal commutation, fixed permanent magnets, and rotating electromagnets.
Brushed dc motors have the advantage of generally low initial cost and simple control of the motor speed. However, there are some drawbacks. At certain periods during the dc motor rotation, the commutator must reverse the current, causing reduced motor life due to arcing and friction. Consequently, brushed dc motors require more maintenance such as frequently replacing the springs and brushes which carry the electrical current, as well as replacing or cleaning the commutator. These components are important for transferring electrical power from outside the motor to the spinning coil windings of the rotor inside the motor.
Brushless dc (BLDC) motors, on the other hand, do away with mechanical commutation in favor of electronic commutation, which eliminates the mechanical wear and tear involved with brushed dc motors. In BLDC motors, the permanent magnet is housed in the rotor and the coils are placed in the stator. The coil windings produce a rotating magnetic field because they’re separated from each other electrically, which enables them to be turned on and off. The BLDC’s commutator does not bring the current to the rotor. Instead, the rotor’s permanent magnet field trails the rotating stator field, producing the rotor field.
Field-wound dc motors use a coil to generate a magnetic field and are used where high power and constant horsepower is required. Among the field-wound motors there are series-wound, shunt-wound, and compound-wound configurations. The armature and the field coils in a shunt-wound motor are connected in a parallel formation that causes the field current to be proportional to the load on the motor, while in series-wound motors, the armature and field coils are placed in series and the current passes only through the field coils. The compound-wound configuration combines both the series-wound and shunt-wound types and uses both configurations.
Lastly, the permanent magnet (or PM) motor uses permanent magnets such as high-energy neodymium to generate a magnetic field. These motors tend to be smaller and weigh less than the other common types of dc motors and so are well suited for applications where size and weight are important factors. One limitation of PM motors is that they cannot be speed controlled because the magnetic field is fixed.
Content provided by Design World.