DIRECT CURRENT (DC) ELECTRIC MOTOR

A direct current (DC) electric motor is an electrical machine that converts direct current (DC) electrical energy to mechanical energy.

The main components of a direct current (DC) electric motor are; stator, rotor, armature and commutator.

The stator is the stationary part of the motor that provides a magnetic field.

The rotor is the rotating part of the motor that interacts with the stators magnetic field.

The armature is the coil of wire that carries the current.

The commutator is the mechanical device that switches the direction of the current flowing through the armature.

The direct electric motor in its principle of operation consists of a rotor mounted windings or armature and a stationary windings or fixed poles. Current is conducted to the armature windings through carbon brushes that slide over a set of copper surfaces called a commutator, which is mounted on the rotor. This fact is true for all dc motor except permanent magnet motors and the brushless dc electric motors.

The brush and commutator combination makes a sliding switch that energizes particular portion of the armature based on the position of the rotor. This creates north and south magnetic poles on the rotor that are attracted to or repelled by the north and south poles on the stator, which are formed by passing direct current through the field windings.  It is this magnetic attraction and repulsion that causes the rotor to rotate.

Direct current (DC) electric motors are either brush or brushless in their construction and features.

There are various types of brush direct current (DC) motors and they are; shunt wound, separately excited, series wound, compound wound, permanent magnet, servo and universal motors.

Shunt wound motors have their armature and field windings connected in parallel. They are characterized by low starting torque, high speed and high efficiency in their operation.

Their high efficiency and easy speed control makes them ideal for lathe machines, drilling machines and printing presses.

Separately excited DC electric motors have their field windings connected to a separate power source. They are characterized by high efficiency, high reliability and easy speed control in their operation. They are most suited for applications in industrial automation, aerospace and automotive industries.

Series wound DC electric motors have their armature and field windings connected in series. They are characterized by high starting torque, high speed and low efficiency at high speeds in their operation. They are most suited for use in hoist, cranes and conveyor belt applications.

Compound wound DC electric motors have a combination of series and shunt wound motors. They are characterized by high starting torque, high speed and high efficiency in their operation. They are most suited for heavy duty applications such as motors of hoist, cranes and conveyor belts.

Permanent magnet DC electric motors use permanent magnets instead of electromagnets. They are characterized by high efficiency, high reliability and low maintenance in their operation. They are most suited for use in automotive, aerospace and consumer electronics.

Servo DC electric motor uses a positional feedback mechanism to control rotation. They are characterized by high precision, high torque and low speed in their operation. They are suitable for use in robotics, CNC machines and industrial automation applications.

Universal DC electric motors are DC motors that can operate on both DC and AC electric power sources. They are based on brush construction and are series wound motor design. They are characterized by high starting torque, low maintenance and simple construction making them inexpensive to manufacture. On the contrary their low efficiency, limited speed range of only up to 3000rpm and their susceptibility to noise and vibration make them only useful as electric motors for power tools, appliances, industrial equipment and various automotive application such as windshield wipers, power windows and fuel pumps.

Brushless DC electric motor uses a controller to switch the flow of current to the motor windings eliminating the need of brushes and a commutator. They are characterized by high efficiency, high reliability, long life, high speed and low noise. They find application in the aerospace, automotive, medical and consumer appliance industries.

The main advantages of direct current (DC) electric motors are that in most cases they generate a high starting torque, easy speed control using only variable electronic controllers, they generally have high efficiency as high as 90% and above and they require minimal maintenance because of their fewer moving parts.

Their obvious disadvantages are their high brush and commutator wear rate necessitating frequent changes. They also have limited overload capacity making them unsuitable for applications that require overload capacity. They are also very sensitive to voltage fluctuation as well the need to convert from grid AC supply using some sort of rectification to DC.

Direct current (DC) electric motors find widespread application in the following industries: industrial automation as motors for conveyor belts and pumps. Transportation as motors for electric vehicles, and other road vehicles. They also find use in the aerospace and marine industries as motors for various applications. They are useful in Industrial and construction industries as motors for cranes, hoist and forklifts etc. finally they also find application for consumer appliances as motors for vacuum cleaners, power tools, electronics etc.

The future of direct current electric motor is based on the following assumptions; advances in materials that would improve its power density hence its overall efficiency and performance. Increase use of electric vehicles which will go a long way to ginger a sleeping industry in all aspects such as research and development and the integration of direct current motor with other emerging technologies such as artificial intelligence and internet of things.

 

SOURCES:

• Electric motors by Uday A. Bakshi and Mayuresh V. Bakshi.
• DC motors by Irving L. Kosaw.
• Electric motor control: DC,AC and BLDC motors by Sang-Hoon Kim.
• Electric motor handbook by Syed A. Nasar.
• Electric motor design and application by Richard H. Engelmam.