As mentioned before, DC Motor is a category of electric motors which converts electric energy to mechanical energy. Most DC motors use magnetic field for this purpose. If look at the history of electric motors, you would find an argument between two great minds of all times, Nikola Tesla and Thomas Edison. Edison was the ultimate fan of DC motor and due to this eagerness and work, DC motor exists today.
Each DC electric motor consist of a stator and a rotor. Stator is the stationary part of the motor and rotor is the rotating part.
Stator of DC Motors
Stator is stationary. Electric current flowing through stator winding, a magnetic field is generated. Some DC motors use a permanent magnet as stator. Stator has a core and a winding. The core is responsible for focusing the magnetic field which is generated by the winding.
Rotor of DC Motors
As the name suggests, rotor is the rotating part. Rotor generates a magnetic field which in interaction with stator, make it rotate in a certain direction to make the mechanical energy. Rotor consist of core, commutator and coil.
We are going to talk more about stator and rotor in the introduction to AC motors.
DC motors has three sub-categories: brushed motors, brushless motors and servo motors.
Brushed Motors
In this type, magnetic field is generated by electric current flowing through carbon commutators and brushes. On DC and most AC motors the purpose of the commutator is to insure that the current flowing through the rotor windings is always in the same direction, and the proper coil on the rotor is energized in respect to the field coils. By mechanically positioning the brushes on the commutator, an angle of displacement can be set up between the magnetic force of the field windings and the magnetic force of the rotor windings.
Brushes are in contact with commutator and connects the winding to the external current. Brushes should have a good contact with the commutator to ensure orderly flow of the current. Not sufficient pressure can cause electric arc and failure of the motor. On the other hand, too much pressure can cause commutator over heating.
Brushless Motors
This type of motors employs a permanent magnet as the external rotor but the mechanism for stator remains the same as brushed motors. Accuracy and efficiency of this type of DC electric motors are higher than brushed motors. Also, as there is no carbon brush in the motor, there would be no electric arc and much less electric noise. Another advantage for these motors is their getting easier.
Servo Motors
Due to their low inertia, speed of servo motors can change fast and in a short time. This is the reason for these motors more usage. Utilizing a feedback control system, position, power and speed of servo motors can be changed. A Closed-loop Control System, also known as a feedback control system is a control system which uses the concept of an open loop system as its forward path but has one or more feedback loops (hence its name) or paths between its output and its input. Servo motors are divided into different types, such as series servo motors, split series motors, shunt control motors and permanent magnet shunt motors. Some advantages of servo motors include: accurate control of rotation speed in both directions, high torque to inertia ratio, quick response time and the ability of rotation direction.
Servo motors are used in transportation, elevators and lifts, fans, pumps, cranes and other industrial machineries.
In general, DC motors have the following pros and cons:
Pros:
- High starting torque
- High acceleration rate
- Accuracy of speed control in a large range
- Engineering possibility for manufacturing them in different sizes
Cons:
- Permanent maintenance needs
- High cost
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