DC Motors: Working, Types, Uses and Advantages

What is a DC Motor?

A direct current (DC) motor is an electric machine that converts electrical energy into mechanical energy through the production of magnetic fields powered by direct current.

When a DC motor is energized, it produces a magnetic field within the stator of the motor that interacts with magnets on the rotor, causing it to rotate.

DC motors have a commutator which uses brushes that are connected to the power source to send current into the winding of the motor so the rotor can continue to rotate.

DC motors are used more than most other motor types because of their ability to control speed, which is important for industrial applications.

They can instantly start, stop, and reverse. All of these capabilities are important for controlling the operation of production equipment.

How DC Motors Work?

A DC motor is based on the fact that a conductor carrying current in a magnetic field produces mechanical force. Using the left-hand rule, one can determine the direction of the mechanical force to be in.

DC motors are constructed similarly to DC generators, which allows them to be used interchangeably.

AC is converted to DC in heavy electrical applications, such as steel mills and electric trains, because the speed, torque, and operational characteristics of DC will be found superior to AC.

For industrial applications, DC motors have been as widely applied as three-phase induction motors.

Stator

The stator is the stationary primary portion of the motor that supports and encases the motor. It produces rotational magnetic fields to rotate the armature or rotor.

The stator is stationary as it contains the field windings, and electrical power is supplied to the windings by the terminals on the stator.

Shaft

In a DC motor, the windings and the commutator turn the shaft, which is a major component of the motor and is typically manufactured from hardened steel for the load of the application.

Commutator bars are connected to a plate which is secured to the shaft through the process of plastic molding.

The torque created by the winding is passed through to the shaft which is held by the stator. The shaft passes through the stator and mechanically connects the motor to the application.

Terminals

There are two terminals on a DC motor, positive and negative.To make the motor rotate in a clockwise direction, the positive wire is connected to the positive terminal and the negative wire is connected to the negative terminal.

Reversing the wires will cause the motor to rotate counterclockwise. The terminals are connected to the two brushes and brush arms found in the back cover to provide power to the motor.

Magnets

The magnets used in DC motors are called permanent magnets, meaning the magnetic field is always active. Magnets have two poles, opposite poles attract each other and like poles repel each other.

A magnets magnetic field travels from the south pole, through the magnet, to the north pole, the strongest field is located at the ends of the magnet.

Two magnets are used in DC motors for a strong field. The placement of the magnets are set around the rotor so the strong magnetic field goes through the rotor and enhances the motor performance.

Rotor

The rotor, or armature, consists of multiple disks that have insulating laminated sheets insulating them from each other. This reduces the ability of large eddy currents to be formed and slows the impact of eddy currents.

Eddy currents are present, but will not impede the motors performance like it would if the laminated sheets were not used.

Larger disks would impact motors performance, and to improve motors performance the disks needs to be the smallest they can possibly get. The rotor is the moving part of the motor that creates mechanical revolutions.

Coil Windings

The coil windings were wound around the rotor. The wire was coiled and thus created a strong, as well as a powerful magnetic field. All types of wire produce a weak magnetic field when electricity flows through the wire.

By coiling the wire, each coiled piece has the same weak magnetic field. When all the different coiled wire is added together, the magnetic field created becomes strong.

The rotor becomes smoother as more coils are turned and added. All DC motors have three coils at minimum, because two coils can jam, stopping the motor. There is a 120^o distance between each coil.

Brushes

The brushes in a DC MOTOR supply power to the coils, and they are made out of metal.  One side of the brushes is made out of a conductive material, generally carbon, and the other part is a pin for connecting the brushes to the power supply.

The spring of the brushes keeps them pressed against the commutator, and the brush arms hold the brushes in position. The brushes are connected directly to the terminal, or electrical supply, of the motor.

Commutator

The commutator is made up of small copper plates that are located on the shaft, and rotate with the shaft. The commutator connects the coils to the power supply as the shaft turns and reverses the polarity of current.

Each coil has two commutator plates that are electrically isolated from one another by are connected by the coils.

When the positive and/or negative terminals of the power supply is on one of these commutator plates, current will flow easily and create an electromagnetic field.

Types Of DC Motors

To appreciate the advantages of DC motors in an application, a grasp of the many varieties of DC motors available is also integral.

Each type of DC motor possesses beneficial properties that must be considered prior to purchase and subsequent application.

Two of DC motors advantages over alternating current (AC) motors are their relatively simple installation and they usually require little maintenance.

DC motors are classified on the basis of the connection between the field winding and armature.

The field winding may be connected in parallel with the armature, it may be connected in series with the armature or in some instances, it may be a combination of both a parallel connection with the armature and series connection with the armature.

A further classification of DC motors is how the rotor is energized; brushed or brushless. In a brushed DC motor, the brushes are used to transmit current to the rotor. In the brushless DC motor, the rotor has a permanent magnet.

DC motors are utilized in a large array of applications, and different types of motors can be properly implemented to fulfill their rather specific purpose.

They are so widely used, it is necessary to have an understanding of each type of DC motor, because they impact many areas of our everyday lives.

Brushed DC Motor

In a brushed DC motor, the magnetic field is generated by current travelling through a commutator and brushes leading to the rotor. The brushes although they use carbon are either separately excited or self-excited.

The motor components are found within the stator, and the magnetic field is created in the stator, which surrounds the rotor.

The windings of the rotor coil can be wired in series or parallel, as such the two varieties of DC motors can physically be referred to as a series(-wound) DC motor or Concurrent(-wound) DC motor.

The commutator serves the function of an electrical switch by establishing the current direction between the rotor and the source of external power.

The commutator regulates electrical current to the windings and creates a consistent rotational torque by reversing the current direction.

The commutator is made up of several sections which are connected to the rotor windings with contact bars embedded in the shaft of the motor.

Three types of DC motors exist: separately excited, self-excited, or permanent magnet. Separately excited and self-excited motors utilize an electromagnet in the stator configuration. The permanent magnet configuration uses a powerful magnet to generate the magnetic field.

Self-excited DC motors are classified into three broad categories by the fields windings: shunt, series, and compound. The compound motor is classified further by cumulative and differential, all which incorporate long and short shunts.

Separately Excited DC Motor

The separately excited DC motor reference to a motor type having a separate electrical supply to the armature winding and field winding.

The field winding and armature winding are electrically isolated and the operation of the armature current will have no impact on the field current.  However, the total input power for the instruction will be the sum of both currents.

Permanent Magnet DC Motor

The permanent magnet type of DC motor does have an armature winding, but does not have a field winding. It employs a permanent magnet affixed to the inside surface of the stator core for the magnetic field.

A permanent magnet DC motor is commonly classified as a normal armature with a commutator and brushes.

Permanent magnet dc motors are generally smaller and less expensive.They use rare earth magnets, such as samarium cobalt or neodymium iron boron, for performance improvement.

Self Excited DC motor

In self excited DC motors, the field and armature windings use the same power supply. The connections are either parallel or series, with parallel connections termed shunt wound and the series connections called series wound.

Shunt

In a shunt wound DC motor, the armature and field windings are connected in parallel, and the field winding is across the terminal voltage, meaning the terminal voltage is across the field winding.

The armature and field winding are both subjected to the same supply voltage but draw different currents. This model gives consistent speed which is resistant to fluctuations due to mechanical load.

Series

In a series wound DC motor, the field and armature windings are connected in series, meaning the same current is flowing through the armature and field windings.

Due to this connection, the series type design allows the motor to be powered in both AC and DC voltage and is therefore a universal motor.

Series motors will rotate in the same direction with either the AC or the DC voltage. The speed will fluctuate with the mechanical load.

Compound

A compound DC motor utilizes armature winding in series and field winding in parallel (shunt) form.

Compound DC motors are either cumulative or differential form. In cumulative compound DC motors, the extra flux created by the shunt field enhances the flux created by the armature series field, and both fields are in the same direction.

On the other hand, in differential compound DC motors, while the shunt field generates flux, this flux opposes flux from the series field.

Both kinds of compound DC motors (cumulative and differential) can have either long or short shunts based on how the shunt field winding is connected.

Brushless DC Motor (BLDC)

Brushless DC Motors (BLDC motors) are permanent magnetic synchronous motors that run on direct current (DC) and utilize electronic commutation.

This essentially changes the phase currents to produce rotational torque. Some researchers refer to BLDC motors as trapezoidal permanent magnet motors.

In BLDC motors, the rotor is permanent magnets and the stator is a series of coils. Unlike brushed DC motors, where mechanical contact occurs with the rotor, BLDC motors have electronic commutation. The rotor turns when current-carrying conductors on the stator remain stationary.

In a typical BLDC motor, the armature coils are switched electronically using transistors, which are regulated using Hall sensors on the stator to detect the rotor position.

Hall sensors used for feedback control will indicate when to switch the current in the armature coils to create torque, resulting in rotor rotation.

The lack of brushes in the construction of the motor makes it more reliable and quieter, with efficiencies between 85 and 90 percent. Moreover, the simplicity of design eliminates brush wear over a extended period and little heat is produced by the rotating magnet generates little heat in a BLDC motor.

Brushless DC Motor Construction

There are many versions of BLDC motors depending on their winding arrangment of the stator (1-phase, 2-phase, or 3- phase). The majority of BLDC motors have a three-phase permanent magnet rotor.

All BLDC motors have a stator having the same number of windings, and are divided into 2 different types, those being inrunners and outrunners.

With an inrunner brushless motor, the permanent magnets are contained inside the electromagnets, while in an outrunner motor, the permanent magnets are located outside the electromagnets. Although the configurations are different, both are based on the same principles.

Stator

The stator is a component that generates the magnetic energy to rotate the rotor in a brushless DC motor. The stator can be in either of 2 locations – the stator can be surrounded by the rotor or encasinate the rotor.

The stator is built from laminated steel stampings that are stacked into a magnetic core, the coils of wire are wound about the core, and connected to the motor controller.

The stator’s steel can be slotting or slotless. Slotless cores support high speed motors due to their low inductance, but they are typically more expensive because they require more turns of coil.

Rotor

The rotor will have a permanent magnet, ranging from 2 to 8, pair, with alternate south and north poles. The magnet material is selected carefully in order to create the necessary magnetic field density. Magnet types are either ferrite or neodymium.

Rtors can be configured to be either round or rectangular cores with permanent magnets placed on the circumference, or a round core with rectangular magnets.

Hall Sensor

Hall sensors play an important role inecting the position of the rotor, coordinating the excitation of the stator armature. Hall sensors allow for electronic commutation of BLDC motors by controlling the sequence in which the stator windings are energized to move the rotor.

Before energizing any winding, a Hall sensor first determines the position of the rotor. The typical BLDC motor has three Hall sensors located in the stator that output a low and high digital signal as the rotor poles are in close proximity to them.

Advantages of BLDC Motor

• No mechanical commutators to wear out
• High efficiency
• High speeds both loaded and unloaded
• Small motor geometry with lower total motor weight
• Long life
• Faster dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Quiet operation and reduced emitted noise

Servo DC Motor

A servo DC motor consists of four main components: a DC motor, gearbox, control circuit and a position sensing device. With a gearbox, the high-speed input is converted into a manageable slower output speed.

The control circuit acts as an error detecting amplifier and receives feedback about the position of the shaft from the position sensing device creating If there is a disparity between the actual shaft position and the reference position, the error detecting amplifier produces an error signal correcting the difference.

Applications of DC motors

DC motors are used in many applications because they have such excellent starting torque, especially compared to that of an induction motor. Brushed DC motors are small, have exact rotational control, and are very efficient.

Brushless DC motors are reliable, life long and need no service because there are no brushes to wear out, require little maintenance, and are no more noisy than AC motors.

Even though the designs are different these motors have found unlimited applications and have provided mechanical power for more than 130 years.

These type of motors are used in so many respects and processes, for everything from ceiling fan motors to drive motors for large commercial printing presses.

The following is a very short list of just a few products that DC motors are used in.

Diesel-Electric Locomotive

In a diesel electrolyte locomotive , the combustion product from the diesel engine is converted to rotational energy to be coupled with a generator to generate electrical energy, which is then fed to the DC motors coupled with the wheels of the engine.

Electric vehicle

Brushed DC motors are used in electric vehicles for retracting and positioning electrically powered windows. Because brushed motors do oxidize quickly, brushless motors have become prevalent in electric vehicle applications, as they are silent and have a long lifetime.

Brushless DC motors are common applications in windshield wipers and CD players. All of the new hybrid electric vehicle applications have embraced brushless DC motors.

Cranes

With applications where the motor is used for overhauling loads, where the motor must hold the full load at zero speed without mechanical brakes, we can choose DC motors that are economical and safe. The enormous size and weight savings are significant advantages. DC motors are suited for these applications.

Conveyor Systems

Conveyor systems for constant speed with high torque is another good application for DC motors. As DC motors have high start torque and provide a constant speed during run time, they mesh with conveyor systems.

Brushless DC motors in conveyor applications are also desirable for their noiseless operation and high level of control required to make an efficient conveying system. 

Ceiling Fans

Another type of application using DC motors is ceiling fans. DC motors have become popular in ceiling fans due to the reduced power requirement and the quick start torque of the ceiling fan.

The alternating current of homes or offices specific transformer changes the alternating current to DC – which reduces the amount of power being used by a ceiling fan. Brushless DC motors are common in ceiling fans due to their efficiency and noiseless operation.

Pump Drives

Variable speed control has attracted DC motors for many years as reliable pump drives. Because of the simple controllable system also, DC motors have good starting torque, which makes them good for pump systems, and good transient response.

Until the invent of permanent magnet DC motors and brushless DC motors, we typically used brushed DC motors.

Elevators

In high-speed elevators, there are many instances where AC motors can not slow and stop smoothly, and accurately level with the desired floor. DC motors provide a means of controlling speed more precisely by varying the current in to the armature.

Like the ceiling fan application, DC motors use a transformer to convert incoming alternating current to direct current for optimal operating characteristic.

Advantages Of DC Motors

DC motors are in increasingly in demand especially for the 12 V and 24 V applications. The explosion of markets utilizing solar, marine, and truck mounted equipment providing a reliance on a costly, yet effective, DC motor technology.

While dated compared to AC motor technology, DC motor manufacturers continuously develop and engineer developments to reduce the maintenance requirements to DC motors and to increase operational life.

DC motors have many types and they can adapt to all applications. Therefore, it is important to do your research and use the DC motor that is capable of performing under the particular load conditions.

Startup Torque

DC motors are typically recognized as having a large startup torque. DC motors work well where speed is constant with a variable torque.

Linear Speed Torque

The torque-speed curve explains the torque-power relationship where the torque-speed curve explains how fast the motor can spin while concurrently showing how much torque it can produce.

DC motors have an exceptional, more linear speed-torque curve compared to both universal and AC motors.

No Harmonics

The harmonic effects of a power system can severely reduce the performance of that system causing safety risks, systems damage, and equipment damage. In any case, DC motors do not operate by harmonic effects, therefore it operates as it should without the complications of harmonic distortion.

Speed Control

DC motors are recognized for speed control. Critical speed control is required for heavy-loaded systems. This is also one reason why DC motors are popular in roll mills and paper mills. Therefore, DC motors typically provide a constant speed throughout an operational period.

Installation

DC motors are easy to install. DC motors require fewer electronic alterations the power system needs. When it comes to connecting DC motors, the installation can be direct to the power source. After this is completed, the motor is operational.

Maintenance

Since the design of DC motors is simple, they can be easily repaired or replaced. Having been in use for 130 years as an electric motor, DC motors are well known by technicians and electrician staff therefore maintenance costs are relatively low.

For as long as DC motors have been implemented, intelligent input is present with the ability of diagnosing and resolution.

Field excitation is not required for DC motor service. Brushes and speed selections can be easily replaced. Terminal voltage can be adjusted, following repairs, by simply adjusting the potentiometer of the control system.

Repairs- Cost

Cost is a primary reason for selecting a DC motor. Generally, DC motors are less expensive than AC motors, except brushless and permanent magnet DC motors, and can be even higher in cost; however, brushless motors tend to last longer than their initial investment.

As opposed to keeping the low cost of a brushed motor, with a shorter operational life and higher repair period. Maintenance costs of the brush DC motor’s electrical system are typically low and can sustain repair and maintenance from a longevity viewpoint.