DC Motors

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Introduction

This article takes an in depth look at DC motors.

You will learn more about topics such as:

• What is a DC motor?
• Types of DC motors
• How DC motors work
• Uses for DC motors
• And much more&#;

Chapter One &#; What is a DC Motor?

A DC motor, or direct current motor, is an electrical machine that converts electrical energy into mechanical energy through the creation of a magnetic field powered by direct current. When a DC motor is energized, it generates a magnetic field in its stator. This magnetic field interacts with magnets on the rotor, causing it to rotate. To ensure continuous rotation of the rotor, the commutator, connected to brushes that are linked to the power source, supplies current to the motor&#;s wire windings.

DC motors are often preferred over other types of motors due to their precise speed control, which is crucial for industrial machinery. They can start, stop, and reverse instantly, providing essential control over the operation of production equipment.

Chapter Two &#; What are the different types of DC motors?

In order to appreciate the benefits of DC motors, it is important to understand the various types. Each type of DC motor has beneficial characteristics that must be examined before purchase and use. Two of the main advantages of DC motors over alternating current (AC) motors are how easy they are to install and that they require little maintenance.

DC motors are classified based on the connection between the field winding and the armature. The field winding may be connected in parallel with the armature, in series with it, or in some cases, a combination of both parallel and series connections.

A further distinction of DC motors is how the rotor is powered; it can be brushed or brushless. In brush DC motors, current is applied to the rotor by brushes. In a brushless DC motor, the rotor has a permanent magnet.

DC motors are widely used across various applications, with different types available to suit specific needs. Understanding each type is important, as DC motors play a role in many aspects of everyday life.

Brushed DC Motor

In a brush DC motor, the magnetic field is created by the current flowing through a commutator and brushes connected to the rotor. The brushes, typically made of carbon, can be either separately excited or self-excited. The stator houses the motor's components and generates the magnetic field. The rotor's coil winding can be configured in series or parallel, resulting in either a series-wound or shunt-wound DC motor.

The commutator acts as an electrical switch, reversing the current between the rotor and the external power source. This mechanism applies electrical current to the windings, producing consistent rotational torque by alternating the current direction. The commutator sections are linked to the rotor windings through contact bars embedded in the motor's shaft.

There are three main types of DC motors: separately excited, self excited, or permanent magnet. In the separately excited and self excited, an electromagnet is used in the stator structure. With the permanent magnet type, a powerful magnet generates the magnetic field.

Self-excited DC motors are categorized into three main types: shunt, series, and compound. Compound-excited motors are further divided into cumulative and differential, with each type featuring both short and long shunts.

Separately Excited DC Motor

In a separately excited DC motor, the armature winding and field winding have separate electrical supplies, making them electrically independent. The operations of the armature current and field current do not affect each other, but the total input power is the sum of both currents.

Permanent Magnet DC Motor

A permanent magnet DC motor features an armature winding but lacks a field winding. Instead, it uses a permanent magnet mounted on the inner surface of the stator core to generate the magnetic field. This type of motor includes a standard armature with a commutator and brushes.

Permanent magnet DC motors are typically smaller and more cost-effective. They utilize rare earth magnets, such as samarium cobalt or neodymium iron boron, to enhance performance.

Self Excited DC Motor

In self-excited DC motors, the field and armature windings share a single power supply. The connections can be either parallel or series: parallel connections are referred to as shunt-wound, while series connections are known as series-wound.

Shunt

In a shunt-wound DC motor, the field and armature windings are connected in parallel, with the field winding exposed to the terminal voltage. Although both windings receive the same supply voltage, they draw different currents. This configuration results in a constant speed that remains stable despite changes in mechanical load.

Series

In a series-wound DC motor, the field and armature windings are connected in series, so the same current flows through both windings. This design allows the motor to operate with both AC and DC voltage, making it a universal motor. Series motors always rotate in the same direction regardless of the voltage source, but their speed varies with the mechanical load.

Compound

A compound DC motor combines features of both series and shunt field windings. In this design, the armature winding is connected in series, while the field winding is connected in parallel (shunt).

Compound DC motors are further categorized into cumulative and differential types. In cumulative compound DC motors, the flux from the shunt field enhances the flux from the series field, with both fields working in the same direction. Conversely, in differential compound DC motors, the flux from the shunt field opposes the flux from the series field. Both cumulative and differential compound DC motors can have either long or short shunts, depending on the configuration of the shunt field winding.

Brushless DC Motor (BLDC)

Brushless DC motors (BLDC motors) are a type of permanent magnet synchronous motor that operates on direct current with an electronically controlled commutation system. This system produces rotational torque by sequentially changing the phase currents. BLDC motors are sometimes referred to as trapezoidal permanent magnet motors.

Unlike brushed DC motors, which rely on mechanical contact with the rotor, BLDC motors use electronic commutation. In a BLDC motor, the rotor is made of permanent magnets, while the stator contains a series of coils. The rotor rotates as the stator's current-carrying conductors remain fixed in position.

The armature coils in a BLDC motor are electronically switched by transistors based on the rotor's position, which is detected by Hall sensors mounted on the stator. This feedback mechanism ensures the correct timing for switching the current in the armature, resulting in rotor rotation.

The absence of brushes in BLDC motors enhances their reliability and reduces noise, with efficiency ratings between 85 and 90 percent. The design eliminates brush wear and minimizes heat production, as the rotating magnet generates very little heat.

Brushless DC Motor Construction

BLDC motors come in various configurations, primarily differentiated by their stator windings, which can be single-phase, two-phase, or three-phase. Most BLDC motors feature a three-phase design with a permanent magnet rotor. Regardless of the type, all BLDC motors have a stator with the same number of windings.

BLDC motors are also categorized into inrunner and outrunner types. In an inrunner brushless motor, the permanent magnets are located inside the electromagnets, whereas in an outrunner motor, the permanent magnets are situated outside the electromagnets. Despite these different configurations, both designs operate on the same fundamental principle.

Stator

The stator generates the magnetic force that drives the rotation of the rotor in a brushless DC motor. It can be positioned either inside and surrounded by the rotor or outside encasing the rotor. The stator is composed of laminated steel stampings stacked to create a magnetic core. Coils of wire are wound around this core and connected to the motor controller.

The stator's steel components can be either slotted or slotless. Slotless cores are designed to produce high-speed motors due to their lower inductance but are more expensive because they require more coil turns.

Rotor

The rotor contains a permanent magnet with two to eight pairs of poles with alternate south and north poles. The magnetic material for the rotor is carefully chosen in order to produce the required magnetic field density. The types of magnets for the rotor can be ferrite or neodymium.

Core configurations for rotors can be circular with permanent magnets arranged on the periphery or circular with rectangular magnets.

Hall Sensor

Hall sensors play a crucial role in synchronizing the excitation of the stator armature by detecting the rotor's position. They enable electronic commutation of BLDC motors by sequencing the energization of the stator windings to rotate the rotor. Before a winding is energized, the Hall sensor determines the rotor's position. Most BLDC motors are equipped with three Hall sensors located in the stator. Each sensor produces a low and high signal as the rotor poles pass in proximity to them.

Benefits of BLDC Motor

• Absence of mechanical commutator to avoid wear
• High efficiency
• High speed of operation in loaded and unloaded conditions
• Smaller motor geometry and lighter weight
• Long life
• Higher dynamic response because of low inertia and carrying windings in the stator
• Less electromagnetic interference
• Low noise and quiet operation

Servo DC Motor

A servo DC motor consists of four main components: a DC motor, a gearbox, a control circuit, and a position sensing unit. The gearbox converts high-speed input into a more manageable, slower output speed. The control circuit, which functions as an error detector amplifier, receives feedback about the shaft's position. This feedback creates a closed-loop system. If there is any discrepancy between the current shaft position and its reference position, the error detecting amplifier generates an error signal to correct the mismatch.

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Chapter Three &#; How DC Motors Work?

A DC motor operates on the principle that when a current-carrying conductor is placed within a magnetic field, it generates mechanical force. The direction of this force is determined using the left-hand rule. DC motors and DC generators have similar constructions and can be used interchangeably.

In large electrical applications, such as steel mills and electric trains, alternating current (AC) is often converted to DC because DC motors offer superior speed and torque characteristics compared to AC motors. For industrial uses, DC motors are as commonly employed as three-phase induction motors.

Stator

The stator is the stationary main component of the motor, providing support and protection. It generates a rotating magnetic field that drives the armature or rotor. As the static part of the motor, it houses the field windings and receives electrical power through its terminals.

Shaft

The windings and commutator work together to rotate the shaft, which is centrally located in the motor and typically made of hardened steel to handle application loads. The commutator bars are attached to a plate fixed to the shaft through plastic molding. The torque generated by the windings is transferred to the shaft, which is supported by the stator. The shaft extends through the stator, connecting the motor to the application.

Terminals

A DC motor features two terminals: positive and negative. Connecting the positive wire to the positive terminal and the negative wire to the negative terminal makes the motor rotate clockwise. Reversing the connections causes the motor to rotate counterclockwise. These terminals supply power to the motor and are linked to the brushes and brush arms within the back cover.

Magnets

The magnets used in DC motors are known as permanent magnets, meaning their magnetic field is always active. Opposite poles of magnets attract each other, while similar poles repel. The magnetic field of a magnet extends from the south pole to the north pole, with the strongest part of the field concentrated at the ends of the magnet.

In a DC motor, two magnets are employed to create a powerful magnetic field. These magnets are positioned around the rotor to ensure that the strong magnetic field passes through it, enhancing the motor's performance.

Rotor

The rotor, or armature, consists of multiple disks that are insulated from one another by laminated sheets. This insulation helps prevent the formation of large eddy currents, which can affect motor performance. While eddy currents are still present, their impact is minimized, allowing the motor to operate efficiently. To enhance motor efficiency, the disks are made as small as possible. The rotor is the dynamic component of the motor responsible for generating mechanical revolutions.

Coil Windings

The coil windings are wrapped around the rotor. The coiling of the wire creates a strong and powerful magnetic field. Every type of wire creates a weak magnetic field when electricity passes through it. Due to coiling of the wire, each turned section has the same weak magnetic field. When combined with all the different coiled wire, a strong magnetic field is created. As more coils are added to the rotor, its rotation becomes smoother. All DC motors have a minimum of three coils to ensure smooth rotation since two coils tend to jam and stop the motor. Each coil is 120o from the previous coil.

Brushes

The brushes in a DC motor deliver power to the coils and are constructed from metal with spring-like properties. One side of the brushes is made of a conductive material, typically carbon, while the other side has a pin for connecting to the power supply. The spring action of the brushes keeps them pressed against the commutator, and they are secured in place by the brush arms. The brushes are directly connected to the motor's terminals or electrical supply.

Commutator

The commutator consists of small copper plates mounted on the shaft that rotate along with it. As the shaft turns, the commutator shifts the power supply to the coils, changing the polarity of the current. Each coil is linked to two commutator plates, which are electrically isolated from each other but connected through the coils. When the positive and negative terminals are connected to these commutator plates, current flows easily, generating an electromagnetic field.

Chapter Four &#; What are the uses for DC motors?

DC motors are employed in a wide range of applications due to their high starting torque compared to induction motors. Brushed DC motors are compact and offer precise rotational control and high efficiency. Brushless DC motors, on the other hand, are durable with a longer lifespan due to the absence of brush wear, require minimal maintenance, and operate quietly.

DC motors are ubiquitous in various applications and processes, having served as mechanical power sources for over 130 years. They are used in diverse settings, from powering ceiling fans to driving large printing presses.

Below is a list describing a few of the many applications of DC motors.

Diesel Electric Locomotives

In a diesel-electric locomotive, the combustion from the diesel engine is converted into rotational energy, which is then coupled with a generator to produce electrical energy. This electrical energy is supplied to DC motors that are connected to the wheels of the engine.

Electric Vehicles

Brushed DC motors are used in electric vehicles for retracting and positioning electrically powered windows. Since brushed motors tend to wear out rapidly, many electric vehicle applications use brushless motors due to their long life span and noiselessness. Brushless DC motors are used for windshield wipers and CD players. All of the recent hybrid electric vehicles depend on brushless DC motors.

Cranes

For applications involving overhauling loads, where the motor must hold a full load at zero speed without mechanical brakes, DC motors are a cost-effective and safe choice. They offer significant advantages in terms of size and weight, making them ideal for such uses.

Conveyor Systems

Conveyor systems require constant speed and high torque, which makes DC motors an excellent fit. DC motors provide high torque at startup and maintain consistent speed throughout operation. Brushless DC motors are particularly favored in conveyor applications for their noiseless operation and precise control, essential for efficient conveying systems.

Ceiling Fans

Ceiling fans equipped with DC motors have gained popularity due to their lower power consumption and rapid startup torque. The alternating current in homes or offices is converted to DC power by a transformer, which reduces the power required by the fan. Brushless DC motors are most commonly used in ceiling fans for their efficiency and quiet performance.

Pump Drives

DC motors have long been used in pump drives due to their variable speed control, simple control systems, high starting torque, and good transient response. While brushed DC motors were traditionally used, the development of permanent magnet DC motors and brushless DC motors now provides more efficient options for pump systems.

Elevators

High-speed elevators face challenges with AC motors, such as difficulties in decelerating smoothly and accurately leveling with floors. DC motors offer a solution by allowing precise speed control through variations in the current supplied to the armature. Like ceiling fans, DC motors in elevators convert incoming AC power to DC using a transformer to ensure optimal performance.

Chapter Five &#; What are the advantages of using DC motors?

There is an ever growing demand for DC motors, especially 12 V and 24 V models. The expanding market of solar, marine, and truck mounted equipment have come to depend on DC motor technology as an exceptionally cost effective solution. Though DC motor technology is older than AC motor technology, DC motor manufacturers are constantly developing and engineering methods to reduce motor maintenance and extend motor life.

DC motors come in various types, each adaptable to a range of applications. To ensure optimal performance, it's crucial to conduct thorough research and select the DC motor best suited to handle the specific workload.

Startup Torque

DC motors are renowned for their high startup torque. They are well-suited for applications requiring constant and consistent speed with variable torque.

Linear Speed Torque

The relationship between torque and speed, known as the torque-speed curve, reveals how quickly a motor can spin and how much torque it can produce. DC motors exhibit an exceptional, more linear speed-torque curve compared to other motor types.

No Harmonic Effects

Harmonic effects can degrade a power system's performance, posing safety risks and potentially damaging equipment. DC motors operate without these issues, ensuring reliable performance without the complications of harmonic distortion.

Speed Control

DC motors are valued for their precise speed control. This capability is critical for heavy load systems and is why DC motors are commonly used in applications like paper and rolling mills, where consistent speed is essential.

Installation

DC motors are straightforward to install, requiring fewer electronic adjustments and rectifications in the power system. They can be quickly put into operation by connecting them directly to the power source.

Maintenance

The simple design of DC motors makes them easy to repair and replace. With over 130 years of use, these motors are well-known to technicians and electricians, which helps keep repair costs low. Their long history means that diagnosing and fixing issues is straightforward.

When servicing a DC motor, there is no need for field excitation. Components such as brushes and speed settings are easily replaceable. For issues with the control system, the terminal voltage can be adjusted using a potentiometer.

Low Cost

Cost is a significant factor when choosing DC motors. DC motors are generally less expensive than AC motors, though brushless and permanent magnet DC motors come at a higher cost. However, the extended lifespan of brushless motors often justifies the higher initial investment. On the other hand, while brushed DC motors are more affordable, they have a shorter lifespan and require more frequent repairs. Fortunately, the cost of repairing brushed motors is relatively low, which can offset their shorter operational life.

Conclusion

• A DC motor or direct current motor is an electrical machine that transforms electrical energy into mechanical energy by creating a magnetic field that is powered by direct current.
• One of the reasons that DC motors are preferred over other types of motors is their ability to precision control their speed, which is a necessity for industrial machinery.
• Two of the main advantages of DC motors over alternating current (AC) motors are how easy they are to install and that they require little maintenance.
• A DC motor is based on the idea that when a current carrying conductor is placed in a magnetic field, it produces mechanical force.
• There is a limitless number of applications where DC motors are used since they have a high starting torque compared to induction motors.

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How a DC motor actually works

The rotor is normally located on the inside of the motor, while the stator is located on the outside. The rotor contains coil windings that are powered by the DC current and the stator contains either permanent magnets or electromagnetic windings. When the motor is powered by DC current, a magnetic field is created within the stator, attracting and repelling the magnets on the rotor. This causes the rotor to start rotating. To keep the rotor rotating, the motor has a commutator. When the rotor aligns with the magnetic field, it would stop spinning, but in this case the commutator would reverse the current through the stator and this way reverse the magnetic field. This way the rotor can keep spinning. See the image on the right for a schematic display of how the dc motor works.

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