DC Motors

Chapter One – What is a DC Motor?

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. When a DC motor is powered, a magnetic field is created in its stator. The field attracts and repels magnets on the rotor; this causes the rotor to rotate. To keep the rotor continually rotating, the commutator that is attached to brushes connected to the power source supply current to the motors wire windings.

One of the reasons DC motors are preferred over other types of motors is their ability to precision control their speed, which is a necessity for industrial machinery. DC motors are able to immediately start, stop, and reverse—an essential factor for controlling the operation of production equipment.

Chapter Two – 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 differentiated by the connections between the field winding and the armature. The field winding can be connected parallel to the armature or connected in a series. In some cases, the connection is both parallel and in a series.

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.

Since DC motors are everywhere and used for a wide variety of applications, there is a different type to meet the needs of every application. Regardless of your need for DC motors, it is important to understand each type since they can be found in every aspect of life.

DC Motors Types

Brushed DC Motor

The magnetic field in a brush DC motor is produced by current sent through a commutator and brush that are connected to the rotor. Brushes are made of carbon and can be separately excited or self excited. The stator is the enclosure that contains the components of the motor and contains the magnetic field. The winding of the coil on the rotor can be in a series or parallel to form either a series wound DC motor or shunt wound DC motor.

The commutator is an electrical switch that reverses the current between the rotor and the external power source. It is a method of applying electrical current to the windings and produces a steady rotating torque by reversing the current direction. The sections of the commutator are attached to the windings on the rotor through a set of contact bars that are set in the shaft of the motor.

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 further divided into shunt, series, and compound. The compound excited type is separated into cumulative and differential with short and long shunts in each type.

Separately Excited DC Motor

In a separately excited DC motor, the motor has separate electrical supplies to the armature winding and field winding, which are electrically separate from each other. The operations of the armature current and field current do not interfere with each other‘s actions, but the input power is their total sum.

Permanent Magnet DC Motor

A permanent magnet DC motor has an armature winding but does not have a field winding. The permanent magnet is mounted on the inner surface of the stator core to produce the magnetic field. It has a regular armature consisting of a commutator and brushes.

Permanent magnet DC motors are smaller and less expensive. They use rare earth magnets such as samarium cobalt or neodymium iron boron.

Self Excited DC Motor

In self excited DC motors, the field and armature windings are connected and have a single supply source. The connections are parallel or series with parallel made as shunt wound while the series version is series wound.

Shunt

In a shunt wound DC motor, the field and armature windings are connected parallel to each other; this exposes the field winding to terminal voltage. Though the supply is the same, the current for the field and armature windings is different. The speed of a shunt DC motor is constant and does not deviate with varying mechanical loads.

Series

The field and armature winding on a series DC motor are connected to the power supply in a series. The same current flows in the field and armature windings. A series wound motor can work with AC and DC voltage supply, which makes it a universal motor. Series motors always rotate in the same direction regardless of the voltage source. Their speed varies with the mechanical load.

Compound

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A compound DC motor uses the features of the series and shunt field windings. The winding for the armature is connected in a series while the winding for the field is a shunt or parallel connection.

Compound DC motors are further divided into cumulative and differential. With cumulative DC motors, the flux of the shunt field helps the flux in the series field. They both move in the same direction while the flux of a differential compound DC motor, for the series and shunt fields, moves in the opposite direction. Cumulative and differential compound DC motors can have long or short shunts; this is based on the shunting of the shunt field winding.

Brushless DC Motor

Brushless DC motors have a permanent magnet rotor and a coil wound stator. This design of DC motor removes the need for brushes. The benefit of a brushless DC motor is the elimination of wear and tear of brushes since very little heat is produced by the rotating magnet. The efficiency of brushless DC motors makes them more expensive.

Servo DC Motor

A servo DC motor has four parts: a DC motor, gearbox, control circuit, and position sensing unit. The gearbox changes high speed input into slower practical speed. The control circuit is an error detector amplifier. The position of the shaft gives feedback to the control circuit and is in a closed loop. With a servo DC motor, if there is any mismatch between the current position of the shaft and its reference position, an error signal is sent to the error detecting amplifier.

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Chapter Three – How DC Motors Work

A DC motor is based on the idea that when a current carrying conductor is placed in a magnetic field, it produces mechanical force. The direction of the force is determined by the left hand rule. Since DC motors and DC generators have the same construction, they can be used interchangeably.

For large electrical applications, such as steel mills and electric trains, alternating current (AC) is converted into DC current since the speed and torque characteristics of a DC motor are superior to an AC motor. In the case of industrial applications, DC motors are as widely used as three phase induction motors.

How DC Motors Work

Stator

The stator is the unmoving main body of the motor, and it provides support and protection for the motor. The stator provides a rotating magnetic field that drives the armature or rotor. It is the static part of the motor that houses the field windings and receives the electrical supply through its terminals.

Shaft

The windings and the commutator rotate the shaft, which is at the center of the motor and made of a hardened metal, usually steel, to withstand the loads of the application. The commutator bars are attached to the plate that is affixed to the shaft by plastic molding. The torque that is produced by the winding is transferred to the shaft supported by the stator. The shaft protrudes through the stator and connects the motor to the application.

Terminals

A DC motor has two terminals: positive and negative. When the positive wire is connected to the positive terminal and the negative wire connected to the negative terminal, the motor rotates clockwise. When they are reversed, the motor rotates counter clockwise. The terminals provide the power supply for the motor and are connected to the brushes and brush arms inside the back cover.

Magnets

The magnets used in DC motors are referred to as permanent magnets; this means their magnetic field is always active. Opposite ends of magnets attract while similar ends repel. The magnetic field of a magnet runs from the south pole to the north pole. The most powerful part of a magnet‘s magnetic field is at its ends.

Two magnets create a very strong field; this is why two magnets are included in a DC motor around the rotor such that the strong magnetic field passes through the rotor.

Rotor

The rotor or armature is made of multiple disks that are insulated from each other by laminated sheets. The multiple disks prevent the creation of a large eddy current. (Eddy currents are the reason the plates are insulated.) Eddy currents are still present but are much smaller and do not interfere with the operation of the motor; they are a necessary part of the motor's operation. For greater motor efficiency, the disks of the rotor are made as small as possible. The rotor is the dynamic part of the motor that is used to create the 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 of a DC motor provide the coils with power and are metal pieces that act like springs. On one side, they have a conductive material made of carbon. On the other side, they have a pin where the power supply is applied to the motor. The brushes are pushed by their spring action against the commutator, are held in place by the brush arms, and are directly connected to the terminals or electrical supply.

Commutator

The commutator is made of small copper plates that are mounted on the shaft and rotate as the shaft rotates. The rotation of the rotor causes the poles of the power supply to the coils to change. Each coil is connected to two commutator plates, which are electrically isolated from each other but connected by the coils. With positive and negative terminals connected to two commutator plates, current easily flows and an electromagnetic field is generated.

Chapter Four – Uses for DC Motors

DC motors are used in any number of applications since they have a high starting torque compared to induction motors. Brushed DC motors are easy to miniaturize, and they provide good rotational control as well as high efficiency. Brushless DC motors have a long life due to their lack of wear from brushes, are easy to maintain, and are noiseless.

It is easy to find DC motors since they are all around us in multiple applications and processes. DC motors have been used as a mechanical power source for over 130 years. The variations in their use run from providing power to a ceiling fan in a bedroom to supplying mechanical energy to a large printing press.

The list below provides descriptions of a few of the millions of uses for DC motors.

DC Motors Usages

Diesel Electric Locomotives

In a diesel electric locomotive, combustion from the diesel engine is converted into rotational energy by the diesel engine, which is coupled with a generator that converts it to electrical energy. The converted electrical energy is fed to DC motors that are coupled with the wheels on 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

In applications with overhauling loads, it is important for the motor to have the ability to hold a full load at zero speed where mechanical brakes may not be required. In those situations, DC motors are the most cost effective and safest option. A major benefit of their use is their size and weight.

Conveyor Systems

Conveyor systems require constant speed and high torque, which makes DC motors an ideal option. As has been found with other applications, DC motors have high torque at start up and even consistent speed. Brushless DC motors are the most commonly used for conveyor applications. They are noiseless and can be easily controlled, a major requirement for conveying systems.

Ceiling Fans

Ceiling fans made with DC motors have become extremely popular. They use less power and have a rapid start up torque. The alternating current in a home or office is easily converted to DC power by a transformer, an effect that decreases the amount of power required by the fan. As with other DC motor applications, brushless DC motors are most commonly used in ceiling fans.

Pump Drives

DC motors have been the main driving force behind pumps for several decades because of their variable speed control, simple control system, high starting torque, and good transient response. For many years, pumping systems depended on brushed DC motors as their primary source of energy. The development of permanent magnet DC motors and brushless DC motors have offered a more beneficial option for pump system operations.

Elevators

In high speed elevators, AC motors are impractical due to their difficulty decelerating and accurately leveling with the floor. These problems are overcome with DC motors because they allow for infinite control of their speed by varying the current supplied to the armature. As with ceiling fans, the operation of a DC motor for elevators depends on changing the incoming AC current to DC current through the use of a transformer.

Chapter Five – 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.

The many types of DC motors are adaptable and adjustable to fit a wide variety of applications. It‘s important to do sufficient research to find the correct DC motor to fit the workload.

Advantages of Using DC Motors

Startup Torque

Constantly discussed in regard to DC motors is their high startup torque. For applications that need constant and consistent speed with variable torque, DC motors are the ideal choice.

Linear Speed Torque

The curve between the torque and speed of a motor explains how fast the motor spins and how much torque it can generate. DC motors generate an exceptional speed to torque curve that is more linear than other motors.

No Harmonic Effects

Harmonic effects degrade a power system‘s performance and reliability and may become a safety problem. When harmonic effects exist, they must be immediately identified and corrected. Damage to equipment can cause metal components to heat up and become dangerous. This particular issue is not a problem in the operation of DC motors.

Speed Control

Another factor that is regularly discussed regarding DC motors is the ability to monitor and control their speed. When working with a heavy load system, the ability to control speed precisely and accurately ensures the success of the job. It is for this reason that DC motors are often found in paper and rolling mills where consistent speed is a necessity.

Installation

When a DC motor is installed, it requires fewer electronic rectifications in the power system and fewer adjustments in general. Once a DC motor is installed, it can be used immediately by feeding power to it directly from the power source.

Maintenance

The design of DC motors is simple, which makes them easy to repair or replace. DC motors have been around for over 130 years and are familiar to technicians and electricians. The many years they have been used makes them easy to diagnose and repair at very low cost.

When repairing a DC motor, there is no need for field excitation, and brushes, speed settings, and other components are easy to replace. If there is a problem with the control system, the terminal voltage can be adjusted using a potentiometer.

Low Cost

The obvious final reason for using DC motors is their low cost; they are cheaper than AC motors, though brushless and permanent magnet DC motors are more expensive. The cost advantage of brushless motors is their exceptionally long life span. Though brush motors are less expensive, they tend to have a shorter life span and require regular repair, a negative aspect that is balanced by their low cost of repair.

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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