DC motor
Encyclopedia
A DC motor is an electric motor
that runs on direct current
(DC) electricity.
generates torque directly from DC power supplied to the motor by using internal commutation, stationary magnets (permanent
or electromagnet
s), and rotating electrical magnets.
Like all electric motors or generators, torque is produced by the principle of Lorentz force
, which states that any current-carrying conductor placed within an external magnetic field experiences a torque or force known as Lorentz force. Advantages of a brushed DC motor include low initial cost, high reliability, and simple control of motor speed. Disadvantages are high maintenance and low life-span for high intensity uses. Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator
. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor.
. This design is simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor. Advantages of brushless motors include long life span, little or no maintenance, and high efficiency. Disadvantages include high initial cost, and more complicated motor speed controllers. Some such brushless motors are sometimes referred to as "synchronous motors" although they have no external power supply to be synchronized with, as would be the case with normal AC synchronous motors.
Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This has restricted the practical application of this type of motor.
and field windings
in series with a common D.C. power source. This motor has poor speed regulation since its speed/torque response varies with the load. However, a series DC motor has very high starting torque and is commonly used for starting high inertia loads, such as trains, elevators or hoists. The series DC motor is dangerous to operate unloaded because as its load decreases, its speed increases. In a no-load condition, the motor will increase its speed until the motor mechanically destroys itself. This is called a runaway condition.
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
that runs on direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) electricity.
Brush
The brushed DC electric motorBrushed DC electric motor
A brushed DC motor is an internally commutated electric motor designed to be run from a direct current power source.-Simple two-pole DC motor:The following graphics illustrate a simple, two-pole, brushed, DC motor.DC Motor Rotation...
generates torque directly from DC power supplied to the motor by using internal commutation, stationary magnets (permanent
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
or electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
s), and rotating electrical magnets.
Like all electric motors or generators, torque is produced by the principle of Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
, which states that any current-carrying conductor placed within an external magnetic field experiences a torque or force known as Lorentz force. Advantages of a brushed DC motor include low initial cost, high reliability, and simple control of motor speed. Disadvantages are high maintenance and low life-span for high intensity uses. Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...
. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor.
Brushless
Brushless DC motors use a rotating permanent magnet or soft magnetic core in the rotor, and stationary electrical magnets on the motor housing. A motor controller converts DC to ACAlternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
. This design is simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor. Advantages of brushless motors include long life span, little or no maintenance, and high efficiency. Disadvantages include high initial cost, and more complicated motor speed controllers. Some such brushless motors are sometimes referred to as "synchronous motors" although they have no external power supply to be synchronized with, as would be the case with normal AC synchronous motors.
Uncommutated
Other types of DC motors require no commutation.- Homopolar motorHomopolar motorA homopolar motor is an electric motor that works without the need for a commutator, by rotating along a fixed axis that is parallel to the external magnetic field produced by a permanent magnet. The name homopolar indicates that the electrical polarity of the motor does not change...
– A homopolar motor has a magnetic field along the axis of rotation and an electric current that at some point is not parallel to the magnetic field. The name homopolar refers to the absence of polarity change.
Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This has restricted the practical application of this type of motor.
- Ball bearing motorBall bearing motorA ball bearing motor is an unusual electric motor that consists of two ball-bearing-type bearings, with the inner races mounted on a common conductive shaft, and the outer races connected to a high current, low voltage power supply. An alternative construction fits the outer races inside a metal...
– A ball bearing motor is an unusual electric motor that consists of two ball bearingBall bearingA ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit...
-type bearings, with the inner races mounted on a common conductive shaft, and the outer races connected to a high current, low voltage power supply. An alternative construction fits the outer races inside a metal tube, while the inner races are mounted on a shaft with a non-conductive section (e.g. two sleeves on an insulating rod). This method has the advantage that the tube will act as a flywheel. The direction of rotation is determined by the initial spin which is usually required to get it going.
Connection types
There are three types of connections used for DC electric motors: series, shunt and compound. These types of connections configure how the motor's field and armature windings are connected together. The type of connection is significant because it determines the characteristics of the motor and is selected for speed/torque requirements of the load.Series connection
A series DC motor connects the armatureArmature (electrical engineering)
In electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine–generally in a motor or generator, but it may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay....
and field windings
Field coil
A field coil is a component of an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. A current-carrying coil is used to generate a magnetic field....
in series with a common D.C. power source. This motor has poor speed regulation since its speed/torque response varies with the load. However, a series DC motor has very high starting torque and is commonly used for starting high inertia loads, such as trains, elevators or hoists. The series DC motor is dangerous to operate unloaded because as its load decreases, its speed increases. In a no-load condition, the motor will increase its speed until the motor mechanically destroys itself. This is called a runaway condition.
Shunt connection
A shunt DC motor connects the armature and field windings in parallel or shunt with a common D.C. power source. This type of motor has good speed regulation even as the load varies, but does not have as high of starting torque as a series DC motor. It is typically used for industrial, adjustable speed applications, such as machine tools, winding/unwinding machines and tensioners.Compound connection
A compound DC motor connects the armature and fields windings in a shunt and a series combination to give it characteristics of both a shunt and a series DC motor. This motor is used when both a high starting torque and good speed regulation is needed. The motor can be connected in two arrangements: cumulatively or differentially. Cumulative compound motors connect the series field to aid the shunt field, which provides higher starting torque but less speed regulation. Differential compound DC motors have good speed regulation and are typically operated at constant speed. They are commonly used in elevators, air compressors, conveyors and punch presses.External links
- DC Motor - Interactive Java Tutorial National High Magnetic Field Laboratory
- Make a working model of dc motor at sci-toys.com
- How to select a DC motor at MICROMO