DC
motor
From
Wikipedia, the free encyclopedia
A DC
motor is a mechanically
commutated electric motor powered from direct current (DC). The stator is stationary in
space by definition and therefore so is its current. The current in the rotor
is switched by the commutator to
also be stationary in space. This is how the relative angle between the stator
and rotor magnetic flux is maintained near 90 degrees, which generates the
maximum torque.
DC motors have a rotating armature winding
(winding in which a voltage is induced) but non-rotating armature magnetic
field and a static field winding (winding that produce the main magnetic flux)
or permanent magnet. Different connections of the field and armature winding
provide different inherent speed/torque regulation characteristics. The speed
of a DC motor can be controlled by changing the voltage applied to the armature
or by changing the field current. The introduction of variable resistance in
the armature circuit or field circuit allowed speed control. Modern DC motors
are often controlled by power
electronics systems
called DC drives.
The introduction of DC motors to run
machinery eliminated the need for local steam or internal combustion engines,
and line shaft drive systems. DC motors can operate
directly from rechargeable batteries, providing the motive power for the first
electric vehicles. Today DC motors are still found in applications as small as
toys and disk drives, or in large sizes to operate steel rolling mills and paper
machines.
Contents
[hide] ·
1 Brush |
[edit]Brush
Main
article: Brushed DC electric motor
_80_degree_split_ring.gif){kind=small}
A
brushed DC electric motor generating torque from DC power supply by using
internal mechanical commutation, space stationary permanent magnets form the
stator field. Torque is produced by the principle of Lorentz
force, which states that any current-carrying conductor placed within an
external magnetic field experiences a force known as Lorentz
force. The actual (Lorentz) force (
and also torque since torque is F x l where l is rotor radius) is a
function for rotor angle and so the green arrow/vector actually changes
length/magnitude with angle known as torque ripple) Since this is a single
phase two pole motor the commutator consists of a
split ring, so that the current reverses each half turn ( 180 degrees).
http://en.wikipedia.org/wiki/DC_motor
A
brushed DC electric motor generating torque from DC power supply by using
internal mechanical commutation, space stationary permanent magnets form the
stator field. Torque is produced by the principle of Lorentz force, which
states that any current-carrying conductor placed within an external magnetic
field experiences a force known as Lorentz force. The actual (Lorentz) force (
and also torque since torque is F x l where l is rotor radius) is a function for
rotor angle and so the green arrow/vector actually changes length/magnitude
with angle known as torque ripple) Since this is a single phase two pole motor
the commutator consists of a split ring, so that the current reverses each half
turn ( 180 degrees).
The brushed DC electric motor generates torque directly from DC
power supplied to the motor by using internal commutation, stationary magnets (permanent or electromagnets),
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.Brushes are made of conductors.
[edit]Brushless
Main
articles: Brushless DC electric motor and Switched reluctance motor
Typical brushless DC motors use a rotating
permanent magnet in the rotor, and stationary electrical current/coil magnets
on the motor housing for the rotor, but the symmetrical opposite is also
possible. A motor controller converts DC to AC. 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.
[edit]Uncommutated
Other types of DC motors require no
commutation.
·
Homopolar motor – 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 motor – A 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 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.
[edit]Connection types
See
also: Excitation (magnetic)
There are three types of electrical
connections between the stator and rotor possible for DC electric motors:
series, shunt/parallel and compound ( various blends of series and
shunt/parallel) and each has unique speed/torque characteristics appropriate
for diffent loading torque profiles/signatures.[1]
[edit]Series connection
Main
article: Universal motor
A series DC motor connects the armature and field windings in series with a common D.C. power
source. The motor speed varies as a non-linear function of load torque and
armature current; current is common to both the stator and rotor yielding I^2
(current) squared behavior[citation needed]. A series motor
has very high starting torque and is commonly used for starting high inertia
loads, such as trains, elevators or hoists.[2] This speed/torque characteristic is
useful in applications such as dragline excavators, where the digging tool
moves rapidly when unloaded but slowly when carrying a heavy load.
With no mechanical load on the series motor,
the current is low, the counter-EMF produced by the field winding is weak, and
so the armature must turn faster to produce sufficient counter-EMF to balance
the supply voltage. The motor can be damaged by over speed. This is called a
runaway condition.
Series motors called "universal
motors" can be used on alternating current. Since the armature voltage and
the field direction reverse at (substantially) the same time, torque continues
to be produced in the same direction. Since the speed is not related to the line
frequency, universal motors can develop higher-than-synchronous speeds, making
them lighter than induction motors of the same rated mechanical output. This is
a valuable characteristic for hand-held power tools. Universal motors for
commercial power frequency are usually small, not more than about
1 kW output. However, much larger universal motors were used for electric
locomotives, fed by special low-frequency traction power networks to avoid problems with commutation
under heavy and varying loads.
[edit]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
the starting torque of a series DC motor.[3] It is typically used for industrial,
adjustable speed applications, such as machine tools, winding/unwinding
machines and tensioners.
[edit]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.[4] 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.
[edit]See also
·
Torque and speed of a DC motor
[edit]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
·
DC motor model
in Simulink at File
Exchange - MATLAB Central
[edit]References
1.
^ Herman, Stephen.
Industrial Motor Control. 6th ed. Delmar, Cengage Learning, 2010. Page
251.
2.
^ Ohio Electric Motors. DC Series
Motors: High Starting Torque but No Load Operation Ill-Advised. Ohio Electric Motors, 2011. Archived 20 July 2011 at WebCite
3.
^ Laughton M.A. and Warne D.F., Editors. Electrical
engineer's reference book. 16th ed. Newnes, 2003. Page 19-4.
4.
^ William H. Yeadon, Alan W. Yeadon. Handbook of
small electric motors. McGraw-Hill Professional, 2001. Page 4-134.