DC MOTORS
A DC
motor is an electric motor that runs on direct current (DC) electricity. In any
electric motor, operation is based on simple electromagnetism. A current-carrying conductor
generates a magnetic field; when this is then placed in an external magnetic
field, it will experience a force proportional to the current in the conductor, and to the
strength of the external magnetic field. As you are well aware of from playing
with magnets as a kid, opposite (North and South) polarities attract, while
like polarities (North and North, South and South) repel. The internal configuration
of a DC motor is designed to harness
the magnetic interaction between a current-carrying conductor
and an external magnetic field to generate rotational motion.
A
dc motor can be broadly classified into two distinguished types of motors
namely -:
· Brushed
dc motor
· Brush-less dc motor
As
per our requirements (for building a quad-copter) we will be concentrating more on the concept of brush-less dc
motor.
BRUSHED DC MOTOR
A brushed
DC motor is an internally commutated electric motor designed to
be run from a DC power source. The brushed dc electric motor generates torque directly
from DC power supplied to the motor by using internal commutation, stationary
permanent magnets, 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.
When a current passes
through the coil wound around a soft iron core, the side of the positive pole
is acted upon by an upwards force, while the other side is acted upon by a
downward force. According to Fleming’s left hand rule, the forces cause a turning effect on the
coil, making it rotate. To make the motor rotate in a constant direction,
"direct current" commutators make the current reverse in
direction every half a cycle (in a two-pole motor) thus causing the motor to
continue to rotate in the same direction.
When a current passes
through the coil wound around a soft iron core, the side of the positive pole
is acted upon by an upwards force, while the other side is acted upon by a
downward force. According to Fleming’s left hand rule, the forces cause a turning effect on the
coil, making it rotate. To make the motor rotate in a constant direction,
"direct current" commutators make the current reverse in direction
every half a cycle (in a two-pole motor) thus causing the motor to continue to
rotate in the same direction.
A problem with the motor
shown above is that when the plane of the coil is parallel to the magnetic
field i.e. when the rotor poles are 90 degrees from the stator poles—the torque
is zero. In the pictures above, this occurs when the core of the coil is
horizontal—the position it is just about to reach in the last picture on the
right. The motor would not be able to start in this position. However, once it
was started, it would continue to rotate through this position by inertia.
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| Working of brushed DC motor |
FLEMING’S
LEFT HAND RULE
Fleming's left hand rule (for motors),
is a visual mnemonics that is used for working out which way
an electric motor will turn. The term was coined by John Ambrose Fleming in the late 19th century. When an
electric current flows in a wire, and an external magnetic field is applied
across that flow, the wire experiences a force perpendicular both to that
field, and to the direction of the current flow. A left hand can be held, as
shown in the illustration, so as to represent three mutually orthogonal axes on
the thumb, first finger and middle finger. It is then just a question of
remembering which finger represents which quantity (electric current, magnetic
field and mechanical force), and whether the right hand should be used instead
of the left.
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| Fleming's left hand rule |
BRUSH-LESS DC MOTOR
Brush-less
DC motors (BLDC motors, BL
motors) also known as electronically
commutated motors (ECMs, EC
motors) are synchronous electric
motors powered by direct-current (DC) electricity and electronic commutation
systems, rather mechanical commutators and brushes.
The
current-to-torque and frequency-to-speed relationships of BLDC motors are
linear.BLDC motors may be described as stepper
motors, with fixed permanent
magnets and possibly more poles
on the rotor than the stator, or reluctance motors. The latter may be
without permanent magnets, just poles that are induced on the rotor then pulled
into alignment by timed stator winding's However, the term stepper
motor tends to be used for
motors that are designed specifically to be operated in a mode where they are
frequently stopped with the rotor in a defined angular
position; this page describes more general BLDC motor principles, though there
is overlap.Now the movement of the magnet in the center depends on the
direction of flow of current in the coil as shown in the above figure. The
continuous movement of the magnet is ensured by Left hand rule for the coils.
LEFT HAND RULE FOR THE COILS
The left hand rule states that
Grasp the coil in your left hand, with your finger wrapped around in the direction of the current. Your thumb will point towards the north pole of the
coil.
 |
Left hand rule for coils
|
The term out-runner refers to a type of brush-less motor primarily used in electrically
propelled, radio-controlled model aircraft. This type of motor spins its outer
shell around its windings, much like motors found in ordinary CD-ROMs computer drives. In fact, CD-ROM
motors are frequently rewound into brush-less out-runner motors for small park flyer aircraft. Parts to aid in converting
CD-ROM motors to aircraft use are commercially available.
Out-runners
spin much slower than their in-runner counterparts with their more
traditional layout (though still considerably faster than ferrite motors) while producing far
more torque. This makes an
out-runner an excellent choice for directly driving electric aircraft propellers since they eliminate the extra weight,
complexity, inefficiency and noise of a gearbox.
Out-runner
motors have quickly become popular and are now available in many sizes. They
have also become popular in personal, electric transportation applications such
as electric bikes and scooters due to their compact size and favorable
power-to-weight ratios.
The stationary (stator) windings of an out-runner motor are
excited by conventional DC brushless motor controllers. A direct current
(switched on and off at high frequency for voltage modulation) is typically
passed through three or more non-adjacent windings together, and the group so
energized is alternated electronically based upon rotor position feedback.
The number of permanent magnets in the rotor does not match
the number of stator poles, however. The difference between the number of
magnet poles and the number of stator poles provides an effect that can be
understood as similar to planetary gearing. The number of magnet poles divided
by 2 gives the ratio of magnetic field rotation speed to motor rotation speed.
Consequently the advance of the electromagnetic impulse around the motor axis
proceeds much faster than the rotor turns. With more magnet poles the
maximum torque is increased, while the speed of rotor advance is decreased in
proportion to the ratio of magnet poles to stator poles.
In an out-runner the outer shell constituting the magnets
are rotated around the windings (coils).An out runner is mounted below the surface of the quad
consolidating the centre of gravity and providing more stability. The out
runner is basically used to counter the inverse pendulum problem due to which
if any stacking or mounting is done on top of the quad copter it can be
stabilized through out runners.
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| Out Runner motor that we used. |
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