FOUR QUADRANT DC MOTOR CONTROL
WITHOUT MICROCONTROLLER
ABSTRACT
The project is designed to develop a four quadrant
control system for a DC motor. The motor is operated in four quadrants i.e.
clockwise; counter clock-wise, forward brake and reverse brake.
The four quadrant operation of the dc motor is best
suited for industries where motors are used and as per requirement as they can
rotate in clockwise, counter-clockwise and also apply brakes immediately in
both the directions. In case of a specific operation in industrial environment,
the motor needs to be stopped immediately. In such scenario, this proposed
system is very apt as forward brake and reverse brake are its integral
features.
Instantaneous
brake in both the directions happens as a result of applying a reverse voltage
across the running motor for a brief period. 555 timer used in the project develops
required pulses. Push buttons are provided for the operation of the motor which
are interfaced to the circuit that provides an input signal to it and in turn
controls the motor through a driver IC. Optionally speed control feature can be
achieved (but not provided in this project) by push button operation.
This project can be enhanced by using higher power
electronic devices to operate high capacity DC motors. Regenerative braking for
optimizing the power consumption can also be incorporated..
DC motor
A DC motor is an
electric motor that runs on direct current (DC) electricity.
DC Motor
Connections
Figure shows
schematically the different methods of connecting the field and armature
circuits in a DC Motor. The circular symbol represents the armature circuit,
and the squares at the side of
the circle represent the brush commutator system. The direction of the arrows indicates
the direction of the magnetic fields.
Figure shows
schematically the different methods of connecting the field and armature
circuits in a DC Motor. The circular symbol represents the armature circuit,
and the squares at the side of
the circle represent the brush commutator system. The direction of the arrows indicates
the direction of the magnetic fields.
Brushed
The brushed DC
motor generates torque directly from DC power supplied to the motor by using
internal commutation, stationary permanent magnets, and rotating electrical
magnets.It works on 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.
Brushed DC motor
Brushless
Brushless DC motors
use a rotating permanent magnet in the rotor, and stationary electrical magnets
on the motor housing. 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.
Torque and speed of
a DC motor
The torque of an
electric motor is independent of speed. It is rather a function of flux and
armature current.
Characteristics of
DC motors
DC motors respond
to load changes in different ways, depending on the arrangement of the
windin
Shunt wound motor
A shunt wound motor
has a high-resistance field winding connected in parallel with the armature. It
responds to increased load by trying to maintain its speed and this leads to an
increase in armature current. This makes it unsuitable for widely-varying
loads, which may lead to overheating.
Series wound motor
A series wound
motor has a low-resistance field winding connected in series with the armature.
It responds to increased load by slowing down and this reduces the armature
current and minimizes the risk of overheating. Series wound motors were widely
used as traction motors in rail transport of every kind, but are being phased
out in favor of AC induction motors supplied through solid state inverters. The
counter-emf aids the armature resistance to limit the current through the
armature. When power is first applied to a motor, the armature does not rotate.
At that instant the counter-emf is zero and the only factor limiting the
armature current is the armature resistance. Usually the armature resistance of
a motor is less than 1 Ω; therefore the current through the armature would be
very large when the power is applied. Therefore the need arises for an
additional resistance in series with the armature to limit the current until
the motor rotation can build up the counter-emf. As the motor rotation builds
up, the resistance is gradually cut out.
Permanent magnet
motor
A permanent magnet
DC motor is characterized by its locked rotor (stall) torque and its no-load
angular velocity (speed).
Principles of
operation
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.
Let's start by
looking at a simple 2-pole DC electric motor (here red represents a magnet or
winding with a "North" polarization, while green represents a magnet
or winding with a "South" polarization).
Every DC motor has
six basic parts -- axle, rotor (a.k.a., armature), stator, commutator, field
magnet(s), and brushes. In most common DC motors (and all that Beamers will
see), the external magnetic field is produced by high-strength permanent
magnets. The stator is the stationary part of the motor -- this includes the
motor casing, as well as two or more permanent magnet pole pieces. The rotor
(together with the axle and attached commutator) rotates with respect to the
stator. The rotor consists of windings (generally on a core), the windings
being electrically connected to the commutator. The above diagram shows a
common motor layout -- with the rotor inside the stator (field) magnets.
The geometry of the
brushes, commutator contacts, and rotor windings are such that when power is
applied, the polarities of the energized winding and the stator magnet(s) are
misaligned, and the rotor will rotate until it is almost aligned with the
stator's field magnets. As the rotor reaches alignment, the brushes move to the
next commutator contacts, and energize the next winding. Given our example
two-pole motor, the rotation reverses the direction of current through the
rotor winding, leading to a "flip" of the rotor's magnetic field,
driving it to continue rotating.
In real life,
though, DC motors will always have more than two poles (three is a very common
number). In particular, this avoids "dead spots" in the commutator.
You can imagine how with our example two-pole motor, if the rotor is exactly at
the middle of its rotation (perfectly aligned with the field magnets), it will
get "stuck" there. Meanwhile, with a two-pole motor, there is a
moment where the commutator shorts out the power supply (i.e., both brushes
touch both commutator contacts simultaneously). This would be bad for the power
supply, waste energy, and damage motor components as well. Yet another
disadvantage of such a simple motor is that it would exhibit a high amount of
torque "ripple" (the amount of torque it could produce is cyclic with
the position of the rotor).
So since most small
DC motors are of a three-pole design, let's tinker with the workings of one via
an interactive animation.
You'll notice a few
things from this -- namely, one pole is fully energized at a time (but two
others are "partially" energized). As each brush transitions from one
commutator contact to the next, one coil's field will rapidly collapse, as the
next coil's field will rapidly charge up (this occurs within a few
microsecond). We'll see more about the effects of this later, but in the
meantime you can see that this is a direct result of the coil windings' series
wiring:
The use of an iron
core armature (as in the Mabuchi, above) is quite common, and has a number of
advantages. First off, the iron core provides a strong, rigid support for the
windings -- a particularly important consideration for high-torque motors. The
core also conducts heat away from the rotor windings, allowing the motor to be
driven harder than might otherwise be the case. Iron core construction is also
relatively inexpensive compared with other construction types.
But iron core
construction also has several disadvantages. The iron armature has a relatively
high inertia which limits motor acceleration. This construction also results in
high winding inductances which limit brush and commutator life.
In small motors, an
alternative design is often used which features a 'coreless' armature winding.
This design depends upon the coil wire itself for structural integrity. As a
result, the armature is hollow, and the permanent magnet can be mounted inside
the rotor coil. Coreless DC motors have much lower armature inductance than
iron-core motors of comparable size, extending brush and commutator life.
DC motor behavior
High-speed output
This is the
simplest trait to understand and treat -- most DC motors run at very high
output speeds (generally thousands or tens of thousands of RPM). While this is
fine for some BEAM bots (say, photo poppers or solar rollers), many BEAM bots
(walkers, heads) require lower speeds -- you must put gears on your DC motor's
output for these applications.
Back EMF
Just as putting
voltage across a wire in a magnetic field can generate motion, moving a wire
through a magnetic field can generate voltage. This means that as a DC motor's
rotor spins, it generates voltage -- the output voltage is known as back EMF.
Because of back EMF, a spark is created at the commutator as a motor's brushes
switch from contact to contact. Meanwhile, back EMF can damage sensitive
circuits when a motor is stopped suddenly.
Noise (ripple) on
power lines
A number of things
will cause a DC motor to put noise on its power lines: commutation noise (a
function of brush / commutator design & construction), roughness in
bearings (via back EMF), and gearing roughness (via back EMF, if the motor is
part of a gearmotor) are three big contributors.
Even without these
avoidable factors, any electric motor will put noise on its power lines by
virtue of the fact that its current draw is not constant throughout its motion.
Going back to our example two-pole motor, its current draw will be a function
of the angle between its rotor coil and field magnets:
Since most small DC
motors have 3 coils, the coils' current curves will overlay each other:
Added together,
this ideal motor's current will then look something like this:
Reality is a bit
more complex than this, as even a high-quality motor will display a current
transient at each commutation transition. Since each coil has inductance (by
definition) and some capacitance, there will be a surge of current as the
commutator's brushes first touch a coil's contact, and another as the brushes
leave the contact (here, there's a slight spark as the coil's magnetic field
collapses).
As a good example,
consider an oscilloscope trace of the current through a Mabuchi FF-030PN motor
supplied with 2 V (1ms per horizontal division, 0.05 mA per vertical division):
In this case, the
peak-to-peak current ripple is approximately 0.29 mA, while the average motor
current is just under 31 mA. So under these conditions, the motor puts about
less than 1% of current ripple onto its power lines (and as you can see from
the "clean" traces, it outputs essentially no high-frequency current
noise). Note that since this is a 3-pole motor, and each coil is energized in
both directions over the course of a rotor rotation, one revolution of the
rotor will correspond to six of the above curves (here, 6 x 2.4 ms = 0.0144
sec, corresponding to a motor rotation rate of just under 4200 RPM).
Motor power ripple
can wreak havoc in Nv nets by destabilizing them inadvertently. Fortunately,
this can be mitigated by putting a small capacitor across the motor's power
lines (you'll only be able to filter out "spikey" transients this
way, though -- you'll always see curves like the ones above being imposed on
your power). On the flip side of this coin, motor power ripple can be put to
good use -- as was shown above, ripple frequency can be used to measure motor
speed, and its destabilizing tendencies can be used to reverse a motor without
the need for discrete "back-up" sensors.
600mA OUTPUT CURRENT CAPABILITY PER CHANNEL
1.2A PEAK OUTPUT CURRENT (non repetitive) PER CHANNEL ENABLE
FACILITY
OVERTEMPERATURE PROTECTION LOGICAL "0" INPUT
VOLTAGE UP TO 1.5 V
(HIGH NOISE IMMUNITY) INTERNAL CLAMP DIODES
DESCRIPTION
The Device is a monolithic integrated high voltage, high
current four channel driver designed to
accept standard DTL or TTL logic levels and drive inductive
loads (such as relays solenoides, DC
and stepping motors) and switching power transistors. To
simplify use as two bridges each pair of channels is equipped with an enable
input. A separate supply input is provided for the logic, allowing operation at
a lower voltage and internal clamp diodes
are included. This device is suitable for use in
switching applications at frequencies up to 5 kHz.
The L293D is assembled in a 16 lead plastic packaage
which has 4 center pins connected together
and used for heatsinking. The L293DD is assembled in a 20
lead surface
mount which has 8 center pins connected together and used
for heatsinking.
DIAGRAM of L293D
Block Diagram of L293D
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