INTRODUCTION
In our system the main feature is to
control the DC shunt motor in four quadrants to rotate in clock wise direction
and in anti-clock wise and to apply brakes in both forward and reverse.
The four quadrant operation is done by the current and
voltage control methods. In first quadrant the voltage and current will be
positive and the motor runs in forward direction. In second quadrant the
current will be positive but the voltage will be negative. So the motor breaks.
In third quadrant the voltage and current will be negative and the motor runs
in reverse direction. In fourth quadrant the voltage will be positive but the
current will be negative. So the motor brakes in reverse direction.
BLOCK DIAGRAM
Fig 2 Block Diagram
BLOCK DIAGRAM DESCRIPTION
The step down voltage is given to the
rectifier by using step down transformer. Rectifier converts the AC voltage
into DC voltage. The voltage from the rectifier is regulated using voltage
regulator. The output from the voltage regulator is given to the driver
circuit, relay, latching circuit, and timer circuit. The latching circuit can
be set to forward and reveres to run the motor in forward or reverse direction.
The push button operates the relay according to the latch set. The relay drives
the contactor set of motor drive circuit which drives the motor in forward or reverse
direction.
3.1 REGULATED POWER SUPPLY
The AC voltage,
typically 220V Rms, is connected to a transformer, which steps that ac voltage
down to the level of the desired DC output. A diode rectifier provides a
full-wave rectified voltage that is initially filtered by a simple capacitor
filter to produce a dc voltage. The resulting DC voltage usually has some
ripple or AC voltage variation.
A regulator circuit
removes the ripples and also remains the same dc value even if the input dc
voltage varies, or the load connected to the output dc voltage changes. This
voltage regulation is usually obtained using one of the popular voltage
regulator IC units.
3.2
TRANSFORMER
A transformer is an electrical device that transfers
energy between two circuits through electromagnetic
induction. Transformers may be used in step-up or
step-down voltage
conversion, which 'transforms' an AC voltage from one
voltage level on the input of the device to another level at the output
terminals. This special function of transformers can provide control of
specified requirements of current level as an alternating current source, or it
may be used for impedance
matching between mismatched electrical circuits to effect maximum
power transfer between the circuits.
A transformer is a
four-terminal device that transforms an alternating current (AC) input voltage
into a higher or lower AC output voltage Transformers range in size from
thumbnail-sized used in microphones to units weighing hundreds of tons
interconnecting the power grid. A wide range of transformer designs are used in
electronic and electric power applications. Transformers are essential for the
transmission, distribution, and utilization of electrical energy.
3.3
RECTIFIER
A rectifier is an electrical device that converts alternating
current (AC), which periodically reverses direction, to direct current (DC),
which flows in only one direction. The process is known as rectification. Diode
Bridge is an arrangement of four (or more) diodes in a bridge circuit
configuration that provides the same polarity of output for either polarity of
input. A bridge rectifier makes use of four diodes in a bridge arrangement to
achieve full-wave rectification. This is a widely used configuration, both with
individual diodes wired as shown and with single component bridges where the
diode bridge is wired internally.
3.4 VOLTAGE REGULATOR
A voltage regulator
is designed to automatically maintain a constant voltage level. A voltage
regulator may be a simple "feed-forward" design or may include
negative feedback control loops. It may use an electro-mechanical mechanism, or
electronic components. Depending on the design, it may be used to regulate one
or more AC or DC voltages. Voltage regulator is used for regulating the voltage
variations in the rectified voltage. A regulator removes the ripples and also
remains the same dc value even if the input dc voltage varies, or the load
connected to the output dc voltage changes. This voltage regulation is usually
obtained using one of the popular voltage regulator IC units.
3.5 RELAY
A relay is an electrical switch
that opens and closes under the control of another electrical circuit. In the
original form, the switch is operated by an electromagnet to open or close one
or many sets of contacts. It was invented by Joseph Henry in 1835. Because a
relay is able to control an output circuit of higher power than the input
circuit, it can be considered to be, in a broad sense, a form of an electrical
amplifier.
3.6 ELECTRONIC TIMER
A timer is a specialized type of
clock for measuring time intervals. A device which countdown from a specified
time interval is more usually called a timer.
Timers
may be free standing or incorporated into appliances and machines. Their
operating mechanism may be mechanical, electro-mechanical, or purely electronic
(counting cycles of an electronic oscillator). Timing functionality can be
provided by software, typically in a computer, the program is often called a
“timer”. The timer produces time duration of 72 milli seconds at braking.
3.7 DC
MOTOR
Dc shunt motor
has its field winding parallel to the armature. The armature supply is given to
the rotor by brushes and commutators. The DC motor relies on the fact that like magnet
poles repels and unlike magnetic poles attracts each other. A coil of wire with
a current running through it generates an electromagnetic field aligned with
the center of the coil. By switching the current on or off in a coil its magnet
field can be switched on or off or by switching the direction of the current in
the coil the direction of the generated magnetic field can be switched 180°.
Fig
4 circuit diagram
CIRCUIT OPERATION
In our project the supply is directly
given to the motor field winding. The supply to the armature winding is given
through the contactors. According to the relay control the contactors are
operated and the motor is controlled in four quadrant stages.
Now firstly the single phase supply is
fed as the input. In this circuit there are three steps down transformers are
used. Two steps down are used for control circuit and center tapped transformer
as the motor supply.
In control circuit the 15V transformer
is connected to the relay coil. The output lead of the relay coil is connected
to the timer IC. The contacts of the relay are used to connect the contactor
coil to the 24V supply.
The toggle switch, start push button
and the stop push button are arranged in the circuit. When the toggle switch is
set to forward condition, the motor runs in forward direction when the start
push button is pressed the motor runs in forward direction. When the stop push
button is pressed the motor is braked by giving the reverse supply to the motor
for a brief period. Similarly the reverse operation can be done.
HARDWARE REQUIREMENTS
6.1 CONTACTOR
A contactor is an electrically
controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. A
contactor is controlled by a circuit which has a much lower power level than
the switched circuit.
Contactors come in many forms with
varying capacities and features. Unlike a circuit breaker, a contactor is
not intended to interrupt a short circuit current.
Contactors range from those having a breaking current of several amperes to
thousands of amperes and 24 V DC to many kilovolts. The physical size of
contactors ranges from a device small enough to pick up with one hand, to large
devices approximately a meter (yard) on a side. Contactors are used to control
the electric motor, lighting, heating, capacitor banks, thermal evaporators,
and other electrical loads.
6.1.1 CONSTRUCTION
A contactor has three components.
The contacts are the current carrying part of the contactor. This
includes power contacts, auxiliary contacts, and contact springs. The
electromagnet (or "coil") provides the driving force to close
the contacts. The enclosure is a frame housing the contact and the
electromagnet. Enclosures are made of insulating materials like Bakelite, Nylon 6, and thermosetting
plastics to protect and insulate the contacts and to provide some measure of
protection against personnel touching the contacts. Open-frame contactors may
have a further enclosure to protect against dust, oil, explosion hazards and
weather.
Magnetic blowouts use blowout
coils to lengthen and move the electric arc. These are especially useful in DC
power circuits. AC arcs have periods of low current, during which the arc can
be extinguished with relative ease, but DC arcs have continuous high current,
so blowing them out requires the arc to be stretched further than an AC arc of
the same current. The magnetic blowouts in the pictured Albright contactor
(which is designed for DC currents) more than double the current it can break,
increasing it from 600 A to 1,500 A.
Sometimes an economizer circuit is also
installed to reduce the power required to keep a contactor closed; an auxiliary
contact reduces coil current after the contactor closes. A somewhat greater
amount of power is required to initially close a contactor than is required to
keep it closed. Such a circuit can save a substantial amount of power and allow
the energized coil to stay cooler. Economizer circuits are nearly always
applied on direct-current contactor coils and on large alternating current
contactor coils.
A basic contactor will have a coil
input (which may be driven by either an AC or DC supply depending on the
contactor design). The coil may be energized at the same voltage as a motor the
contactor is controlling, or may be separately controlled with a lower coil
voltage better suited to control by programmable
controllers and lower-voltage pilot devices. Certain contactors
have series coils connected in the motor circuit; these are used, for example,
for automatic acceleration control, where the next stage of resistance is not cut
out until the motor current has dropped.
6.1.2 OPERATING PRINCIPLE
Unlike general-purpose relays,
contactors are designed to be directly connected to high-current load devices.
Relays tend to be of lower capacity and are usually designed for
both normally closed and normally open applications.
Devices switching more than 15 amperes or in circuits rated more than a few
kilowatts are usually called contactors. Apart from optional auxiliary low
current contacts, contactors are almost exclusively fitted with normally open
("form A") contacts. Unlike relays, contactors are designed with
features to control and suppress the arc produced when interrupting heavy motor
currents.
When current passes through the electromagnet, a magnetic field
is produced, this attracts the moving core of the contactor. The electromagnet
coil draws more current initially, until its inductance increases
when the metal core enters the coil. The moving contact is propelled by the
moving core; the force developed by the electromagnet holds the moving and
fixed contacts together. When the contactor coil is de-energized, gravity or a
spring returns the electromagnet core to its initial position and opens the
contacts.
For contactors energized with alternating
current, a small part of the core is surrounded with a shading coil, which
slightly delays the magnetic flux in the core. The effect is to average out the
alternating pull of the magnetic field and so prevent the core from buzzing at
twice line frequency.
Because arcing and consequent damage
occurs just as the contacts are opening or closing, contactors are designed to
open and close very rapidly; there is often an internal tipping
point mechanism to ensure rapid action.
Rapid closing can, however, lead to
increase contact bounce which causes
additional unwanted open-close cycles. One solution is to have bifurcated
contacts to minimize contact bounce; two contacts designed to close
simultaneously, but bounce at different times so the circuit will not be
briefly disconnected and cause an arc.
A slight variant has multiple contacts
designed to engage in rapid succession. The first to make contact and last to
break will experience the greatest contact wear and will form a high-resistance
connection that would cause excessive heating inside the contactor. However, in
doing so, it will protect the primary contact from arcing, so a low contact
resistance will be established a millisecond later.
Another technique for improving the
life of contactors is contact
wipe; the contacts move past each other after initial contact on order to wipe
off any contamination.
6.1.3 ARC
SUPPRESSION
Without adequate contact
protection, the occurrence of electric current arcing causes
significant degradation of the contacts, which suffer significant damage. An
electrical arc occurs between the two contact points (electrodes) when they
transition from a closed to an open (break arc) or from an open to a closed
(make arc). The break arc is typically more energetic and thus more destructive.
The heat energy contained in the
resulting electrical arc is very high, ultimately causing the metal on the
contact to migrate with the current. The extremely high temperature of the arc
(tens of thousands of degrees Celsius) cracks the surrounding gas molecules
creating ozone, carbon monoxide, and other
compounds. The arc energy slowly destroys the contact metal, causing some
material to escape into the air as fine particulate matter. This activity
causes the material in the contacts to degrade over time, ultimately resulting
in device failure. For example, a properly applied contactor will have a life
span of 10,000 to 100,000 operations when run under power; which is
significantly less than the mechanical (non-powered) life of the same device
which can be in excess of 20 million operations.[4]
Most motor control contactors at low
voltages (600 volts and less) are air break contactors; air at atmospheric
pressure surrounds the contacts and extinguishes the arc when interrupting the
circuit. Modern medium-voltage AC motor controllers use vacuum contactors. High
voltage AC contactors (greater than 1000 volts) may use vacuum or an inert gas
around the contacts. High Voltage DC contactors (greater than 600V) still rely
on air within specially designed arc-chutes to break the arc energy. High-voltage
electric locomotives may be isolated from their overhead supply by
roof-mounted circuit
breakers actuated by compressed air; the same air supply may be used to
"blow out" any arc that forms.
6.1.4 RATINGS
Contactors are rated by designed load
current per contact (pole), maximum fault withstand current, duty cycle,
voltage, and coil voltage. A general purpose motor control contactor may be suitable
for heavy starting duty on large motors; so-called "definite purpose"
contactors are carefully adapted to such applications as air-conditioning
compressor motor starting.
6.2 RELAY
A relay is an electrical switch that opens and
closes under the control of another electrical circuit. In the original form, the switch is
operated by an electromagnet to open or close
one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is
able to control an output circuit of higher power than the input circuit, it
can be considered to be, in a broad sense, a form of an electrical amplifier.
6.2.1 OPERATION
When
a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a
moving contact. The movement either makes or breaks a connection with a fixed
contact. When the current to the coil is switched off, the armature is returned
by a force approximately half as strong as the magnetic force to its relaxed
position. Usually this is a spring, but gravity is
also used commonly in industrial motor starters. Most relays are manufactured
to operate quickly. In a low voltage application, this is to reduce noise. In a
high voltage or high current application, this is to reduce arcing.
If
the coil is energized with DC, a diode is frequently
installed across the coil, to dissipate the energy from the collapsing magnetic
field at deactivation, which would otherwise generate a spike of voltage and
might cause damage to circuit components. Some automotive relays already
include that diode inside the relay case. Alternatively a contact protection
network, consisting of a capacitor and resistor in series, may absorb the
surge. If the coil is designed to be energized with AC, a small copper ring can
be crimped to the end of the solenoid. This "shading ring" creates a
small out-of-phase current, which increases the minimum pull on the armature
during the AC cycle.
A
relay is an electrically operated
switch. Current flowing through the coil of the relay creates a magnetic
field which attracts a lever and changes the switch contacts. The coil current
can be on or off so relays have two switch positions and they are double throw switches. Relays allow
one circuit to switch a second circuit which can be completely separate from
the first. For example a low voltage battery circuit can use a relay to switch
a 230V AC mains circuit. There is no electrical connection inside the relay
between the two circuits; the link is magnetic and mechanical.
The
coil of a relay passes a relatively large current, typically 30mA for a 12V
relay, but it can be as much as 100mA for relays designed to operate from lower
voltages. Most ICs (chips) cannot provide this current and a transistor is
usually used to amplify the small IC current to the larger value required for
the relay coil. The maximum output current for the popular 555 timer IC is
200mA so these devices can supply relay coils directly without amplification.
Relays are usually SPDT or DPDT but they can have many more
sets of switch contacts, for example relays with 4 sets of changeover contacts
are readily available. Most relays are designed for PCB mounting but you can
solder wires directly to the pins providing you take care to avoid melting the
plastic case of the relay. The animated
picture shows a working relay with its coil and switch contacts. You can see a
lever on the left being attracted by magnetism when the coil is switched on.
This lever moves the switch contacts. There is one set of contacts (SPDT) in
the foreground and another behind them, making the relay DPDT.
The
relay's switch connections are usually labeled COM, NC and NO:
- COM = Common, always connect to this, it is the moving part of the switch.
- NC = Normally Closed, COM is connected to this when the relay coil is off.
- NO = Normally Open, COM is connected to this when the relay coil is on.
6.3 ELECTRONIC TIMER (MCP 23016)
The MCP23016 device provides 16-bit,
general purpose, parallel I/O expansion for I2C bus applications.
This device includes high-current drive
capability, low supply current and individual I/O configuration. I/O expanders
provide a simple solution when additional I/Os are needed for ACPI, power
switches, sensors, push buttons, LEDs and so on.
The MCP23016 consists
of multiple 8-bit configuration registers for input, output and polarity
selection. The system master can enable the I/Os as either inputs or outputs by
writing the I/O configuration bits. The data for each input or output is kept
in the corresponding.
6.3.1
PINOUT DESCRIPTION
The polarity of the read register can
be inverted with the polarity inversion register. All registers can be read by the
system master.
The open-drain interrupt output is
activated when any input state differs from its corresponding input port
register state. This is used to indicate to the system master that an input
state has changed. The interrupt capture register captures port value at this
time. The Power-on Reset sets the registers to their default values and
initializes the device state machine.
Three
device inputs (A0 - A2) determine the I2C address and allow up to
eight I/O expander devices to share the same I2C bus.
Power-on Reset (POR)
The on-chip POR circuit holds the chip
in RESET until VDD has reached a high enough level to deactivate the POR
circuit (i.e., release RESET). A maximum rise time for VDD is specified in the
electrical specifications.
When the device
starts normal operation, device operating parameters must be met to ensure
proper operation.
Power-up Timer (PWRT)
The Power-up Timer provides a 72 ms
nominal timeout on power-up, keeping the device in RESET and allowing VDD to
rise to an acceptable level.
The power-up time
delay will vary from chip-to-chip due to VDD, temperature and process variation.
See Table 2-4 for details Clock Generator
The MCP23016 uses an
external RC circuit to determine the internal clock speed. The user must
connect R and C to the MCP23016,
6.4
REGULATED POWER SUPPLY
The
ac voltage, typically 220V rms, is connected to a transformer, which steps that
ac voltage down to the level of the desired dc output. A diode rectifier then
provides a full-wave rectified voltage that is initially filtered by a simple
capacitor filter to produce a dc voltage. This resulting dc voltage usually has
some ripple or ac voltage variation.
A regulator circuit removes the ripples
and also remains the same dc value even if the input dc voltage varies, or the
load connected to the output dc voltage changes. This voltage regulation is
usually obtained using one of the popular voltage regulator IC units.
6.5 TRANSFORMER
A transformer is an electrical device that transfers
energy between two circuits through electromagnetic induction. Transformers may
be used in step-up or step-down voltage
conversion, which 'transforms' an AC voltage from one voltage level on the
input of the device to another level at the output terminals. This special
function of transformers can provide control of specified requirements of
current level as an alternating current source, or it may be used for impedance matching between mismatched electrical circuits
to effect maximum power transfer between the circuits.
A transformer most commonly consist of two windings of wire wound around
a common core to induce tight electromagnetic coupling between the windings. The core material
is often a laminated iron core.
The coil that receives the electrical input energy is referred to as the
primary winding, while the output coil is called the secondary winding.
If an alternating electric current flows through the primary winding
(coil) of a transformer, an electromagnetic field is generated that develops
into a varying magnetic flux in the core of the transformer.
Through electromagnetic
induction, this magnetic flux generates a varying electromotive force in the secondary winding, which
induces a voltage across the output terminals. If a load
impedance is connected across the secondary winding, a current flows through
the secondary winding drawing power from the primary winding and its power
source.
A transformer cannot operate with direct current; although, when it is
connected to a DC source, a transformer typically produces a short output pulse
as the voltage rises
The
transformer will step down the power supply voltage (0-230V) to (0-6V) level.
Then the secondary of the potential transformer will be connected to the
precision rectifier, which is constructed with the help of op–amp. The
advantages of using precision rectifier are it will give peak voltage output as
DC; rest of the circuits will give only RMS output.
6.6 RECTIFIER
When four diodes are connected as shown
in figure, the circuit is called as bridge rectifier. The input to the circuit
is applied to the diagonally opposite corners of the network, and the output is
taken from the remaining two corners.
Let us assume
that the transformer is working properly and there is a positive potential, at
point A and a negative potential at point B. the positive potential at point A
will forward bias D3 and reverse bias D4.
The negative potential at point B will
forward bias D1 and reverse D2. At this time D3 and D1 are forward biased and
will allow current flow to pass through them; D4 and D2 are reverse biased and
will block current flow.
The path for current flow is from point
B through D1, up through RL, through D3, through the secondary of the
transformer back to point B. this path is indicated by the solid arrows.
Waveforms (1) and (2) can be observed across D1 and D3.
One-half cycle
later the polarity across the secondary of the transformer reverse, forward
biasing D2 and D4 and reverse biasing D1 and D3. Current flow will now be from
point A through D4, up through RL, through D2, through the secondary of T1, and
back to point A. This path is indicated by the broken arrows. Waveforms (3) and
(4) can be observed across D2 and D4. The current flow through RL is always in
the same direction. In flowing through RL this current develops a voltage
corresponding to that shown waveform (5). Since current flows through the load
(RL) during both half cycles of the applied voltage, this bridge rectifier is a
full-wave rectifier.
One advantage of a bridge rectifier
over a conventional full-wave rectifier is that with a given transformer the
bridge rectifier produces a voltage output that is nearly twice that of the
conventional full-wave circuit.
This may be shown by assigning values to some of the components shown in
views A and B. assume that the same transformer is used in both circuits. The
peak voltage developed between points X and y is 1000 volts in both circuits.
In the conventional full-wave circuit shown—in view A, the peak voltage from
the center tap to either X or Y is 500 volts. Since only one diode can conduct
at any instant, the maximum voltage that can be rectified at any instant is 500
volts.
The maximum voltage that appears
across the load resistor is nearly-but never exceeds-500 v0lts, as result of
the small voltage drop across the diode. In the bridge rectifier shown in view
B, the maximum voltage that can be rectified is the full secondary voltage,
which is 1000 volts. Therefore, the peak output voltage across the load
resistor is nearly 1000 volts. With both circuits using the same transformer,
the bridge rectifier circuit produces a higher output voltage than the
conventional full-wave rectifier circuit.
6.7 DIODES
In electronics, a diode is a
two-terminal electronic component with asymmetric conductance; it has low
(ideally zero) resistance to current in one direction, and high (ideally
infinite) resistance in the other. A semiconductor diode, the most common type
today, is a crystalline piece of semiconductor material with a p–n junction
connected to two electrical terminals. A vacuum tube diode has two electrodes,
a plate (anode) and a heated cathode. Semiconductor diodes were the first
semiconductor electronic devices. The discovery of crystals' rectifying
abilities was made by German physicist Ferdinand Braun in 1874. The first
semiconductor diodes, called cat's whisker diodes, developed around 1906, were
made of mineral crystals such as galena. Today, most diodes are made of
silicon, but other semiconductors such as selenium or germanium are sometimes
used
The most common function of a diode
is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking
current in the opposite direction (the reverse
direction). Thus, the diode can be viewed as an electronic version of a check
valve. This unidirectional behavior is called rectification, and is used to
convert alternating current to direct current, including extraction of
modulation from radio signals in radio receivers—these diodes are forms of
rectifiers.
However,
diodes can have more complicated behavior than this simple on–off action, due
to their nonlinear current-voltage characteristics. Semiconductor diodes begin
conducting electricity only if a certain threshold voltage or cut-in voltage is
present in the forward direction (a state in which the diode is said to be forward-biased). The voltage drop
across a forward-biased diode varies only a little with the current, and is a
function of temperature; this effect can be used as a temperature sensor or
voltage reference.
Semiconductor
diodes' current–voltage characteristic can be tailored by varying the
semiconductor materials and doping, introducing impurities into the materials.
These are exploited in special-purpose diodes that perform many different
functions. For example, diodes are used to regulate voltage, to protect
circuits from high voltage surges (avalanche diodes), to electronically tune
radio and TV receivers, to generate radio frequency oscillations, and to
produce light. Tunnel diodes exhibit negative resistance, which makes them
useful in some types of circuits.
6.8 VOLTAGE REGULATOR
Voltage regulators comprise a class of
widely used ICs. Regulator IC units contain the circuitry for reference source,
comparator amplifier, control device, and overload protection all in a single
IC. IC units provide regulation of either a fixed positive voltage, a fixed
negative voltage, or an adjustably set voltage. The regulators can be selected
for operation with load currents from hundreds of milli amperes to tens of
amperes, corresponding to power ratings from milli watts to tens of watts.
A fixed three-terminal voltage
regulator has an unregulated dc input voltage, Vi, applied to one input
terminal, a regulated dc output voltage, Vo, from a second terminal, with the
third terminal connected to ground.
The series 78 regulators provide fixed
positive regulated voltages from 5 to 24 volts. Similarly, the series 79
regulators provide fixed negative regulated voltages from 5 to 24 volts.
6.9 CAPACITORS
The
capacitors function is to store electricity, or electrical energy. The capacitor
also functions as a filter, passing alternating current (AC), and blocking
direct current (DC). This symbol –II- is used to indicate a capacitor in a
circuit diagram. The capacitor is constructed with two electrode plates phasing
each other, but separated by an insulator.
When
DC voltage is applied to capacitor, an electric charge is stored on each
electrode. While the capacitor is charging up, current flows. The current will
stop flowing when the capacitor is fully charged. The value of a capacitor (the
capacitance), is designated in units called the farad (F).
The capacitance of a capacitor is
generally very small so units such as the micro farad (10-6 F), nano
farad (10-9 F), and Pico farad (10-12) are used. This has
an insulator (the dielectric) between two sheets of electrodes. Different kinds
of capacitors use different material as dielectric. Aluminium is used for the
electrodes by using a thin oxidization membrane large values of capacitance can
be obtained in comparison with the size of the capacitor, because the
dielectric used is thin. The most important characteristics of electrolytic
capacitor are that they have polarity.
They
have a positive and a negative electrode. [Polarized] this mean that it is very
important which way round they are connected. If the capacitor is subjected to
voltage exceeding its working voltage, or if it is connected with incorrect
polarity, it may burst. It is extremely dangerous, because it can quit
literally explode.
Generally, in the circuit diagram, the
positive side is indicated by a “+” (pulse) symbol. Electrolytic capacitors
range in value in value from about 1
F to thousand of
F mainly this type of capacitor is used
as a ripple filter in a power supply circuit, or as a filter to bypass low
frequency signal, etc
DIFFERENT
TYPES OF CAPACITORS
1. Tantalum
capacitor
2.
Ceramic capacitor
3.
Multilayer ceramic capacitor
4.
Polystyrene film capacitors
5.
Electric double layer capacitors (super
capacitors)
6.
Polypropylene capacitors
7. Mica
capacitors
6.10 RESISTORS
The resistance value of the resistor is not
the only thing to consider when selecting a resistor for use in a circuit. The
“tolerance” and the electric power rating of the resistor are also important.
The tolerance of a resistor denotes how close it is to the actual rated
resistance value. For the example, a
5% tolarence would indicate a resistor that is
with in
5% of the specified resistance value.
The power rating indicates how much power the resistor can safely tolerate the
maximum rated power of the resistor is specified in watts. Power is calculated using
square of the current (12) x the resistance value (R) of the resistor. If the
maximum rating of the resistor is exceeded, it will become extremely hot and
even burn. Resistors in electronic circuit are typically rated 1/8w, 1/4w, and
1/2w. 1/8w is almost always used in signal circuit application.
6.11 TRANSISTORS
Theoretical considerations
The bipolar
transistor is an electronic device with amplification in current. For this the
transistor must by polarized correct, that is the junction base-emitter must be
polarized direct and the junction base- collector must be polarized reverse. Working
modes of the transistor is so called “active normal region” are:
Blocked transistor
The transistor is
characterized in this situation by:
VBE
= 0 V(
the base – emitter tension is smallest
than the opening tension of the junction) and VCE>0 – in this
situation both junctions are blocked, so the current does not pass between
emitter and collector.
-
between emitter and
collector appears o very high resistance : Ic - current of residual collector,
of a very low value, of order μA;
-
Ib – base 0 current.
The transistor in conduction
The transistor is
characterized in this situation by:
-
VBE> VBE0 (the oppening tension
is specified in the catalog for that transistor) and
VCE>
VCesat (specified in the
catalog) – in this situation the
junction base – emitter is polarized direct,
we have the base current IB defferent from 0; the electrones whom
reach from emitter in the base have a kinetic energy sufficiently large such
as to “pass” the potential barrier of the emitter – base junction which is
still polarized reverse;
-
Between
emitter and collector appears a
resistance which is smaller and smaller, as the base current is growing,
so the current established between emitter and collector is IC=β*IB.
We say that the transistor is in the linear conduction area.
Saturated transistor
-
VBE> 0.7V, VCE = 0.2V (the
values are de used ones, the real ones are specified in the catalog );
-
as we increase the tension VBE, at some time
moment it will be reach at the saturation of emitter – base junction and properly at the very strong increasment of the base current (and implicitly
the collector current). The tension Vce is reaching a very low value, and the two
junctions ( base – emitter , emitter –
base ) are direct polarized. In this situation, the collector current is limitated only by the external
resistances.
In case we want to amplify any signal
it will be used the transitorin the linear region of the normal active regime and if we want to use it like am element of comutation it will be
used in blocked – saturated regime.
6.12 PUSH BUTTONS
Push buttons are connected together by a mechanical linkage so that the
act of pushing one button causes the other button to be released. In this way,
a stop button can "force" a start button to be released. This method
of linkage is used in simple manual operations in which the machine or process
have no electrical circuits for control.
Pushbuttons are often color-coded to associate them with their function so that
the operator will not push the wrong button in error. Commonly used colours are red
for stopping the machine or process and green for starting the machine or
process.
Red pushbuttons can also have
large heads for easy operation and to facilitate the stopping of a machine.
These pushbuttons are called emergency stop buttons
and are mandated by the electrical code in many jurisdictions for increased
safety. This large mushroom shape can also be found in buttons for use with
operators who need to wear gloves for their work
and could not actuate a regular flush-mounted push button.
As an aid for operators and users in
industrial or commercial applications, a pilot light is
commonly added to draw the attention of the user and to provide feedback if the
button is pushed. Typically this light is included into the center of the
pushbutton and a lens replaces
the pushbutton hard center disk. The source of the energy to illuminate the
light is not directly tied to the contacts on the back of the pushbutton but to
the action the pushbutton controls. In this way a start button when pushed will
cause the process or machine operation to be started and a secondary contact
designed into the operation or process will close to turn on the pilot light
and signify the action of pushing the button cause. A push-button (also
spelled pushbutton) or simply button is a simple switch mechanism for controlling
some aspect of a machine or
a process. Buttons are typically made out of hard material,
usually plastic or metal.[1] The
surface is usually flat or shaped to accommodate the human finger or hand, so
as to be easily depressed or pushed. Buttons are most often biased switches, though
even many un-biased buttons (due to their physical nature) require a spring to return to their un-pushed state.
Different people use different terms for the "pushing" of the button,
such as press, depress, mash, and punch.
APPLICATIONS
Ø The
"push-button" has been utilized in calculators, push-button telephones, kitchen appliances, and various other mechanical and electronic
devices, home and commercial.
Ø In
industrial and commercial applications, push buttons can be conned the
resultant process or action to start.
6.13
TOGGLE SWITCH
In electrical
engineering, a switch is an electrical component that can break an electrical
circuit, interrupting the current or diverting it from one conductor to
another.
The most
familiar form of switch is a manually operated electromechanical device with
one or more sets of electrical contacts, which are connected to external
circuits. Each set of contacts can be in one of two states: either
"closed" meaning the contacts are touching and electricity can flow
between them, or "open", meaning the contacts are separated and the
switch is non conducting. The mechanism actuating the transition between these
two states (open or closed) can be either a "toggle" (flip switch for continuous "on" or
"off") or "momentary"
(push-for "on" or push-for "off") type.
A switch may
be directly manipulated by a human as a control signal to a system, such as a
computer keyboard button, or to control power flow in a circuit, such as a
light switch. Automatically operated switches can be used to control the
motions of machines, for example, to indicate that a garage door has reached
its full open position or that a machine tool is in a position to accept
another work piece. Switches may be operated by process variables such as
pressure, temperature, flow, current, voltage, and force, acting as sensors in
a process and used to automatically control a system. For example, a thermostat
is a temperature-operated switch used to control a heating process. A switch
that is operated by another electrical circuit is called a relay. Large
switches may be remotely operated by a motor drive mechanism. Some switches are
used to isolate electric power from a system, providing a visible point of
isolation that can be padlocked if necessary to prevent accidental operation of
a machine during maintenance, or to prevent electric shock.
An ideal
switch would have no voltage drop when closed, and would have no limits on
voltage or current rating. It would have zero rise time and fall time during
state changes, and would change state without "bouncing" between on
and off positions.
Practical
switches fall short of this ideal; they have resistance, limits on the current
and voltage they can handle, finite switching time, etc. The ideal switch is
often used in circuit analysis as it greatly simplifies the system of equations
to be solved, but this can lead to a less accurate solution. Theoretical
treatment of the effects of non-ideal properties is required in the design of
large networks of switches, as for example used in telephone exchanges.
A toggle
switch is a class of electrical switches that are manually actuated by a
mechanical lever, handle, or rocking mechanism.
Toggle
switches are available in many different styles and sizes, and are used in
countless applications. Many are designed to provide the simultaneous actuation
of multiple sets of electrical contacts, or the control of large amounts of
electric current or mains voltages.
The word
"toggle" is a reference to a kind of mechanism or joint consisting of
two arms, which are almost in line with each other, connected with an
elbow-like pivot. However, the phrase "toggle switch" is applied to a
switch with a short handle and a positive snap-action, whether it actually
contains a toggle mechanism or not. Similarly, a switch where a definitive
click is heard is called a "positive on-off switch. Multiple toggle
switches may be mechanically interlocked to prevent forbidden combinations.
An electrical
contact is an electrical circuit component found in electrical switches, relays
and breakers. It is composed of two pieces of electrically conductive metal
that pass electrical current or insulate when the gap between them is closed or
open. The gap must be an insulating medium of air, vacuum, oil, SF6
or other electrically insulating fluid. Contacts maybe operated by humans in
pushbuttons and switches, by mechanical pressure in sensors or machine cams,
and electromechanically in relays. Contact materials are usually composed of
superior conduction materials such as silver or gold. Cheaper metals may be
used to reduce costs for the main contact bump and plated with superior metals.
The contact arms are typically spring metals to allow operation motion without breaking.
6.14 DC MOTOR
A DC motor
relies on the fact that like magnet poles repels and unlike magnetic poles
attracts each other. A coil of wire with a current running through it generates
a electromagnetic field aligned with the center of the coil. By switching the
current on or off in a coil its magnet field can be switched on or off or by
switching the direction of the current in the coil the direction of the
generated magnetic field can be switched 180°. A simple DC motor typically has a stationary set of magnets in the stator
and an armature with a series of two or more windings of wire wrapped in
insulated stack slots around iron pole pieces (called stack teeth) with the
ends of the wires terminating on a commutators. The armature includes the
mounting bearings that keep it in the center of the motor and the power shaft
of the motor and the commutators connections.
The winding
in the armature continues to loop all the way around the armature and uses
either single or parallel conductors (wires), and can circle several times
around the stack teeth. The total amount of current sent to the coil and the
coils size and what it wrapped around dictates the strength of the
electromagnetic field created. The sequence of turning a particular coil on or
off dictates what direction the effective electromagnetic fields are pointed.
By turning on
and off coils in sequence a rotating magnetic field can be created. These
rotating magnetic fields interact with the magnetic fields of the magnets
(permanent or electromagnets) in the stationary part of the motor (stator) to
create a force on the armature which causes it to rotate. In some DC motor
designs the stator fields use electromagnets to create their magnetic fields
which allow greater control over the motor. At high power levels, DC motors are
almost always cooled using forced air.
The commutators
allow each armature coil to be activated in turn. The current in the coil is
typically supplied via two brushes that make moving contact with the commutators.
Now, some brushless DC motors have electronics that switch the DC current to
each coil on and off and have no brushes to wear out or create sparks.
Different number
of stator and armature fields as well as how they are connected provides
different inherent speed/torque regulation characteristics. The speed of a DC
motor can be controlled by changing the voltage applied to the armature. 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 which adjust the voltage by "chopping" the DC
current into on and off cycles which have an effective lower voltage.
Since the
series-wound DC motor develops its highest torque at low speed, it is often
used in traction applications such as electric locomotives, and trams. The DC
motor was the mainstay of electric traction drives on both electric and
diesel-electric locomotives, street-cars/trams and diesel electric drilling
rigs for many years. The introduction of DC motors and an electrical grid
system to run machinery starting in the 1870s started a new second Industrial
Revolution. DC motors can operate directly from rechargeable batteries,
providing the motive power for the first electric vehicles and today's hybrid
cars and electric cars as well as driving a host of cordless tools. 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.
If external
power is applied to a DC motor it acts as a DC generator, a dynamo. This
feature is used to slow down and recharge batteries on hybrid car and electric
cars or to return electricity back to the electric grid used on a street car or
electric powered train line when they slow down. This process is called
regenerative braking on hybrid and electric cars. In diesel electric
locomotives they also use their DC motors as generators to slow down but
dissipate the energy in resistor stacks. Newer designs are adding large battery
packs to recapture some of this energy.
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.
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 carbon
brushes and springs which carry the electric current, as well as cleaning or
replacing the commutators. These components are necessary for transferring
electrical power from outside the motor to the spinning wire windings of the
rotor inside the motor. Brushes consist of conductors.
Typical brushless DC motors use a
rotating permanent magnet in the rotor, and stationary electrical current/coil
magnets on the motor housing for the stator, 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.
A
Permanent Magnet motor does not have a field winding on the stator frame,
instead relying on PMs to provide the magnetic field against which the rotor
field interacts to produce torque. Compensating windings in series with the
armature may be used on large motors to improve commutation under load. Because
this field is fixed, it cannot be adjusted for speed control. Permanent Magnet
fields (stators) are convenient in miniature motors to eliminate the power
consumption of the field winding. Larger DC motors are of the
"dynamo" type, which have stator windings. Historically, Permanent Magnets
could not be made to retain high flux if they were disassembled; field windings
were more practical to obtain the needed amount of flux. However, large Permanent
Magnets are costly, as well as dangerous and difficult to assemble; this favors
wound fields for large machines.
To minimize
overall weight and size, miniature Permanent Magnet motors may use high energy
magnets made with neodymium or other strategic elements; most such are
neodymium-iron-boron alloy. With their higher flux density, electric machines
with high-energy Permanent Magnets are at least competitive with all optimally
designed singly fed synchronous and induction electric machines. Miniature
motors resemble the structure in the illustration, except that they have at
least three rotor poles (to ensure starting, regardless of rotor position) and
their outer housing is a steel tube that magnetically links the exteriors of
the curved field magnets.
ADVANTAGES
Ø Very
simple in construction
Ø Small
in size
Ø Very
low cost
Ø Higher
efficiency
Ø Maintenance
is very easy
APPLICATIONS
Ø Used
in industries were clockwise; counter clock-wise, forward braking and reverse
braking is needed.
Ø Used
in lifts.
Ø Used
in cranes etc….
CONCLUSION
The
progress in science & technology is a non-stop process. Now things and new
technology are being invented. As the technology grows day by day, we can
imagine about the future in which thing we may occupy every place.
The
proposed system is found to be more compact, user friendly and less complex,
which can readily be used in order to perform several tedious and repetitive
tasks. Though it is designed keeping in mind about the need for industry, it
can extended for other purposes such as commercial & research applications.
The feature makes this system is the base for future systems.
The
principle of the development of science is that “nothing is impossible”.
BIBLIOGRAPHY
BOOKS
1. N
N Bhargav – Basic electronics and linear circuits,
2. A
K Shawney – Electrical machine design,
3. Stan
Giblisco – Teach yourself electronics and electricity,
WEB SITES
· www.howstuffworks.com
·
www.nptel.com
· www.automation.schneir-electric.com
