ABSTRACT
This deals with the control system of a computer
numeric control machine and its associated interaction with electromechanically
devices. Also, here we shall be looking at the various components such as
circuit breakers, contactors, relays etc that makes this control system
possible. This component’s description, operation, types and uses shall as well
be discussed.
INTRODUCTION
The
use of simple electrical components for the designing and constructing of a
panel capable of controlling an electrical machine both in the forward and
reverse directions has led to automation engineering and subsequently to
Computer Numeric Control machine technology. The computer numeric control (CNC)
panel is the automated design part of a CNC machine where electrical inputs are
fed and controlled. Such control includes the spindle motor, the X,Y, motors,
the coolant motors etc. It is made up of both electrical and electronics
components. Here emphasis shall be laid on the electrical components as much of
it does the control system.
Below is an outline of the various components used
in a CNC control panel;
Circuit breaker
Control transformer
Motor protection switch
Contactors
Rectifier
Relays e. t. c.
CIRCUIT
BREAKERS
DESCRIPTION
break An early form of circuit breaker was
described by Thomas Alva Edison in an 1879 patent application, although
his commercial power distribution system used fuses.[1]
Its purpose was to protect lighting circuit wiring from accidental
short-circuits and overloads.
OPERATION OF A
CIRCUIT BREAKER
All
circuit breakers have common features in their operation, although details vary
substantially depending on the voltage class, current rating and type of the
circuit breaker. The circuit breaker must detect a fault condition; in
low-voltage circuit breakers this is usually done within the breaker enclosure.
Circuit breakers for large currents or high voltages are usually arranged with pilot
devices to sense a fault current and to operate the trip opening mechanism.
The
trip solenoid that releases the latch is usually energized by a separate
battery, although some high-voltage circuit breakers are self-contained with
current transformers, protection relays, and an internal control power source.
Once a fault is detected, contacts within the circuit breaker must open to
interrupt the circuit.
The
circuit breaker contacts must carry the load current without excessive heating,
and must also withstand the heat of the arc produced when interrupting the
circuit.
Contacts
are made of copper or copper alloys, silver alloys, and other materials.
Service life of the contacts is limited by
the erosion due to interrupting the arc. Miniature and molded case circuit
breakers are usually discarded when the contacts are worn, but power circuit
breakers and high-voltage circuit breakers have replaceable contacts.
POLES OF A CIRCUIT
BREAKER
Circuit breaker poles are usually expressed
in 1p, 2p, 3p, 4p, e.t.c. this indicates the terminal(s) of the breaker.
TYPES OF CIRCUIT
BREAKER.
Low voltage circuit breakers
Magnetic circuit breaker
Thermal magnetic circuit breaker Common trip
breakers
High-voltage circuit breakers
Sulfur hexafluoride (SF6)
high-voltage circuit-breakers
Other breakers
Breakers for protections against earth faults
too small to trip an over-current device:
Residual-current device (RCD, formerly known
as a residual current circuit breaker) — detects current imbalance, but
does not provide over-current protection.
Residual current breaker with over-current
protection (RCBO) — combines the functions of an RCD and an MCB in one package. In
the United States and Canada, panel-mounted devices that combine ground
(earth) fault detection and over-current protection are called Ground Fault
Circuit Interrupter (GFCI) breakers; a wall mounted outlet device providing
ground fault detection only is called a GFI.
Earth leakage circuit breaker (ELCB) — This detects earth current directly
rather than detecting imbalance. They are no longer seen in new installations
for various reasons
Autorecloser — A type of circuit
breaker which closes again after a delay. These are used on overhead power
distribution systems, to prevent short duration faults from causing
sustained outages.
Polyswitch (polyfuse) — A small device commonly
described as an automatically resetting fuse rather than a circuit breaker
CONTROL TRANSFORMER
A
control transformer is a device used to transform or "step down" a
high main circuit voltage to a lower voltage which is then used to operate the
control or switching components of the main circuit. These devices are commonly
used in industrial starter circuits where the main circuit voltage is not
suitable for use in the control circuit and where a separate control circuit
feed would not be practical. For example in a starter panel designed to start a
500 volt electric motor, the contactors and relays used to switch the motor on
or off would typically use electromagnetic coils rated for a far lower voltage.
USES OF CONTROL
TRANFORMER
To supply this voltage without the need for a
separate power feed, power is tapped off the main incoming 500 volt feed and
passed through a control transformer which would then supply the lower control
circuit voltage.
Heavy
electrical machinery that starts automatically or remotely generally makes use
of contactors which rely on an electromagnetic force to close them to
start the machinery. This force is created by an electric coil placed in the
center of a laminated steel core. These coils are typically designed to operate
at fairly low voltages, ranging from 110 volts to as low as 12 volts. As these machines
themselves typically run on far higher voltages, this creates the need for a
separate control voltage feed.
Instead of having to run separate cables or
install extra sets of bus bars, it is far simpler to use the main circuit
voltage and step it down with a control transformer to the appropriate control
voltage. Low control circuit voltages are used for various reasons including
the fact that parts of the control circuit include push buttons in a remote
control room, on the starter panel door, and at the machine itself. It would
not be wise to have high voltages used in these applications for obvious safety
reasons. It is also undesirable to have densely packed control wiring carrying
very high voltages inside the starter panel either. For these reasons, lower
voltages are typically used in control circuits.
Another
benefit of using a control transformer is the inherent stability of the voltage
supplied from a transformer as well as its ability to handle extreme peaks in
demand. When the start button on a motor starter is pushed and the contactor
coil energizes, there is a very brief (typically 30 to 50 milliseconds)
surge in current demand known as an "inrush current". This
peak can exceed 10 times the normal current flow, and transformers handle these
peaks far more efficiently than a conventional supply. Using a control
transformer to supply control power thus allows a lower, safer and more
efficient control circuit voltage to be used in high working voltage
applications. The excellent inrush current handling characteristics of
transformer supplied power also makes for a more efficient power supply.
Lastly, the use of lower voltages in a control circuit make for far safer use
by workers using stop and start buttons in hazardous environments.
Motor protection
switch
Mainly used for overload and short-circuit
protections of motor in AC 50/60Hz, up to 660V, 0.1 to 80A power circuit
Full voltage starter to start and cut-off motor
Under AC-3 load or for the overload and short-circuit
protection of circuit and power equipment in power distribution network
Conform to IEC60947.2 and IEC 60947-4.1 as
well as the EN60947-1 standard
Mainly used for overload and short-circuit protections
of motor in AC 50/60Hz, up to 660V, 0.1 to 80A power circuit
Full voltage starter to start and cut-off motor
Under AC-3 load or for the overload and short-circuit
protection of circuit and power equipment in power distribution network
CONTACTOR
Contactors are electrical devices used to
control electric motors, lighting, heating, and other
electrical loads.
A contactor
is an electrically controlled switch used for switching a power circuit,
similar to a relay except with higher current ratings.[1]
A contactor is controlled by a circuit which has a much lower power level than
the switched circuit. A contactor is composed of three different items. The
contacts are the current carrying part of the contactor. This includes power
contacts, auxiliary contacts, and contact springs. The electromagnet 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.
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 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, which 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.
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 motor controllers use vacuum contactors. High voltage
contactors (greater than 1000 volts) may use vacuum or an inert gas around the
contacts.
APPLICATIONS/USES
Lighting control
Contactors are often used to provide central
control of large lighting installations, such as an office building or retail
building. To reduce power consumption in the contactor coils, latching
contactors are used, which have two operating coils. One coil, momentarily
energized, closes the power circuit contacts, which are then mechanically held
closed; the second coil opens the contacts.
Magnetic starter
A magnetic starter is a contactor designed to
provide power to electric motors. The magnetic starter has an overload
relay, which will open the control voltage to the starter coil if it detects an
overload on a motor.[
Overload relays may rely on heat produced by
the motor current to operate a bimetal contact or release a contact held
closed by a low-melting-point alloy. The overload relay opens a set of contacts
that are wired in series with the supply to the contactor feeding the
motor. The characteristics of the heaters can be matched to the motor so that
the motor is protected against overload. Recently, microprocessor-controlled
motor digital protective relays offer more comprehensive protection of
motors.
PICTURE OF A
CONTACTOR
The picture below shows the nature,
appearance and connection of a typical contactor.
Motor control contactors can be fitted with
short-circuit protection (fuses or circuit breakers), disconnecting means,
overload relays and an enclosure to make a combination starter. Several
combination starters and other switchgear and control devices can be grouped in
a common enclosure called a motor control center/panel.
RECTIFIER
A rectifier
is an electrical device that converts alternating current (AC), which
periodically reverses direction, to direct current (DC), which is in
only one direction, a process known as rectification.
Rectifiers have many uses including as components of power supplies and
as detectors of radio signals. Rectifiers may be made of solid
state diodes, vacuum tube diodes, mercury arc valves,
and other components.
A device which performs the opposite function
(converting DC to AC) is known as an inverter.
When only one diode is used to rectify AC (by
blocking the negative or positive portion of the waveform), the
difference between the term diode and the term rectifier is
merely one of usage, i.e., the term rectifier describes a diode
that is being used to convert AC to DC. Almost all rectifiers comprise a number
of diodes in a specific arrangement for more efficiently converting AC to DC
than is possible with only one diode. Before the development of silicon
semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium
rectifier stacks were used
HALF-WAVE
RECTIFICATION.
In half wave rectification, either the
positive or negative half of the AC wave is passed, while the other half is
blocked. Because only one half of the input waveform reaches the output, it is
very inefficient if used for power transfer. Half-wave rectification can be
achieved with a single diode in a one-phase supply, or with three diodes in a three-phase
supply. The output DC voltage of a half wave rectifier can be calculated with
the following two ideal equations:[1]
FULL-WAVE
RECTIFICATION.
A full-wave rectifier converts the whole of
the input waveform to one of constant polarity (positive or negative) at its
output. Full-wave rectification converts both polarities of the input waveform
to DC (direct current), and is more efficient.
However, in a circuit with a non-center
tapped transformer, four diodes are required instead of the one
needed for half-wave rectification.
For single-phase AC, if the transformer is
center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode)
can form a full-wave rectifier. Twice as many windings are required on the
transformer secondary to obtain the same output voltage compared to the bridge
rectifier above.
Full-wave rectifier using a center tap
transformer and 2 diodes.
The
average and root-mean-square output voltages of an ideal single
phase full wave rectifier can be calculated as
Vr.m.s
= Vp/√2
Vdc,Vav - the average or DC
output voltage,
Vp - the peak value of half wave,
Vrms - the root-mean-square value of output
voltage.
π = ~ 3.14159
RELAYS
DESCRIPTION
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 most have double throw
(changeover) switch contacts as
shown in the diagram.
OPERATION OF A
RELAY
Relays allow one circuit to switch a second
circuit which can be completely separated 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. 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.
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.
Connect to COM and NO if you want the
switched circuit to be on when the
relay coil is on.
Connect to COM and NC if you want the
switched circuit to be on when the
relay coil is off.
Protection diodes
for relays
Transistors and ICs must be protected from
the brief high voltage produced when a relay coil is switched off. The diagram
shows how a signal diode (eg 1N4148) is connected 'backwards' across the
relay coil to provide this protection.
Current flowing through a relay coil creates
a magnetic field which collapses suddenly when the current is switched off. The
sudden collapse of the magnetic field induces a brief high voltage across the
relay coil which is very likely to damage transistors and ICs. The protection
diode allows the induced voltage to drive a brief current through the coil (and
diode) so the magnetic field dies away quickly rather than instantly. This
prevents the induced voltage becoming high enough to cause damage to
transistors and ICs.
USES/APPLICATIONS
OF RELAYS
Relays are used to switch AC and DC
Relays can switch higher voltages.
Relays are often a better choice for
switching large currents
(> 5A).
Relays can switch many contacts.
CONCLUSION
With
the use of control panel, the production of convectional machine has become
uncommon in the field of engineering and technology and subsequently makes
machine automatic in operation. Today over 50% of our machines in the workshop
can be operated with little or no human intervention. Therefore, for an
efficient and effective maintenance of computer numeric control machines, a
sound knowledge of paneling is essential.
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