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LW1DSE > TECH 17.01.18 15:32l 399 Lines 19813 Bytes #999 (0) @ WW
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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.
Contents:
1) Operation
2) Types of relay
2.1) Latching relay
2.2) Reed relay
2.3) Mercury-wetted relay
2.4) Polarized relay
2.5) Machine tool relay
2.6) Contactor relay
2.7) Solid state contactor relay
2.8) Buchholz relay
2.9) Forced-guided contacts relay
2.10) Solid-state relay
2.11) Overload protection relay
3) Pole & Throw
4) Applications
5) Relay application considerations
6) Protective relay
6.1) Overcurrent relay
7) Distance relay
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.
By analogy with the functions of the original electromagnetic device, a
solid-state relay is made with a thyristor or other solid-state switching
device. To achieve electrical isolation an optocoupler can be used which is a
light-emitting diode (LED) coupled with a photo transistor.
2) Types of relay
2.1) Latching relay
A latching relay has two relaxed states (bistable). These are also
called 'keep' relays. When the current is switched off, the relay remains in
its last state. This is achieved with a solenoid operating a ratchet and cam
mechanism, or by having two opposing coils with an over-center spring or
permanent magnet to hold the armature and contacts in position while the coil
is relaxed, or with a remanent core. In the ratchet and cam example, the first
pulse to the coil turns the relay on and the second pulse turns it off. In
the two coil example, a pulse to one coil turns the relay on and a pulse to
the opposite coil turns the relay off. This type of relay has the advantage
that it consumes power only for an instant, while it is being switched, and
it retains its last setting across a power outage.
2.2) Reed relay
A reed relay has a set of contacts inside a vacuum or inert gas filled
glass tube, which protects the contacts against atmospheric corrosion. The
contacts are closed by a magnetic field generated when current passes through
a coil around the glass tube. Reed relays are capable of faster switching
speeds than larger types of relays, but have low switch current and voltage
ratings. See also reed switch.
2.3) Mercury-wetted relay
A mercury-wetted reed relay is a form of reed relay in which the
contacts are wetted with mercury. Such relays are used to switch low-voltage
signals (one volt or less) because of its low contact resistance, or for
high-speed counting and timing applications where the mercury eliminates
contact bounce. Mercury wetted relays are position-sensitive and must be
mounted vertically to work properly. Because of the toxicity and expense of
liquid mercury, these relays are rarely specified for new equipment. See also
mercury switch.
2.4) Polarized relay
A Polarized Relay placed the armature between the poles of a permanent
magnet to increase sensitivity. Polarized relays were used in middle 20th
Century telephone exchanges to detect faint pulses and correct telegraphic
distortion. The poles were on screws, so a technician could first adjust them
for maximum sensitivity and then apply a bias spring to set the critical
current that would operate the relay.
2.5) Machine tool relay
A machine tool relay is a type standardized for industrial control of
machine tools, transfer machines, and other sequential control. They are
characterized by a large number of contacts (sometimes extendable in the
field) which are easily converted from normally-open to normally-closed
status, easily replaceable coils, and a form factor that allows compactly
installing many relays in a control panel. Although such relays once were the
backbone of automation in such industries as automobile assembly, the
programmable logic controller mostly displaced the machine tool relay from
sequential control applications.
2.6) Contactor relay
A contactor is a very heavy-duty relay used for switching electric
motors and lighting loads. With high current, the contacts are made with pure
silver. The unavoidable arcing causes the contacts to oxidize and silver
oxide is still a good conductor. Such devices are often used for motor
starters. A motor starter is a contactor with overload protection devices
attached. The overload sensing devices are a form of heat operated relay where
a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring
to operate auxiliary contacts. These auxiliary contacts are in series with
the coil. If the overload senses excess current in the load, the coil is
de-energized. Contactor relays can be extremely loud to operate, making them
unfit for use where noise is a chief concern.
2.7) Solid state contactor relay
A solid state contactor is a very heavy-duty solid state relay,
including the necessary heat sink, used for switching electric heaters, small
electric motors and lighting loads; where frequent on/off cycles are required.
There are no moving parts to wear out and there is no contact bounce due to
vibration. They are activated by AC control signals or DC control signals from
Programmable logic controller (PLCs), PCs, Transistors sources, or other
microprocessor controls.
2.8) Buchholz relay
A Buchholz relay is a safety device sensing the accumulation of gas in
large oil-filled transformers, which will alarm on slow accumulation of gas
or shut down the transformer if gas is produced rapidly in the transformer
oil.
2.9) Forced-guided contacts relay
A forced-guided contacts relay has relay contacts that are mechanically
linked together, so that when the relay coil is energized or de-energized, all
of the linked contacts move together. If one set of contacts in the relay
becomes immobilized, no other contact of the same relay will be able to move.
The function of forced-guided contacts is to enable the safety circuit to
check the status of the relay. Forced-guided contacts are also known as
"positive-guided contacts", "captive contacts", "locked contacts", or "safety
relays".
2.10) Solid-state relay
A solid state relay (SSR) is a solid state electronic component that
provides a similar function to an electromechanical relay but doesn't have
any moving components, increasing long-term reliability. With early SSR's,
the tradeoff came from the fact that every transistor has a small voltage
drop across it. This collective voltage drop limited the amount of current a
given SSR could handle. As transistors improved, higher current SSR's, able
to handle 100 to 1,200 amps, have become commercially available. Compared to
electromagnetic relays, they may be falsely triggered by transients.
2.11) Overload protection relay
One type of electric motor overload protection relay is operated by a
heating element in series with the electric motor. The heat generated by the
motor current operates a bi-metal strip or melts solder, releasing a spring
to operate contacts. Where the overload relay is exposed to the same
environment as the motor, a useful though crude compensation for motor
ambient temperature is provided.
3) Pole & Throw
"C" denotes the common terminal in SPDT and DPDT types.
Since relays are switches, the terminology applied to switches is also
applied to relays. A relay will switch one or more poles, each of whose
contacts can be thrown by energizing the coil in one of three ways:
* Normally-open (NO) contacts connect the circuit when the relay is activated;
the circuit is disconnected when the relay is inactive. It is also called a
Form A contact or "make" contact.
* Normally-closed (NC) contacts disconnect the circuit when the relay is
activated; the circuit is connected when the relay is inactive. It is also
called a Form B contact or "break" contact.
* Change-over, or double-throw, contacts control two circuits: one
normally-open contact and one normally-closed contact with a common
terminal. It is also called a Form C contact or "transfer" contact. If this
type of contact utilizes a "make before break" functionality, then it is
called a Form D contact.
The following types of relays are commonly encountered:
SPST - Single Pole Single Throw. These have two terminals which can be
connected or disconnected. Including two for the coil, such a relay
has four terminals in total. It is ambiguous whether the pole is
normally open or normally closed. The terminology "SPNO" and "SPNC" is
sometimes used to resolve the ambiguity.
SPDT - Single Pole Double Throw. A common terminal connects to either of two
others. Including two for the coil, such a relay has five terminals in
total.
DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent
to two SPST switches or relays actuated by a single coil. Including two
for the coil, such a relay has six terminals in total. It is ambiguous
whether the poles are normally open, normally closed, or one of each.
DPDT - Double Pole Double Throw. These have two rows of change-over terminals.
Equivalent to two SPDT switches or relays actuated by a single coil.
Such a relay has eight terminals, including the coil.
QPDT - Quadruple Pole Double Throw. Often referred to as Quad Pole Double
Throw, or 4PDT. These have four rows of change-over terminals.
Equivalent to four SPDT switches or relays actuated by a single coil,
or two DPDT relays. In total, fourteen terminals including the coil.
4) Applications
Relays are used:
* to control a high-voltage circuit with a low-voltage signal, as in some
types of modems,
* to control a high-current circuit with a low-current signal, as in the
starter solenoid of an automobile,
* to detect and isolate faults on transmission and distribution lines by
opening and closing circuit breakers (protection relays),
* to isolate the controlling circuit from the controlled circuit when the two
are at different potentials, for example when controlling a mains-powered
device from a low-voltage switch. The latter is often applied to control
office lighting as the low voltage wires are easily installed in partitions,
which may be often moved as needs change. They may also be controlled by
room occupancy detectors in an effort to conserve energy,
* to perform logic functions. For example, the boolean AND function is
realised by connecting NO relay contacts in series, the OR function by
connecting NO contacts in parallel. The change-over or Form C contacts
perform the XOR (exclusive or) function. Similar functions for NAND and NOR
are accomplished using NC contacts. The Ladder programming language is often
used for designing relay logic networks.
Early computing. Before vacuum tubes and transistors, relays were used
as logical elements in digital computers. Examples of this are ARRA
(computer), Harvard Mark II, Zuse Z2, and Zuse Z3.
Safety-critical logic. Because relays are much more resistant than
semiconductors to nuclear radiation, they are widely used in safety-critical
logic, such as the control panels of radioactive waste-handling machinery.
* to perform time delay functions. Relays can be modified to delay opening or
delay closing a set of contacts. A very short (a fraction of a second)
delay would use a copper disk between the armature and moving blade
assembly. Current flowing in the disk maintains magnetic field for a short
time, lengthening release time. For a slightly longer (up to a minute)
delay, a dashpot is used. A dashpot is a piston filled with fluid that is
allowed to escape slowly. The time period can be varied by increasing or
decreasing the flow rate. For longer time periods, a mechanical clockwork
timer is installed.
5) Relay application considerations
* Number and type of contacts: normally open, normally closed, changeover
(double-throw). In the case of changeover, there are two types. This style
of relay can be manufactured two different ways. "Make before Break" and
"Break before Make". The old style telephone switch required
Make-before-break so that the connection didn't get dropped while dialing
the number. The railroad still uses them to control railroad crossings.
* Rating of contacts: small relays switch a few amperes, large contactors are
rated for up to 3000 amperes, alternating or direct current;
* Voltage rating of contacts: typical control relays rated 300 or 600 VAC,
automotive types to 50 VDC, special high-voltage relays to about 15,000 V;
* Coil voltage: machine-tool relays usually 24 VAC or 120 VAC, relays for
switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a
few milliamperes;
* Package/enclosure: open, touch-safe, double-voltage for isolation between
circuits, explosion proof, outdoor, oil-splash resistant;
* Mounting: sockets, plug board, rail mount, panel mount, through-panel mount,
enclosure for mounting on walls or equipment;
* Switching time: where high speed is required;
* "Dry" contacts: when switching very low level signals, special contact
materials may be needed such as gold-plated contacts;
* Contact protection: suppress arcing in very inductive circuits;
* Coil protection: suppress the surge voltage produced when switching the coil
current;
* Isolation between coil circuit and contacts;
* Aerospace or radiation-resistant testing, special quality assurance;
* Expected mechanical loads due to acceleration: some relays used in aerospace
applications are designed to function in shock loads of 50 g or more
* Accessories such as timers, auxiliary contacts, pilot lamps, test buttons
* Regulatory approvals
* Stray magnetic linkage between coils of adjacent relays on a printed circuit
board.
6) Protective relay
A protective relay is a complex electromechanical apparatus, often with
more than one coil, designed to calculate operating conditions on an
electrical circuit and trip circuit breakers when a fault was found. Unlike
switching type relays with fixed and usually ill-defined operating voltage
thresholds and operating times, protective relays had well-established,
selectable, time/current (or other operating parameter) curves. Such relays
were very elaborate, using arrays of induction disks, shaded-pole magnets,
operating and restraint coils, solenoid-type operators, telephone-relay style
contacts, and phase-shifting networks to allow the relay to respond to such
conditions as over-current, over-voltage, reverse power flow, over- and under-
frequency, and even distance relays that would trip for faults up to a
certain distance away from a substation but not beyond that point. An
important transmission line or generator unit would have had cubicles
dedicated to protection, with a score of individual electromechanical devices.
The various protective functions available on a given relay are denoted
by standard ANSI Device Numbers. For example, a relay including function 51
would be a timed overcurrent protective relay.
These protective relays provide various types of electrical protection
by detecting abnormal conditions and isolating them from the rest of the
electrical system by circuit breaker operation. Such relays may be located at
the service entrance or at major load centers.
Design and theory of these protective devices is an important part of
the education of an electrical engineer who specializes in power systems.
Today these devices are nearly entirely replaced (in new designs) with
microprocessor-based instruments (numerical relays) that emulate their
electromechanical ancestors with great precision and convenience in
application. By combining several functions in one case, numerical relays
also save capital cost and maintenance cost over electromechanical relays.
However, due to their very long life span, tens of thousands of these "silent
sentinels" are still protecting transmission lines and electrical apparatus
all over the world.
6.1) Overcurrent relay
An "Overcurrent Relay" is a type of protective relay which operates
when the load current exceeds a preset value. The ANSI Device Designation
Number is 50 for an Instantaneous OverCurrent (IOC), 51 for a Time OverCurrent
(TOC). In a typical application the overcurrent relay is used for overcurrent
protection, connected to a current transformer and calibrated to operate at
or above a specific current level. When the relay operates, one or more
contacts will operate and energize a trip coil in a Circuit Breaker and trip
(open) the Circuit Breaker.
7) Distance relay
It is a protective relay used to protect power transmission and
distribution lines against different fault types. The relay monitors line
impedance by measuring line voltage and current. Once a fault occurs, the
voltage drops to zero and thus the measured impedance become less than the
setting value "reach". As a result the relay issues a trip command.
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