[1] Electromechanical relay principle
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Relay
Relay
[1] A relay is an electrical switch with a set of input terminals for 1/many control signals and a set of operating contact terminals. It may have some contacts in many contact forms (e.g. make contacts, break contacts, or combinations thereof).
They control a circuit by an independent low-power signal and many circuits by 1 signal and were first for long-distance telegraph circuits as signal repeaters sending refreshed incoming signals copies to other circuit. Relays were used alot in phone exchanges and old computers for logical operations.
Components
Components
[7], [8] Relays' key parts vary between electromechanical relays (EMRs) and solid-state relays (SSRs).
[1.1] Auxiliary contacts are a type of contacts let low current to flow between a circuit's 2 points by contact, a type of contacts mainly for controls to carry low currents.
Power contacts are another type of contacs carrying higher current (60A here) and voltage, for power transfer in circuits. Both power and auxiliary contacts have normally open and close states.
Manual operation is an action/process of human control, instead of automatic--very hazardous due to electrical, mechanical.
Unlike EMRs, a SSR has an electromagnet coil of copper wound around an iron core. If energized, it creates a magnetic field to move the armature.
Its voltage ratings are often: 5V, 12V, 24V, 120V, 240V (AC/DC).
[10] An armature/Moving contact is a lever/spring-loaded metal piece moving if the coil energizes and connects/disconnects the common (COM) terminal from NO/NC contacts. An armature's one end connectes to a tension spring that pulls the other end of the armature up--the relay in its 'at rest' state, without voltage. The braided bonding strap gives great electrical bond between an armature and yolk, rather than relying on contact between the armature pivot point alone. The coil and contact (or contacts) are then connected to various terminals on the outside of the relay body.
The coil around the core is held in a frame or 'yoke' from which the armature hinges. Yoke and iron core concentrate magnetic field for efficient armature movement.
[6] Note: General contacts carry high voltage and high current. But auxiliary contacts carry low voltage only, with many purposes in motor-operated devices and electrical professionals for safety and protection from electrical faults.
In intelligent, reliable, and efficient electrical products, CHINT offers auxiliary contact that protects your circuit breakers from damage, provides enhanced protection against electrical hazards, minimizes electrical failure risks, and increases circuit breaker life expectancy.
Types
Types
[11]
An electromechanical relays (EMR) has a coil and uses physical moving parts, electrically controlling circuits by opening/closing contacts via an electromagnet and a coil to make an electromagnetic field moving the armature, changing the contacts' position, to switch between circuits and handle high-power loads via a low-power control signal.
A solid-state relays (SSR) have no moving parts, using semiconductors (like thyristors, TRIACs, or MOSFETs).
Contactors are larger higher-power relays made for heavy-duty industrial loads (motors, heaters, lighting), handling up to thousands amps of currents.
[12] Reed relay are special fast switching electromechanical relays using hermetically sealed reed switches (enclosed in glass) to open/close circuits. Known for being compact, it's often for low-power, high-speed, or signal isolation small fast-acting applications, using a sealed magnetic reed switch.
hermetically: in a fully closed way
[3.3] Time delay relays use a programmed delay, switching after a set of time, before closing/opening a circuit, used in automation, motor controls, lighting, and safety systems where timing is critical.
[1.5] Buchholz relays (by German inventor Max Buchholz, 1921) are safety relays for oil-filled transformers to detect and mitigate internal faults (arcing, insulation fault), acting as early warning system, stopping transformer damages.
an EMR
an SSR
a contactor relay
reed relays
a time delay relay
a Buchholz relay
Solid-state and electromechanical relay differences:
[1.6] Coaxial relay
[1.7] High-voltage interface relay
[1.8] Safety relay
[1.9] Shading coil
[1.10] Starter solenoid
[1.11] Uniselector
[1.12] Wire spring relay
[1.13] Mercury relay
[1.14] Numerical relay
Systems
Systems
[R1]
A relay's diagram
If the arm moves the switch closes.
Electromagnets have an iron core.
We can make one with an iron bolt.
In industrial relay logic circuits, switch contacts and relay coils' operating voltage are often 120 V AC.
Lower voltage AC and DC systems are at times made according to ladder diagrams:
So long as the switch contacts and relay coils are all adequately rated, it won't matter what voltage level a system has.
An insulated conductor is wound around a core.
Without current flow, no magnetic field.
Current flow turns the iron core into a magnet--how a junk yard crane works (Fig 2.)
Fig 2.
This electromagnet isn’t energised.
Energizing the magnet moves the arm, completing the circuit.
What if we add a upper contact set?
Energise the coil turns off the red light.
“Normal” is always with the coil de-energised
The 3 symbols are used on all control drawings
This yellow part is the control circuit.
This yellow area is the power circuit.
[14] This is a motor control panel (MCP), used control and protect electric motors. its left side is a power circuit and its right is a control circuit.
Control circuits often:
have lower current
have lower voltage
are more complex small conductors
Power circuits often:
have higher current
have higher voltage
are less complex big conductors
an amplifier
As amplifiers
As amplifiers
An amplifier is an electronic device increasing electrical signals' amplitude.
Big motors/other heavy loads are often controlled by low power control circuits, for use of smaller, more sophisticated, and cheaper controls, with effectively amplified control signal.
Under this ramp is a heating cable system keeping the ramp ice-free, operating at 600 V, 3 phase and draws 96 A.
Cables are controlled by a thermostat/a manual switch and have a pilot light to show they are on, and a pilot light to show if they are off.
thermostat: a regulating device component sensing a physical system's temperature and does actions so that the temperature stays near a point.
manual switch: a switch turned on/off manually by a handle/knob/button
Heating power
Heating power
A power circuit is often simple.
Heating cables maybe multiple sets in parallel.
The conductors of the power circuit maybe a 3 AWG copper minimum.
Heater controls Controls are powered by a 600/120 V transformer. The circuit is grounded and equipped via a fuse Fig 4.)
A thermostat is a regulating device component sensing a physical system's temperature and acts for the temperature stays near a point.
The thermostat controls the M coil and is often drawn as a normally open (NO) or normally closed (NC) responding to temperature changes (Fig 5.)
M coil (motor starter coil or contactor coil) is often used in industrial control circuits to energize motor starters/contactors, that then switches power to the motor.
The M contacts control the heater.
M contacts are auxiliary contacts controlled by an M coil, used to latch a circuit (hold a motor running after start button releases), tell status (e.g., motor ON/OFF), and interlock to other circuits (stop conflicting operations).
The manual switch also controls the M coil.
The pilot light comes on with the heating cables (Fig 6.)
Fig 4.
Fig 5.
Fig 6.
Fig 7.
When is the green (G) pilot light come on?
Does this circuit meet all the requirements?
A coil for such a contactor would draw under a quarter amp at 120 V, consuming under 30 W.
The heaters consume 99.8 kW. The control signal is amplified 3000 times.
History
History
Relays were first made in 19th century for telegraph systems, revolutionizing long-distance communication, originally made for telegraphy. But the issues were electrical signals weakened in long distances due to wire resistance, limiting telegraph range--the solution was relays acted as signal repeaters--detecting a weak incoming signal and retransmitting it at full strength to the next segment of the line.
A weak current from the telegraph line energized an electromagnet in the relay.
The magnet pulled a lever (armature) to close a local circuit with a fresh battery, boosting the signal.
Impact Enabled transcontinental telegraph lines (e.g., Samuel Morse’s system).
Joseph Henry (1835)made the first relay by using an electromagnet to remotely control a high-current circuit.
Samuel Morse (1844) used relays in the Baltimore-Washington telegraph line, proving their practicality.
for railway signaling, relays ensured safe train routing by electrically controlling track switches and signals.
Related pages
Related pages
[R1]
References
References
[1] Wikipedia
[1.1] Electrical contact
[1.2] Solid-state relay
[1.3] Protective relay
[1.4] Reed relay
[1.5] Buchholz relay
[1.6] Coaxial relay
[1.8] Safety relay
[1.9] Shading coil
[1.10] Starter solenoid
[1.11] Uniselector
[1.12] Wire spring relay
[1.13] Mercury relay
[1.14] Numerical relay
[1.15]
[3] “Ladder” Diagrams - All About Circuits
[3.3] Time-delay Relays
[5] Understanding Industrial Relay Contact Configurations: NO, NC, SPDT, and DPDT - Proax
[6] What is the difference between auxiliary and main contacts? - Facebook
[8] Detailed Analysis Between Solid State vs Electromechanical Relays - Ariat Technology
[9] Difference Between Solid-State Relay and Electromechanical Relay - Electrical Technology
Quizzes
Quizzes
[Q1] Flaschards
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