What is the working principle of AC contactor?


The AC contactor is an

electromagnetic AC contactor with a NO main contact, three poles, and air as the arc

extinguishing medium. Its components include: coil, short-circuit ring, static iron

core, moving iron core, moving contact, static contact, auxiliary NO contact, auxiliary

NC contact, pressure spring sheet, reaction spring, buffer spring, arc extinguishing The

cover is composed of original parts. The appearance structure of the common AC contactor

is shown in the figure below:


Electromagnetic system: It includes coil, static iron core and moving iron core

(also called armature).


Contact system: It includes main contacts and auxiliary contacts. The main contact

allows a larger current to pass through and plays the role of connecting and cutting off

the main circuit. Usually, the maximum current allowed by the main contact (ie, the

rated current) is one of the technical parameters of the contactor. Auxiliary contacts

are only allowed to pass small currents, and are generally connected to the control

circuit when used.


The main contacts of AC contactors are generally NO contacts, and the auxiliary

contacts are either NO or NC. A contactor with a smaller rated current has four

auxiliary contacts; a contactor with a larger rated current has six auxiliary contacts.


NO and NC refer to the state of the contacts before the electromagnetic system is

energized. That is, the NO contact means that when the coil is not energized, its moving

and static contacts are in an open state, and the coil is closed after it is energized.

NC contact means that when the coil is not energized, its moving and static contacts are

closed, and when the coil is energized, it is disconnected.


The function of the arc extinguishing device is to quickly cut off the arc when the

main contact is broken. If it is not cut off quickly, the main contact singeing and

welding will occur. Therefore, AC contactors generally have arc extinguishing devices.

For AC contactors with larger capacity, arc extinguishing grids are often used.


The working principle structure of the AC contactor is shown in the figure below.

When the coil is energized, the iron core is magnetized, attracting the armature to move

downward, making the normally closed contact open and the normally open contact closed.

When the coil is de-energized, the magnetic force disappears. Under the action of the

reaction spring, the armature returns to the original position and the contact returns

to the original state.


Impact of the Circuit Breaker and Budget Measures in Response to COVID-19


The COVID-19 pandemic and consequential measures taken to contain the pandemic,

including lockdowns and travel restrictions, have adversely affected economic activity

globally.


In Singapore, the Circuit Breaker

measures, which were necessary to stem the community spread of COVID-19 and save

lives, had a negative impact on the economy. In particular, the closure of most physical

workplace premises from 7 April to 1 June, which had affected businesses that could not

operate remotely from home, is estimated to have reduced Singapore’s annual real GDP by

2.2 per cent.


The Government has introduced four Budgets this year to fight COVID-19, with a total

commitment of $93 billion in economic and social support and public health management

measures. The Budget measures are expected to cushion the fall in employment and

economic output arising from COVID-19. Specifically, the four Budgets are estimated to

avert a loss in real GDP of about 5.5 per cent in 2020, and reduce the rise in resident

unemployment rate by 1.7 percentage-points.


Overall, the four Budgets have supported economic livelihoods and prevented an even

more significant disruption to income and cash flows. They also contribute to the

longer-term objective of helping viable firms stay afloat and facilitating a quicker

recovery. In addition to the economic benefits, there will also be positive

externalities from the public health management measures that have been put in place to

safeguard Singapore from COVID-19.


How Does a Magnetic Motor Starter Work?


Magnetic starters rely on electromagnets to function. They have an

electromagnetically operated set of contacts that starts and stops the attached motor

load, and an overload relay. The overload relay opens the control voltage to the starter

coil if it detects an overload on a motor. A control circuit with momentary contact

devices that are connected to the coil executes the start and stop function.


A 3-pole full-voltage magnetic motor starter has the following workings: a set of

stationary contacts, a set of movable contacts, a solenoid coil, a stationary

electromagnet, pressure springs, a set of magnetic shading coils, and the moving

armature. Magnetic starters use momentary-contact pilot devices (such as switches and

relays) that require a restart after a power loss, or if a low voltage condition causes

the contactor to drop off. They can also be wired to restart motors automatically if the

application requires.


A magnetic starter

contactor is similar to a relay but switches a larger amount of electrical power

and handles higher voltage loads. A contactor has a contact carrier with electrical

contacts to connect the incoming line power contact to the load contact. It also

consists of an electromagnet that provides the force to close the contacts, and the

enclosure, an insulating material that holds the parts together and protects the

components. Contactors are typically made with contacts that stay open unless forced

closed, meaning the power doesn’t go to the load until the coil is activated, closing

the contactor.


When the contactor is closed, the current goes to the electromagnet. This current

can have the same voltage as the power going through the contacts or can have a lower “

control” voltage that is used only to energize the coil. When the coil is energized,

this creates a magnetic coupling between the contacts and the contact carrier, letting

them stay together and current to flow to the motor until the system is shut off by de-

energizing the coil. When de-energized, a spring causes the contacts to separate and

halt the flow of power through the contacts, and the motor turns off.


Some commonly available magnetic motor starters include full voltage (across-the-

line), reduced voltage, and reversing. As the name suggests, a full-voltage or across-

the-line magnetic motor starter gives the motor full voltage. This means that it is

designed to correctly handle the levels of inrush current that happen once the motor

starts. Reduced voltage starters are designed to limit the effects of inrush current

during motor startup and are available in electro-mechanical and electronic formats.

Reversing starters switch the shaft rotation of a 3-phase motor. This action happens

because of the interchanging any two-line conductors that supply the motor load. A

reversing magnetic motor starter has a forward and a reverse starter. It also has

electrical and mechanical interlocks that ensure only the forward or the reverse starter

can be engaged at once.


Thermal Overload Relay Working Principle


Overload thermal

relay works on the heat produced by the excessive overload current. The heat

produced by the overload current is utilized to trip the motor circuit. These are mostly

used for protection of low-voltage squirrel cage induction motors or DC motors of lower

output rating.


The function of a thermal overload relay, used in motor starter circuits is to

prevent the motor from drawing excessive current which is harmful to motor insulation.

It is connected either directly to motor lines or indirectly through current

transformers. It de-energies the starter and stops the motor when excessive current is

drawn.


Thermal Overload Relay Working Principle


Whenever the motor is overloaded, it will draw more current from the line and will

be heated up gradually. The overload relay is intended to protect the motor against

sustained overloads.The overload relay is installed on motor control circuit to make a

contact in the trip circuit or mechanically operate the trip bar thus shutting down the

motor in the event of excessive load.


It consists bimetallic strips. The heat produced by the overload current is utilized

to heat the bimetallic strips.
Under normal operating condition the strip remains straight but under the action of

fault current the strip is heated and bent and the relay contacts get separated which

de-energizes the motor control circuit.
The force required to bend the bimetallic strips can be adjusted by an adjuster. In

other words, it can be adjusted to operate at different overload currents.
The thermal overload relay does not provide short circuit protection as it takes

sufficient time to open the contacts. Therefore, this type of relay is used in

conjunction with fuses to provide overload and short circuit protection to the circuit.
These relays have inverse time characteristics i.e. the tripping time becomes less as

the over load and hence current increases. These are rated in trip class. The trip class

specifies the period of time it will take to operate in an overload condition. The most

common classes are 5, 10, 20 & 30. Class 30, 20, 10 and 5 overload relays will trip

within 30, 20, 10 and 5 seconds respectively at 600% of motor full load amps.


How Does an A/C Contactor Work?


Now that we know it's a switch, the question becomes, "How does

air conditioning

contactor get turned on?"


While light switches need you to physically flip the switch and pressure switches

use air to operate, your contactor is triggered by a solenoid pulling on a small plunger

(the button).


If you look in the picture I've provided above, you'll see the coil windings

in the part that is simply referred to as the "coil." When 24v hits this coil,

it generates just enough force to pull down that button on top of the contactor. When

pulled down, that button is the "drawbridge" closing and allowing electricity

to pass through and run the condenser fan and compressor.


To give you a more complete description, here is the step-by-step of an air

conditioning cycle up to this point.


The thermostat senses it's warmer than the temperature it is set at. As the

temperature rises, so does the little switch inside the stat making the proper 24v

connection to the furnace control board that tells it to turn on the fan.


At the same time, the furnace relays that message through a 24v signal to the 24v

coil in the contactor.


That coil energizes and pulls down the button (plunger) on the contactor.


Once contact is made on both sides of the contactor, power can then pass on to the

motor and compressor and provide you cool air.


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Timer Relays



A timer relay

is a combination of an electromechanical output relay and a control circuit. The

contacts will open or close before or after a pre-selected, timed interval.


Time delay relays are initiated or triggered by one of two methods:


Application of input voltage/auxiliary supply will either initiate the unit or

make it ready to initiate when a trigger signal is applied.


Applying a trigger signal is used to initiate the unit after input voltage has

been applied. This trigger signal can either be a control switch (dry contact switch) or

a power trigger (voltage).



What is Comb Busbar?


Comb busbar

is one of the most basic switch gear instruments. The main purpose of a comb busbar is

to distribute power to switch gear components such as MCBs. There are two types of comp

busbar viz, single phase comp busbar and three phase comb busbar. It can be found in

small distribution boards that you can find at you home. Single phase comb busbar is

used in single phase power distributions and three phase comb busbar is used in three

phase power distribution systems. Two lead comb busbar are also available and are used

in DC system.


The main purpose of using comb busbar is to reduce the number of wire connections.

In distribution boards a output of the isolators or ELCBs are connected to a number of

MCBs or



Residual Current Devices. If comb busbar is not used, numerous wires should be

connected to the ELCB/Isolator output terminals to supply the MCBs. Hence in order to

avoid that a single comb busbar is used. The comb busbar supplies numbers circuit

breakers simultaneously thereby reducing the number of wires used.