HAVELLS RELAY Price List w.e.f 5th September 2013 (VIEW) HAVELLS Customer Care No:+91-120-4771000
L&T RELAY Price List w.e.f 1st November 2013 (VIEW) L&T Customer Care No:18002094545
Introduction of Relay
Relays are electrically actuated switches
– Mechanical relays
– Reed relays
– Solid-state relays
• A relay consists of an electromagnetic coil and one or more pairs of contacts
To make a relay change states, the voltage across of its magnetic coil should be at least within 25 percent of the relay’s specified control voltage rating (Vc ± 0.25×Vc)
• Sudden changes in current will create voltage spike, to avoid this is to use transient suppressors.
Wide range of current ratings – from a few μAto 100A
• Extremely fast switching – 1 to 100 ns
Principle of operation
Thermal motor protection relays contain three bimetal strips together with a trip mechanism in a housing made of insulating material. The bimetal strips are heated by the motor current, causing them to bend and activating the trip mechanism after a certain travel which depends on the current-setting of the relay.
The release mechanism actuates an auxiliary switch that breaks the coil circuit of the motor contactor . A switching position indicator signals the condition “tripped”.
A = Indirectly heated bimetal strips
B = Trip slide
C = Trip lever
D = Contact lever
E = Compensation bimetal strip
The bimetal strips may be heated directly or indirectly. In the first case, the current flows directly through the bimetal, in the second through an insulated heating winding around the strip.
The insulation causes some delay of the heat-flow so that the inertia of indirectly heated thermal relays is greater at higher currents than with their directly heated counterparts. Often both principles are combined.
For motor rated currents over approx. 100 A, the motor current is conducted via current transformers. The thermal overload relay is then heated by the secondary current of the current transformer.
The tripping current of bimetal relays can be set on a current scale – by displacement of the trip mechanism relative to the bimetal strips – so that the protection characteristic can be matched to the protected object in the key area of continuous duty.
The simple, economical design can only approximate the transient thermal characteristic of the motor.
For starting with subsequent continuous duty, the thermal motor protection relay provides perfect protection for the motor. With frequent start-ups in intermittent operation the significantly lower heating time constant of the bimetal strips compared to the motor results in early tripping in which the thermal capacity of the motor is not utilized.
The cooling time constant of thermal relays is shorter than that of normal motors. This also contributes to an increasing difference between the actual temperature of the motor and that simulated by the thermal relay in intermittent operation.
Working of Relay
Relays are switches that open and close circuits electromechanically or electronically. Relays control one electrical circuit by opening and closing contacts in another circuit. As relay diagrams show, when a relay contact is normally open (NO), there is an open contact when the relay is not energized. When a relay contact is normally Closed (NC), there is a closed contact when the relay is not energized. In either case, applying electrical current to the contacts will change their state. Relays are generally used to switch smaller currents in a control circuit and do not usually control power consuming devices except for small motors and Solenoids that draw low amps. Nonetheless, relays can "control" larger voltages and amperes by having an amplifying effect because a small voltage applied to a relays coil can result in a large voltage being switched by the contacts. Protective relays can prevent equipment damage by detecting electrical abnormalities, including over current, undercurrent, overloads and reverse currents. In addition, relays are also widely used to switch starting coils, heating elements, pilot lights and audible alarms.
Electromechanical Relays vs Solid State Relays
Relays are either electromechanical relays or solid-state relays. In electromechanical relays (EMR), contacts are opened or closed by a magnetic force. With solid-state relays (SSR), there are no contacts and switching is totally electronic. The decision to use electromechanical or solid state relays depends on an application's electrical requirements, cost constraints and life expectancy. Although solid-state relays have become very popular, electromechanical relays remain common. Many of the functions performed by heavy-duty equipment need the switching capabilities of electromechanical relays. Solid State Relays switch the current using non-moving electronic devices such as silicon controlled rectifiers.
These differences in the two types of relays result in advantages and disadvantages with each system. Because solid state relays do not have to either energize a coil or open contacts, less voltage is required to "turn" Solid State Relays on or off. Similarly, Solid State Relays turn on and turn off faster because there are no physical parts to move. Although the absence of contacts and moving parts means that Solid State Relays are not subject to arcing and do not wear out, contacts on Electromechanical Relays can be replaced, whereas entire Solid State Relays must be replaced when any part becomes defective. Because of the construction of Solid State Relays, there is residual electrical resistance and/or current leakage whether switches are open and closed. The small voltage drops that are created are not usually a problem; however, Electromechanical Relays provide a cleaner ON or OFF condition because of the relatively large distance between contacts, which acts as a form of insulation.