Relay

Relays have a similar construction to contactors, but since they are usually switching low-current logic signals, they do not have a requirement for the heavy-duty contacts and arc-suppression hardware. Most relay contacts have an AC continuous rating of no more than 10 amperes. They can close on an in-rush current of 150%, but only break 15% at 120 volts AC (vac). A NEMA A600 rating limits the inrush to 7200 voltamperes (va), and a circuit breaking rating of 720 voltamperes. As higher voltages are used, the current capacity goes down proportionately. This difference in make and break ratings closely matches the typi­cal ratio of inrush and holding currents of AC control coils. AC coils have relatively low resistance, allowing high in-rush currents. As the coil is energized, the AC current builds up inductive reactance. Total impedance (ohms), the vector sum of resistance and reactance, limits the continuous holding current. The ratio of in-rush to holding current is often 5 : 1 or more. Maximum impedance is attained when the air gap in the magnetic armature assembly has sealed closed. A failure to close this gap will reduce the inductive reactance, allow more current to flow, overheat the coil, and cause premature coil failure. A shading ring frac­ture will also lead to overheating and coil failure. The same 10 amp contacts are only rated at 0.4 amperes DC, at 125 volts, and 0.2 amperes DC at 250 volts (50 va) because the small air-gaps are not adequate to break a sustained DC arc. Voltages for DC logic controls seldom exceed 24 volts with typical current in the milli-amp ranges.

Relays may have multiple coils for latching and unlatching of the contacts. Contacts may be normally open (NO), and/or normally closed (NC). When we speak of normal, it refers to the condition of the con­tacts when the relay is de-energized. The number of contacts usually varies from one to eight. Some relays use contact cartridges which can be converted for either NO or NC operation. When setting up NC con­tacts, one should try to have an equal number on each side of the relay so that spring-loading does not produce side thrust on the solenoid assembly. Most standard relays will have totally isolated contacts. Some miniature relays have type "C" contacts where a NO and NC contact share a common terminal. This construction requires careful planning to match the schematic wiring diagram to actual relay construction.

Questionnaire:

• Explain about Relays.