LEACH ® INTERNATIONAL
5. HOW TO PICK A RELAY
How to keep 130 people
in the dark
at 30,000 feet
The electromagnetic relay, though easy to understand, seems to be subject to more misapplication "failures" than other components. Part of the reason for this is its very simplicity. Designers tend to discount the relay and do not spend as much time analyzing its operation in their circuits as they do with more complex devices.
A common area of misunderstanding - and misapplication - relates to the fact that there is no single voltage/current rating that applies to a given set of contacts under all circumstances. To ensure the successful, full rated lifetime operation of the relay, it must be chosen in terms of the specific application.
Recently, a transcontinental airliner at 30,000 feet was the scene of a misapplication of this type. As dusk silently fell, the pilot toggled the switch for the inside lights. A low level signal was generated and proceeded down the control lines to the relay coil. The relay actuated and contact was made. There was a short pause as the lights came on - then died. 130 people sat looking darkly about them for the time it took the auxiliary power system to cut in.
And another "failure" was chalked up to relays because the manufacturer had not been given enough information about the application.
CHOOSING THE RELAY
The engineer, when specifying the relay, was aware of the environmental load imposed on equipment by modern aircraft. The relay called out was realistically specified to operate over the temperature range from -65° C to +125° C and to withstand vibrations up to 20 g's at 2000Hz, and shocks to 50g with durations up to 11 milliseconds. He then included a safety factor for the contacts and specified a current carrying capability 50% higher than the steady-state current he expected his lamp load to draw.
Unfortunately, this wasn't nearly adequate to handle the turn-on transients through the filaments, as these might have been as much as 10 or 15 times the steady-state current, and the contacts were burned out shortly after they were energized.
The load-carrying capacity of contacts is normally given as a current value for a resistive load. When a load is nonresistive, energy stored in the circuit or changes in the load characteristics can drastically overload the contacts.
Although there might be momentary arcing between the contacts on make or break, resistive loads draw an almost constant current at all times. As long as the contact ratings are not exceeded, the relay functions reliably. When ratings are exceeded, problems can be expected. If the applied voltage is direct current, excessive arcing results in vaporization of the metal of the contacts and transfer of the material from one contact to the other. This results in what is known as a "pip" and "dimple" - a built-up area on one contact and a crater in the other. Because of the roughness of these areas and because the arcing melts the metal, destruction of the contacts can be expected. If the applied voltage is AC, the arc tends to quench itself as the applied voltage goes through zero and wear on the contacts is less severe than in the DC case. For a given value of current flow, contacts rated for 29 to 32 volts DC can normally be expected to handle 115 VAC at 400Hz.
LAMPS, INDUCTORS, TRANSFORMERS
Lamp filaments are resistive, but change in value by a large factor from their cold state to their operating state. This effect is so great that the inrush current can be expected to be 10 to 15 times greater than the steady-state value as indicated in Figure 1. If this is not taken into account, the consequences described in our example can result, with the contacts either welding shut or being totally destroyed. Normal practice is to derate contacts to 20% of their resistive load capabilities for a lamp load.
Inductors and transformers act as energy storage devices and can cause excessive contact arcing when the relay breaks the circuit. When operated near their maximum capability, the overload added by the arcing will cause deterioration of the contacts. The material of the contacts is vaporized and, if the voltage is DC, is transferred from one contact to the other. The situation is further aggravated by contact bounce and vibration, and a long time constant for the load (measured by the ratio of the load inductance to the resistance of the discharge path: T=L/R). Local heating of the contacts causes either welding or heavy metal transfer. For this type of application, contacts are normally derated to 50% of their resistive load capacity.
MOTORS ARE DIFFERENT
Motors are inductive and will cause arcing on the break, but also draw an initial transient on the make. The peak current can be 5 to 10 times the run current. Contacts are typically derated to 40% of their rating for resistive loads when used to control motors.
Normal derating practices for the contacts have been given, but the best practice is for the designer to discuss his application with the relay manufacturer, as other factors enter into the design of the relay. The amount of arcing and consequent damage can be considerably affected by the rate or amount of contact separation or the amount of contact bounce after the relay is actuated. The designer is the best judge of the appropriate relay for the specific application and should be consulted.
We will continue this series with a variety of techniques for protecting contacts, but in the meantime, please call if you have any questions. The Leach applications group is well staffed and well qualified to help you define your relay requirements. The information we have gathered over the years can be used to prevent a misapplication in your circuits.
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