Control relay
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A control relay is the electromechanical equivalent of a thyristor. Control relays have only two possible operating states: ON or OFF. A control relay is essentially a switch that can be operated from a remote location. Control relays use one or more pairs of contacts to make or break control circuits. For many years, relay logic was the standard method of controlling industrial electronic systems. The principle of relay logic is based on magnetically operated relays which energize and de-energize associated contacts. Relay logic is the predecessor of ladder logic, which is commonly used in Programmable logic controllers.
When electric current passes through a coil, magnetic north and south poles are produced across the gap separating the coil and armature. The relay is actuated, or latched, whenever current of sufficient intensity flows through the coil and the attractive force between the core and armature overcomes the spring tension, as shown in Figure 1. The relay remains latched as long as a specific value of holding current flows through the coil. When the current through the coil is reduced below this value, the core becomes unmagnetized and the armature is pulled up by spring action to its unactuated position.
There are many considerations involved in the correct selection of a control relay for a particular application. These considerations include factors such as speed of operation and relay sensitivity. Although typical control relays operate in the 5 ms to 20 ms range, relays with switching speeds as fast as 100 us are available. In order for a magnetic relay to become energized, the coil current must rise above a certain value, called the pull-in current, to move the armature and switch the contacts. The pull-in current is also known as the make rating, and is the rating applied to the current that can be handled by the contact at the time of contact closure. Since relays are often used in the control of inductive devices, such as motors, they must be able to handle large values of inrush current. Inrush current is often 500% of the normal operating current in inductive circuits.
Once a relay has been pulled in, the coil current drops drastically until it reaches a normal operating value, called the holding current. The break rating of a control relay refers to the value of current that can be interrupted successfully by the contact. The inductive break rating is always lower than the inductive or continuous rating. The continuous rating indicates the amount of load current that can be carried continuously without switching and without exceeding a specific rise in temperature.
[edit] Derating Factors
| Type of load | % of rated value |
|---|---|
| Resistive | 75 |
| Inductive | 35 |
| Motor | 20 |
| Filament | 10 |
| Capacitive | 75 |
Control relays should not be operated above rated temperature because of resulting increased degradation and fatigue. Common practice is to derate 20 degrees Celsius from the maximum rated temperature limit. Relays operating at rated load are also affected by their environment. Oil vapors may greatly decrease the contact tip life, and dust or dirt may cause the tips to burn before their normal life expectancy. Control relay life cycle varies from 50,000 to over one million cycles depending on the electrical loads of the contacts, duty cycle, application, and the extent to which the relay is derated. When a control relay is operating at its derated value, it is controlling a lower value of current than its maximum make and break ratings. This is often done to extend the operating life of the control relay. Table 1 lists the relay and switch derating factors for typical industrial control applications.
[edit] References
Colin Simpson, Principles of Electronics, Prentice-Hall, 2002, ISBN 0-0686860-3-6
