In over current relay, minimum pickup current is taken as double of steady state load current not 1.5 times why?
Hello Debasree,
In this website linked with this answer, the force used to keep the contact in its normal position, either normally opened or closed, is called the controlling force. The force used to move the contact from its normal position is called the deflecting force.
In a relay, a coil is used as the mechanism to create the deflecting force. The pickup current is the current that passes through the coil in creation of the deflecting force. The contact will move once the deflecting force is greater than the controlling force.
In this website, it states that the current setting is usually set at 50% to 200% (1.50 to 2.00). As a designer, you don't have to make the minimum pickup current as double steady state current, it could be 1.50. The parameters of the design will dictate your minimum pickup current; for example, may be your current source can't provide 200% of the steady state current but it could provide 175%. Therefore, the minimum pickup current will be 1.75 of steady state current.
You want the minimum pickup current to be as much as possible because the controlling force could actually overcome the deflecting force and pull the contact back to its normal position even though the relay has been actuated.
I hope this helps.
james
http://www.electrical4u.com/pick-up-current-current-setting-plug-setting-multiplier-and-time-setting-multiplier-of-relay/
For proper discrimination, with speed, between transient current, fault current and overload current, to avoid mal operation , as transient over current((inrush) remains for few cycles.....
Hello Debasree,
In this website linked with this answer, the force used to keep the contact in its normal position, either normally opened or closed, is called the controlling force. The force used to move the contact from its normal position is called the deflecting force.
In a relay, a coil is used as the mechanism to create the deflecting force. The pickup current is the current that passes through the coil in creation of the deflecting force. The contact will move once the deflecting force is greater than the controlling force.
In this website, it states that the current setting is usually set at 50% to 200% (1.50 to 2.00). As a designer, you don't have to make the minimum pickup current as double steady state current, it could be 1.50. The parameters of the design will dictate your minimum pickup current; for example, may be your current source can't provide 200% of the steady state current but it could provide 175%. Therefore, the minimum pickup current will be 1.75 of steady state current.
You want the minimum pickup current to be as much as possible because the controlling force could actually overcome the deflecting force and pull the contact back to its normal position even though the relay has been actuated.
I hope this helps.
james
http://www.electrical4u.com/pick-up-current-current-setting-plug-setting-multiplier-and-time-setting-multiplier-of-relay/
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