26-06-2012, 04:33 AM
Hi, Peter,
Thanks for your kindly reply.
The question is shown below for your reference.
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A d.c. track circuit has the following characteristics:
Length 500.0 metres
Feed transformer-rectifier output: 2.4 volt
Relay coil resistance: 4.0 ohm
Relay pickup current: 100 mA
Ballast resistance: 2.5 ohm km
Feed resistance: adjustable but unknown
Drop shunt: 0.5 ohm
Calculate the value of the feed resistance for these conditions.
Owing to weather conditions, the ballast resistance later rises to 20 ohm km. Calculate the new value of the drop shunt with the same value of feed resistance, and comment on the results.
*****************************************************
As per your reply, I have several questions to ask.
1) As drop shunt resistance is provided in the question, one should use the "drop-away" current approach to find out the feed resistance instead of using "pick-up" current approach?
2) If the drop shunt resistance is not provided, one can use the "pick-up" current to find out the feed resistance?
As you mentioned that the max. feed resistance will be calculated (the relay is just picked up which is not reliable), one can add 10% margin to the pick-up current for reliable operation?
3) If having drop shunt resistance provided, one use the "drop-away" current approach to calculate the feed resistance. Is the calculated resistance is a min. value for the relay to drop away? Should one add a margin on it as well? Or the margin has been already incorporated in multiplying 65% to Current (pick-up) to achieve the drop away current?
My questions may be a bit stupid / redundant....but I hope I could have a better understanding first and use the correct approach to cope with this type of question.
Thanks!
Thanks for your kindly reply.
The question is shown below for your reference.
****************************************************
A d.c. track circuit has the following characteristics:
Length 500.0 metres
Feed transformer-rectifier output: 2.4 volt
Relay coil resistance: 4.0 ohm
Relay pickup current: 100 mA
Ballast resistance: 2.5 ohm km
Feed resistance: adjustable but unknown
Drop shunt: 0.5 ohm
Calculate the value of the feed resistance for these conditions.
Owing to weather conditions, the ballast resistance later rises to 20 ohm km. Calculate the new value of the drop shunt with the same value of feed resistance, and comment on the results.
*****************************************************
As per your reply, I have several questions to ask.
1) As drop shunt resistance is provided in the question, one should use the "drop-away" current approach to find out the feed resistance instead of using "pick-up" current approach?
2) If the drop shunt resistance is not provided, one can use the "pick-up" current to find out the feed resistance?
As you mentioned that the max. feed resistance will be calculated (the relay is just picked up which is not reliable), one can add 10% margin to the pick-up current for reliable operation?
3) If having drop shunt resistance provided, one use the "drop-away" current approach to calculate the feed resistance. Is the calculated resistance is a min. value for the relay to drop away? Should one add a margin on it as well? Or the margin has been already incorporated in multiplying 65% to Current (pick-up) to achieve the drop away current?
My questions may be a bit stupid / redundant....but I hope I could have a better understanding first and use the correct approach to cope with this type of question.
Thanks!