To consider safe operation, need to ensure that the specified drop shunt of 0.5ohm will reduce the current through the relay to less than that needed for the relay to remain up, this being calculated when there is no leakage occuring through the ballast (and thus the ballast resistance is infinite). Hence you want a value which is a bit less than the drop away current.
That calculation permits you to determine the value of the feed resistor; I would be tempted either to calculate this for DS=0.5 and then add about 10% to the resistor value to round up to a integral value so that there is a margin, or alternatively put the margin in another way by calculating the value of the feed resistor for a DS of say 0.7ohm and then making a small rounding to get a suitable value of resistor. [Sometimes the feed resistor is a sliding potentiometer and thus infinitely variable, but more often it is a resistor block with separate resistors of 0.5, 1, 2, 4, 8 etc that can be connected in series to give one of a fixed range of summated values.]
Having decided what the feed resistance would be in that situation, now consider the operation of the track circuit with that setting of feed resistor in the absence of a train but leakage through the poor ballast. For this calculation you want to ensure that the relay will reliably pick even when the ballast is wet, so you'd be looking for a current say 10% more than the pick up value.
That calculation permits you to determine the value of the feed resistor; I would be tempted either to calculate this for DS=0.5 and then add about 10% to the resistor value to round up to a integral value so that there is a margin, or alternatively put the margin in another way by calculating the value of the feed resistor for a DS of say 0.7ohm and then making a small rounding to get a suitable value of resistor. [Sometimes the feed resistor is a sliding potentiometer and thus infinitely variable, but more often it is a resistor block with separate resistors of 0.5, 1, 2, 4, 8 etc that can be connected in series to give one of a fixed range of summated values.]
Having decided what the feed resistance would be in that situation, now consider the operation of the track circuit with that setting of feed resistor in the absence of a train but leakage through the poor ballast. For this calculation you want to ensure that the relay will reliably pick even when the ballast is wet, so you'd be looking for a current say 10% more than the pick up value.
(26-06-2012, 04:33 AM)leochungnet Wrote: 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.
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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!
PJW