General Information on Track Circuits - RGS Online

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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B24 of 25 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits 16.2 Auxilary Winding The impedance available from the simple impedance bond remains a handicap. It is therefore usual to enhance the impedance by parallel resonance of the traction winding, use being made of a step–up (approximately 50:1) transformer to reduce the value of the necessary capacitance to realistic levels. Another solution is to connect the resonating winding to form an auto transformer, refer to 16.2.2. 16.2.1 Resonated Impedance Bonds The induction of the traction winding is tuned to resonance at or near the track circuit operating frequency by use of a parallel capacitor, which raises the rail to rail impedance at the track circuit frequency, and thereby reduces the bond’s effect on the track circuit. For power frequency track circuits (eg. 50 Hz), the value of capacitance required to attain resonance is reduced to an achievable magnitude, by applying the capacitance via an auxiliary winding, as shown in Figure B18. To Next Bond Or Cross Bonding Figure B18 Auxiliary Flux Winding B24 RAILTRACK IRJ IRJ The value of capacitance required to achieve resonance depends on the following: a) Traction winding inductance, which may differ between designs. The capacitance required will vary inversely to the inductance. b) Auxiliary turns ratio, which may differ between designs. The capacitance required will vary inversely as the square of the turns ratio. c) Track circuit frequency, where the capacitance required will vary inversely as the square of the frequency. Resonated impedance bonds are used at the feed and relay ends of jointed audio frequency track circuits and for all intermediate bonds associated with traction cross bonding.
Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B25 of 25 16.2.2 Auto Coupled Impedance Bonds The method used to couple the feed and relay ends of certain designs of track circuit into the track as shown in Figure B19. At the feed end, the reduced voltage appearing across the the traction winding is applied to the rails whilst at the relay end, the current from the track circuit passing through the traction winding is usefully employed to drive the relay. IRJ To Next Bond Or Cross Bonding Figure B19 IRJ To Track Relay Or Feed RAILTRACK B25 OR To Rails To Track Relay Or Feed A double rail A.C. track circuit with auto–coupled impedance bonds is shown in Figure B20. Figure B20 110V Resonant Bond Control Local 110V If the traction current in each rail is not equal, the imbalance results in a net flux in the iron circuit, and if that flux is sufficient to saturate the iron core, the track circuit current will be presented with a short circuit. It is therefore important to make the bond as tolerant as possible of traction current imbalance and this is done by creating an air gap in the magnetic circuit. Such bonds will tolerate 20% imbalance before saturation.
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