Railway Signalling Electrical Point Machine
An electrical point machine is an electrically driven motor used for operation of points in railway yards. The rotary motion of the motor is transmitted through the reduction gears and transmission assembly and converted through linear movement of a toothed rack through a pinion. The gear rack drives switch rails to unlock, change the position from N to R or R to N and lock the switch at the end of the stroke.
Sequence of point machine operation.
* Opening of the detection contacts.
* Unlock the points.
* Move the points to the desired position
* Lock the points.
* Close the detection contacts.
Depending up on the operating voltages
* Low voltage .DC supplies – 24 volt DC.
* High voltage DC supplies -110V DC.
* High voltage AC – 380 AC.
The point machines have split series field windings.
Electrical parameters of point machine
* Rated voltage – 110V DC.
* Working current – 4-6 Amps.
* Obstruction (slipping current) – 6-7 Amps.
* Operating Time – 3 sec.
* Type of machine – Combined type. (Operation, locking and detection)
* Field winding – Split field
* Field connection – Series
* Movement of locking – Rotary type
* Friction clutch – Self adjustable
Main parts of point machine
* DC series split field motor within built in reduction gear unit
* Transmission assembly
* Throw rod, lock slides and detection slides
* Detection and control contact switch assembly
* Hand crank.
* Spring loaded friction clutch.
Need for remote control of point machine:
* To avoid voltage drop in cable cross section.
* Contact of lever /knob/push button do not have the heavy duty current carrying capacity.
* Track locking is to be provided to prevent point operation under wheels.
* Point machine should be protected from over load.
* Facility for sequential operation should be provided.
Two types of rely control units are standardized.
* Siemens point contactor unit.
* Point contactor unit with QBCA relays.