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Methods for Closing and Opening of Contactors in Railway
In electric traction, the control equipment is required for −
Connecting the motors to the supply mains without taking excessive current at start and provide smooth acceleration without causing sudden shock so as to avoid damage to the coupling.
Adjusting the speed as per requirement.
Providing rheostatic or regenerative braking.
The master controller system is used in the electric traction work, where the currents to be dealt are too high for the equipment. In the master controller system, the contact instead of being made or broken by hand are closed or opened by the switches known as contactors.
For closing and opening of these contactors, there are various methods such as −
All-Electric Method
Electro-pneumatic Method
Cam Shaft Method
All-Electric Method
In the all-electric method, the contactors are operated, i.e., closed or opened, by the solenoids which are energized from the master controller. The main drawback of this method is that the wide fluctuations in the supply voltage may cause uncertainty of operation of the contactors. Though, this difficulty can be overcome by using a special low voltage generator to provide a supply at 100 V or lower for operating circuit.
Electro-Pneumatic Method
In the electro-pneumatic method, the contractors are operated by the compressed air. The valves, which admit the compressed air into the operating cylinders, are operated electrically by the solenoids energized from the master controller.
This method of operating contactors is preferred over the all-electric method because of the following reasons −
The contactor groups are more compact and robust.
Each valve magnet requires only a few watts for operation whereas the solenoids in all electric method may require from 100 to 200 watts.
A low-voltage control supply can be used, which simplifies the interlocking and auxiliary contacts on the contactors and their circuits. This reduces the size of the master controller and gives the reliability to the whole of the control equipment and circuits.
In practice, the electric trains are almost universally equipped with compressed air brakes. Therefore, the supply of air for the electro-pneumatic operation is always available.
Cam Shaft Drive Method
In the cam shaft method, the contactors are closed or opened directly by cams, which mounted on a cam shaft and the cam shaft is driven by an electric or pneumatic motor. In this method, the electrical connections are very simple, therefore, this method is widely used.
In this method, the position of the master controller handle governs the angle through which the cam shaft is turned and hence fixed the number of contactors which are closed.
The advantage of the cam shaft drive method includes the following −
- Less wear and tear
- Small size, hence requires less space
- Light in weight
- Maintenance is easy.
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