Contactors are remote-controlled electrical switches with which high currents and voltages can be switched. A control current activates the magnetic drive that closes and reopens the contacts of the load current circuit.

Due to the high loads, contactors need to have a good ability to switch on and off, be able to conduct a high thermal continuous current and also be able to switch off both the open load circuit and a short circuit.

The moment of switching off represents a special challenge. The air between the opening contacts becomes ionised and an electric arc is created in which the current continues to flow. The air between the contacts offers a great deal of electrical resistance, which creates temperatures of up to 10,000°C. To prevent the contacts or even the entire device from being destroyed, the electric arc must be driven out of the contact area into arc chambers to be stretched and cooled until it loses energy and finally dies out.

In order to extinguish the electric arc within a few milliseconds we have incorporated some sophisticated solutions in our contactors: particularly for the special requirements of railway technology. This experience helps us to develop reliable contactors for power plants, computer centres and electrically powered vehicles.

Technical features of Schaltbau contactors

• High quality and performance
• Wide voltage tolerance range (from +25% to -30%)
• Good inverse voltage suppression, with varistors as standard
• Designed to handle rough climatic conditions
• Highly shock- and vibration-resistant
• Optional auxiliary switches available

Permanent magnetic and electromagnetic blowout fields

The redirecting of the electrical arc is done using magnetic fields, also known as magnetic blowout fields: an electric arc is nothing other than a conductor of electricity. When it flows through a magnetic field it is subjected to a force known as the Lorentz force. During this process the direction of the current, the direction of the magnetic field and that of the resulting force all stand at right angles to each other (right-hand rule). Various blowout concepts are applied to influence the electric arc, depending on whether direct or alternating current is being used:

For DC devices that only work in one direction of current, permanent magnetic fields can be used. The permanent magnets are reinforced by pole plates to enable the magnetic field to function throughout the complete contact and extinguishing area.

Advantages of permanent-magnetic blowout:

• The strength of the field does not depend on the flow of current.
• The magnetic field exists prior to the moment the electric arc is ignited and can work at full strength.
• The field can be made stronger by selecting the corresponding magnets. The strength of the magnet depends on the critical current range (as a rule a low current at high voltage).
• The components are relatively inexpensive.

The crucial disadvantage is that the device only works in unidirectional DC operation. If the polarity is incorrect the electric arc flows in the wrong direction and destroys the device.

Electric arcs are easier to extinguish when switching AC loads due to the zero-crossing after each alternation. However, due to the constant changing of the direction of the current, it is not possible to work with a permanent magnetic field because the electric arc would be constantly driven back and forth in the contact zone.

In order to control bidirectional DC or AC, a magnetic field is needed that changes with the direction of the current. That can be achieved by means of an electromagnetic blowout coil mounted laterally with the electric arc. Thus the Lorentz force always works in the right direction – towards the arc chamber.

Disadvantages of electromagnetic blowout:

• Critical current range. The lower the current, the lower the electric arc current and therefore less powerful the effect on the electric arc current.
• The load current cannot constantly flow through the coil. A solution is needed that only activates the coil at the moment of switching off – exactly the moment when an electric arc has to be extinguished.

The new CT concept

The requirements for contactors in railway technology are highly specialised and fundamentally different from those of other branches of industry. For example, European railway systems operate with varying types of supply voltage: Italy and Belgium use direct current, Germany uses 16 2/3 Hertz alternating current and Eastern Europe uses 50 Hz AC.

In many direct current applications the DC has to be switched bidirectionally (e.g. for energy recovery) and AC applications have changing polarities in any case.

As a purely permanent-magnetic blowout system does not work for these requirements and the purely electromagnetic blowout concepts have too many disadvantages, the experts at Schaltbau have developed a process that combines the advantages of permanent-magnetic blowout with those of the electromagnetic solution all in one device.

At the moment of ignition the existing permanent magnetic field takes effect, directing the electric arc into the arc guide shaft. Once in the shaft, the arc activates its own powerful electromagnetic field, which finally drives it into the arc chamber. The simple, robust mechanism enables us to switch both unidirectional and bidirectional direct current and alternating current with one single device.