An electromagnetic drive unit and an electromechanical switching device

Field of the invention The invention relates to the art of designing electromagnetic drive units and to electromechanical switching devices.

Background

In the field of low-voltage switching devices that are adapted to switch electric currents over voltages from 100 V up to 1000 V, electromagnetic trigger units, sometimes referred to as n-triggers, are implemented with an electromagnetic drive unit comprising a coil, a yoke and an armature .

An electromagnetic trigger unit is an unit that limits or breaks the current passing through an electromechanical switching device in the event of short-circuit. The trigger unit uses the force generated by the coil and yoke to push or pull the armature so that the current path is broken or limited. Electromagnetic trigger units are commonly used in circuit breakers and current limiters.

An electromagnetic trigger unit is usually adapted to break or limit the current if the rated current is exceeded. To this end, the coil in the electromagnetic drive unit must be able to generate a strong enough magnet field to pull or push the armature with enough force. For electromechanical switching devices that have small rated currents this may be a problem, since the magnet field is proportional to the current flowing through the coil.

For this reason, coils with a high number of windings need to be used in particular for an electromechanical switching device rated for a small current. Since such electro ^¬ mechanical switching devices tend to be rather small, the cross section of the winding must be small too to accommodate the high number of windings.

A winding having a small cross section is not very stable if large currents are carried through it, and so the copper wire of which the winding has been made tends to get permanently damaged in the case of short circuit.

Summary of the invention

It is a first object of the invention to improve the stability of an electromagnetic drive unit against short circuit. This object can be achieved as set out in claim 1.

A second object of the invention is to improve the stability of an electromechanical switching device against short circuit. This object can be achieved as set out in claim 3.

The dependent claims describe advantageous embodiments of the invention .

Advantages of the invention If the winding of the coil of an electromagnetic drive unit comprising a coil, a yoke and an armature is partly wound in one direction and partly in the other direction, one part of the winding will carry current in the other direction than the other part of the winding. Because of the compensating effect of the other part, a part of the winding may not become magnetically effective under normal conditions. But in the event of short circuit, the inductive resistance of the part of the winding that has been wound in the other direction becomes significant and thus tends to limit the short circuit current.

Similar considerations apply to an electromechanical switching device comprising at least one input terminal, a respective output terminal, and an electromagnetic drive unit according to the first aspect of the invention, if the electromagnetic drive unit is adapted to limit or to break the electrical current between said at least one input terminal .

List of drawings

In the following, the invention is described in more detail with reference to the examples shown in the accompanying drawings, of which:

Figure 1 shows a current limiter; and

Figure 2 illustrates an electromagnetic drive unit.

Detailed description

Figure 1 shows a current limiter 10 comprising a first current input terminal Al and a responsive current output terminal Tl. The limiter 10 is a limiter for a three-phase current, but for simplicity the components for the two other phases have been omitted from Figure 1.

Each input terminal Al of the current limiter 10 receives a phase of a three-phase electric circuit, and passes the electric current via a current rail 11 to a stationary contact piece 12. If the contact bridge 14 is in its normal position, the stationary contact piece 12 is in electrical contact with the movable contact piece 13.

The movable contact piece 13 is in electrical contact with the second movable contact piece 15 via the contact bridge

14. If the contact bridge 14 is still in its normal position, the second movable contact piece 15 is in electrical contact with the second stationary contact piece 16, from which the current may flow via a second current rail 17 to the responsive output terminal Tl.

The electric current passing through the contact bridge 14 passed, advantageously after the second stationary contact 16, through an metal conductor 18 to a coil 21 of an electromagnetic drive unit 20. The electromagnetic drive unit 20 monitors the electric current flowing through the limiter 10, and if it detects an excessive electric current, it limits it by displacing the movable contacts 13, 15 from the

stationary contacts 12, 16 by exerting a force to the armature 23A that moves the plunger 23 connected to the contact bridge 14.

A biasing spring 24 ensures that the movable contacts 13, 15 cannot be displaced from the stationary contacts 12, 16 until an excessive current is reached, i.e. that the electro ^¬ magnetic drive unit 20 generates a force F to the armature 23A that is large enough to exceed the balancing force of the biasing spring 24.

The electromagnetic drive unit 20 comprises a coil 21, a yoke 22 and an armature 23A. The winding 41 of the coil 21, illustrated in Figure 2, is partly wound in one direction and partly in the other direction. The winding 41 may change its direction layerwise, i.e. having one or more layers 51 in one direction and then one or more layers 52 in the other direction .

Even though the invention was described by means of a non- limiting example, the skilled person appreciates that the scope of the invention can be interpreted from the attached patent claims. In particular, even though in the examples the electromechanical switching device was a current limiter, in other applications it may be a circuit breaker.