F O R M 2

THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)

1. Title of the invention: AN IMPROVED SHADING RING ARRANGEMENT FOR ELECTROMAGNET

2. Applicant(s):

(a) NAME : LARSEN & TOUBRO LIMITED
(b) NATIONALITY : An Indian Company
(c) ADDRESS : L & T House, Ballard Estate, Mumbai 400 001,
State of Maharashtra, India

3. PREAMBLE TO THE DESCRIPTION

The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION

The present invention relates to a shading ring arrangement for electromagnet. More particularly, the present invention is concerned about an improved shading ring arrangement for electromagnet enabling more flux to linkage with the moving magnet.

BACKGROUND AND THE PRIOR ART OF THE INVENTION
A contactor is composed of two different items.
1. Contact System
2. Magnet System

Contacts are the current carrying part of the contactor. This includes power contacts, auxiliary contacts, and contact springs. The electromagnet provides the driving force to close the contacts.
A basic contactor will have a coil input (which may be driven by either an AC or DC supply depending on the contactor design). When current passes through the electromagnet, a magnetic field is produced; this attracts the moving core of the contactor. The electromagnet coil draws more current initially, until its inductance increases when moving magnet touches the fixed magnet. The moving contact is propelled by the moving core; the force developed by the electromagnet holds the moving and fixed contacts together. When the contactor coil is de-nergized, a spring returns the electromagnet core to its initial position and opens the contacts.

For contactors energized with alternating current, a small part of the core is surrounded with a shading coil, which slightly delays the magnetic flux in the core. The effect is to average out the alternating pull of the magnetic field and so prevent the core from buzzing at twice the line frequency.

A shading ring is a ring of copper or aluminum , or sometimes a few wraps of wire, placed around half of a ferrous core (such as an electromagnet) by way of a notch in the end of the core. This functions as a shorted secondary coil in the electromagnet, the primary coil being the one connected to the power supply which is powering the electromagnet, which wraps around the entire core of central limb in case of 3 pole electromagnets.

This setup is very similar to a transformer, in which two coils are wrapped around a common ferrous core in such a way that mutual inductance from the AC current in the primary coil will induce a current in the secondary coil.

It is important to note that the current in the secondary coil is a result of the changing magnetic field created by the primary coil, so that as the primary coil's magnetic field is increasing or decreasing in strength at its maximum rate of change (the zero crossings of the sine wave), the current induced in the shading ring is at its peak strength. Thus the alternating magnetic field created by the shading ring is shifted 90 ˚ out of phase with the magnetic field created by the primary coil.

This is why the shading ring only goes halfway around the end of the core, through a notch in the end of it. The side without the shading ring has a magnetic field as dictated by the primary coil of the electromagnet, and the side with the shading ring has a magnetic field shifted 90 ˚ out of phase. Although both sides of the electromagnet have alternating magnetic fields, at no point is the magnetic field ever zero on both sides of the energized electromagnet at once. This is because the magnet flux is the vector resultant of two fluxes produced by the primary coil and the shading ring coil (90 ˚ Out of phase).

In the conventional practice of contactors application is to employ electromagnets with 2 shading rings on two extreme poles of E shaped electromagnet.

Contactors employ one spring called return spring which restores the moving magnet and contact assembly to their initial condition. When the coil is energized the magnetic force produced by the electromagnet attracts the moving core of magnet and hence contact assembly (coupled to moving magnet), this magnet force has to counter act against the contact and return spring forces. In case shading ring is not encountered in electromagnet, the contacts closing is resulted in chattering. The explanation to this phenomenon is that the force produced by magnet is oscillating with double the frequency of main coil voltage, and being proportional to square of flux the force value returns to zero value twice in one cycle. The magnet force while following the falling edge of its waveform is bound to go below the return+contact spring’s force value, this crossing of magnet force curve with constant valued return spring line produces a situation where in the moving core experiences a tendency to return back in influence of spring forces, and when the magnet force follows rising edge of its waveform this becomes more than the spring forces and hence moving core experiences a force towards fixed magnet to close, in influence of magnet force. But occurrence frequency of this opening and closing phenomenon is double the frequency of coil voltage; so contacts don’t open in actual practice but leads to a condition of chatter, which is undesirable and detrimental for contactors operation. So it’s essential to employ shading rings in electromagnet assembly to achieve smoother operation of contactor.

Thus, there is a need to overcome the problems of the prior art. So, present inventors have developed an improved shading ring arrangement for electromagnet. It would enable more flux to linkage with the moving magnet and better product life. It would enable electromagnet to deliver same performance at reduced size of the magnet system in close condition and would increase lower min. peak value of force by significant amount as result of this; minimum to maximum force oscillation reduces which reduces humming noise further.

OBJECTS OF THE INVENTION

An object of the present invention is to overcome the problems/disadvantages of the prior art.

Another object of the present invention is to provide an improved shading ring arrangement for electromagnet.

Another object of the present invention is to provide an improved arrangement for electromagnet adapted to deliver same performance at reduced size of the magnet system in close condition.

Yet another object of the present invention is to provide an improved arrangement for electromagnet adapted to increases lower minimum peak value of force by significant amount as result of this, the minimum to maximum force oscillation reduces which reduces humming noise further.

These and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

According to one of the aspect of the present invention there is provided an improved an improved shading ring arrangement for electromagnet, said arrangement comprising:
a fixed magnet means;
a moving magnet means;
plurality of restrain plate means;
plurality of pole means comprising center pole means and extreme pole means; wherein said each pole means comprising plurality of shading ring means such that total flux produced by all the shade rings of all the poles provide high breaking velocity to minimize losses to said contact means;
wherein said center pole means being attachably and/or detachably arranged on said fixed magnet means by means of said plurality of restrain plate means so as to prevent residual magnetism in the fixed magnet means.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. In the drawings:

Figure 1 illustrates a perspective view of a magnet system in accordance with the present invention.

Figure 2 illustrates a perspective view of the electromagnet system in open condition.

Figure 3 illustrates an exploded perspective view of a portion of the magnet system of figure 1.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWING

According to a preferred embodiment of the invention, there is provided an improved shading ring arrangement for electromagnet which is incorporated three shading rings on all three poles or limbs of E shaped electromagnet which is enabling more flux to linkage with the moving magnet. As shown in figure 1, a magnet system with three shading rings on each pole and the magnet system includes moving magnet 1, fixed magnet 2, extreme pole shading rings 3 and 4, center pole shading ring 5, and restrain plates 6.

Figure 2 shows the magnet system in open condition includes moving magnet 1, fixed magnet 2, extreme pole shading rings 3 and 4, center pole shading ring 5, and restrain plates 6.

Figure 3 shows an exploded view of a portion of magnet system explains geometry of restrain plate 6, used for fixing and all three pole shading rings 3, 4 and5.

The explanation of application of center pole shading ring is as follows:

The present invention involves a unique way of arriving at E shape design of electromagnet. Unlike other conventional E shape electromagnet where central limb is an integral part of Magnet core, this concept uses a separate vertical limb having same numbers of magnet stampings clamped together are placed vertically over short central limb of electromagnet with an insulating spacer of very low thickness is kept in between to prevent residual magnetism in fixed magnet core. This arrangement of having separate central limb is clamped with fixed magnet core using two restrain plates 6, which keep separate limb intact with fixed magnet 2, and makes it as an integral part of fixed magnet 2.

Operation of this concept can be explained as when current passes through the electromagnet coil, a magnetic field is produced; this attracts the moving core of the contactor. The electromagnet coil draws more current initially, until its inductance increases when the metal core enters the coil. The moving contact is propelled by the moving core; the force developed by the electromagnet holds the moving and fixed contacts together. In course of attraction of moving magnet 1, by fixed magnet 2, initially the flux linkage through the core of moving magnet 1 is limited hence the current drawn by coil initially is high, in this period the current through shading rings is also negligible because of the presence of high air gap between moving magnet 1, and fixed magnet 2. So the execution of application of shading rings comes when the moving magnet 1, approaches the fixed magnet 2, and the flux linkage through its core begins producing current in all three shading rings.

The flux produced by third shading ring gives an additional offset to the average magnet holding force. This additionally produced force can be utilized to suffice the requirement of having higher contact spring values to achieve high breaking velocity.

The performance of shading ring actually comes in to picture when moving magnet approaches the fixed magnet 2, and flux linkage starts occurring. Using a high value contact spring can give contacts bounce or may produce chatter at contacts if the magnet holding force is not sufficient. But using this unique concept the chattering can be avoided as the moving magnet 1, approaches fixed magnet, even before moving contacts touch the fixed contacts, the third shading ring 5, along with extreme pole shading rings 3&4 already starts functioning by giving additional offset to magnet force so that before contacts touch each other, there’s an addition pull exist to provide a clear contact mating without any chatter. This way to break higher currents higher force value springs can be employed in contactor application. This improves the contact life and gives multiple advantages like improved contact life, improved breaking of higher currents, better reliability of product, improvised product safety.
Other aspect of using this concept is the economic concern towards product design. As for achieving the same performance coil energy consumption can be reduced as introduction of third shading ring enables magnet force with additional offset. This way coil copper volume or magnet size or both can be reduced, leading to a substantial cost reduction in product manufacturing.

Advantages of 3 shading rings in the present invention:

Introduction of three shading is holds analogy with the situation of three turn shorted secondary coil of an electromagnet, hence increasing the secondary current, resulting more current to be drawn by the primary coil. The increment in primary current provides extra ampere-turns without altering the coil parameters, and in response to those extra pumped ampere-turns the magnet produces a high force as compared to earlier case where contactor was operated with only two shading rings on electromagnets. Following are the advantages that 3-shading ring concept can enhance the contactor performance in following ways:
1. Higher magnet force: The introduction of 3 shading rings on electromagnet enables the electromagnet delivering higher attraction force. In normal practice to achieve higher magnet force the modification will be required as:
a. Higher copper volume (more no. of turns of coil) is required, hence increasing the cost by a substantial factor.
b. Increasing core volume (to achieve higher flux linkage, the pole face area has to be large possible, which leads to an increment in core volume) hence impacting physical size of equipment.

These factors can be compensated just by introduction of one more shading ring on
the center pole of electromagnet.

2. Higher breaking velocity: This is the most advantageous outcome of this concept. A high breaking velocity is required to minimize losses to contacts because of arc. So as in case of higher rated contactors where rating is high but the product size and its element values like copper volume, size of magnets and return/contact spring force value are constraints to provide quick breaking on high currents, all these are constraints against achieving higher breaking velocity. These limitations can be overcome by incorporating 3 shading ring concept in to electromagnets, as with the same volume of copper and iron core we can match the contactor’s hold on and pickup requirements even with the fairly higher contact spring forces and a high breaking velocity for contact system can be achieved.

3. Better product life: The life of contactor mainly depends upon the physical condition of the contacts, the life of which reduces with frequent opening and closing of contacts on higher currents. If breaking is not efficient this may reduce contacts life exponentially. This can be eliminated by providing enhanced and efficient breaking system. This can be achieved by keeping the contact spring force values high, but at the same time closing and hold on conditions of contact system are affected as attraction force of magnet remains same. Using this concept for same magnetic/coil copper volume, spring forces can be increased without affecting the closing and hold on conditions of contact system as introduction of 3rd shading ring suffice the additional requirement of magnet force. This provides better breaking system hence increasing contacts/product life.

4. Cost optimization: This concept without any kind of increment in copper volume of coil, reduction in contact spring force values or increment in iron core area enables the magnet to deliver higher magnet force; hence for achieving same performance of magnet system, the coil copper volume and the size of magnet can be reduced substantially. This way saving cost.

5. Energy saving: A contactor is an electrically controlled switch used for switching a power circuit, and for switching contactor on the supply is given to the coil of magnet. The contactor has to carry current continuously to keep the power circuit healthy; in this manner continuous power consumption is required in the coil. Using this concept the coil parameters can be reduced and contactor requires lower energy to operate to give same performance level. Because of continuous operation of contactor the amount of energy saved even per hour is huge.

The invention has been described in a preferred form only and many variations may be made in the invention which will still be comprised within its spirit. The invention is not limited to the details cited above. The structure thus conceived is susceptible of numerous modifications and variations, all the details may furthermore be replaced with elements having technical equivalence. In practice the materials and dimensions may be any according to the requirements, which will still be comprised within its true spirit.

WE CLAIM:

1. An improved shading ring arrangement for electromagnet, said arrangement comprising:
a fixed magnet means;
a moving magnet means;
plurality of restrain plate means;
plurality of pole means comprising center pole means and extreme pole means; wherein said each pole means comprising plurality of shading ring means such that total flux produced by all the shade rings of all the poles provide high breaking velocity to minimize losses to said contact means;
wherein said center pole means being attachably and/or detachably arranged on said fixed magnet means by means of said plurality of restrain plate means so as to prevent residual magnetism in the fixed magnet means.

2. Arrangement as claimed in any of the preceding claims, wherein said center pole means being operatively arranged approximately at a center portion of said fixed magnet means.

3. Arrangement as claimed any of the preceding claims, wherein said center pole means being operatively arranged between said plurality of restrain plate means.

4. Arrangement as claimed in claim 1, wherein said restrain plate means is substantially ‘L’ shaped.

5. An improved shading ring arrangement for electromagnet as herein substantially described and illustrated with the accompanying drawings.