US2848656A - Windings for electromagnets - Google Patents

Description

Aug. 19, 1958 LQR. NIXCN WINDINGS FOR ELECTROMAGNETS Filed March 25, 1955 I unu- United States Patent WINDINGS FOR ELECTROMAGNETS Leslie Reginald Nixon, deceased, late of Catford, London, England, by Edith Maud Nixon, executrix, Catford, London, England, assignor to J. Stone & Company (Deptford) Limited,Londou, England This invention concerns exciting windings for electromagnets, particularly such for automatic regulators,'relays and the like, and has for its object to .provideby simple means effective compensation for variation of the resistance of such windings with variation of temperature.

According to the invention, an electromagnet comprises, in addition to a main Winding, an auxiliary winding connected in one diagonal of a bridge circuit across whose other diagonal the source of supply of current is to be connected and whose pairs of opposite limbs comprise two parts of a divided main winding, or the main winding and a resistance, and two ballast resistances respectively, the bridge circuit being arranged to be balanced at a normal mean temperature and, on unbalance due to temperature change, to produce in the auxiliary winding a magneto-motive force opposite to and substantially equal to the variation, produced by the temperature change, in the magneto motive force of the main winding. Thus the magneto-motive force of the auxiliary winding can alternatively oppose or assist that of the main winding so as to provide an effective magnetomotive force which remains substantially constant over a range of temperature change.

Various ways of carrying the invention into effect will now be described by way of example and with reference to the accompanying drawing in which:

Figures 1 to 3 are circuit diagrams showing different arrangements.

Fig. 4 is a diagram illustrating the relative disposition on the magnet of the windings shown in the circuit diagram of Fig. 1.

In all of the arrangements illustrated, the magnet is provided with a main winding divided into two equal parts .9, and s and with an auxiliary winding a. The latter is connected in one diagonal of a bridge circuit of which one pair of opposite limbs comprises the two parts s and s of the main winding and the other pair comprises two equal ballast resistances l l The other diagonal of the bridge is connected to the source b of current. The ballast resistances are made substantially insensitive to temperature variation. The magnet windings are preferably all of copper.

The parts s and s of the main Winding are arranged to assist each other. The auxiliary winding a may be unexcited or may assist or oppose the main winding depending upon conditions in the bridge. The resistances of the limbs s s and l 1 are designed so that the bridge will be balanced and the auxiliary winding unexcited approximately at the normal mean temperature in the working range of the electromagnet. If the resistance of the winding parts s s rises, so that the magneto-motive force of the main winding decreases, due to increase of temperature above the value at which the bridge is balanced, a current will flow in one direction in the auxiliary winding a. The winding is so arranged on the magnet that current in this direction will produce a magneto-motive force which boosts the magnetomotive force of the main winding. The former magneto- 2,848,656 Patented Aug. 19,1958

ice

motive force can be made substantially to compensate for the decrease in the magneto-motive force in the main winding. If the resistance of the Winding parts s s falls, due to decrease of temperature, therewill be a current in the opposite direction through the winding a and a magneto-motive force opposing thatof the main winding. In either case, the effective magneto-motive force will remain substantially unaltered.

The relative directions of the. aforesaid magnetizing forces produced by the currents in the windings are indicated inFig. 4. t i

As illustrated in Figure 2, a further ballast resistance 1 may, if desired, be connected in series with the whole bridge. r

If the windings s s and a require to beaexcited' by direct currentand the source of supply. is'alternating,

current, a rectifier may be providedui An arrangement of this kind is shown in Figure 3 in .Whicha;full-wave rectifier c is interposed between the source b and the bridge circuit described with reference to Figure 1. The additional ballast resistance 1 may be provided in this case also, as indicated in broken lines.

If the same compensation is required for variation of winding temperature due to self heating as for that due to ambient changes, it may be advantageous to interleave the main and auxiliary windings. On the other hand, if desired, compensation for the two effects may be made unequal by arranging for unequal temperature rises in the windings due to self heating.

Such arrangements can be advantageously applied to electro-magnetic regulators, for example to the carbonpile regulator described in the specification of the patent application No. 399,974. In such an application, the said arangements have certain advantages over compensating arrangements utilising, for example, a bimetal ele ment acting on the control spring of the regulator: For one thing, the compensation is independent of the setting of the control spring. Moreover, as the net magnetomotive force is substantially unaitected by temperature, undesirable effects due to changes in the force/deflection characteristic of the magnet system are obviated: For instance, a change in the cut-in voltage with temperature is avoided. Transitory errors due 'to sudden changes of ambient temperature can also be avoided, as the thermal time constants of the two windings can be made similar. This is of particular benefit in applications used on aircraft.

For some purposes, instead of dividing the main winding of the electromagnet between two limbs of the bridge circuit, the undivided main winding may be arranged in one limb and an equivalent resistance in the opposite limb of the said circuit, whereby a similar temperaturecompensation effect can be obtained.

However, the invention is applicable with similar advantages to magnet windings of apparatus other than regulators, for instance relays saturable chokes, motor and generator fields, and generally, to any winding whose magneto-motive force requires to be temperature-compensated.

What is claimed is:

1. An electromagnet comprising a magnet and, in addition to a main winding divided into two parts disposed on said magnet and, an auxiliary winding disposed on said magnet and connected in one diagonal of a bridge circuit across Whose other diagonal the source of supply of current is to be connected and Whose pairs of 0pposite limbs comprise the two parts of the main winding and two ballast resistances respectively,. the bridge circuit being arranged to be balanced at a normal mean temperature and, on unbalance due to temperature change, to produce in the auxiliary winding a magnetomotive force opposite to and substantially equal to the variation, produced by- 'the' t'emperature change, in the magneto-motive force on the main winding.

2. An electromagnet comprising a magnet and in addition to a main winding disposed on said magnet, an

auxiliary winding, disposed bni sa'id 'm'agn'et connected in one diagonal of a bridge-"circuit across'whose' other diagonal the-source of supply of current is to be connected and whose'pairs of opposite limbs comprise the balance due tortemperaturechange, to produce in the auxiliarywinding a magneto-motive force opposite to and substantially equal to'the variation, produced by the temperature change, in the magneto-m-otiveforce of the main winding. H

I 3. An ele'ctromagnet as elaimedin claim 1, wherein a further ballast resistance is connected in series with the 1 bridge circuit.

4. An electromagnet as claimed in claim L -wherein a rectifier is interposed between'the source of supply and the bridge circuit.

S. electromagnetic carbon-pile regulator provided "with a temperature-compensated electromagnet comprising a magnet and, in addition to a main winding divided into two parts disposed on said magnet, an auxiliary winding disposed on said magnet connected in one diagonal of a bridge circuiva'crbss Whose other diagonal the source of supply of current is to be connected and whose pairs ofoppositelimbs comprise the two parts of the main winding and two ballast resistances respectively, the bridge-circuit being arranged to' be balanced at a 'normal'mean temper'ature'and, on unbalance due to temperature change, to produce in' the-auxiliary winding a magneto-motive force opposite to and substantially equal to the variation, produced by the temperature change, in the "magneto-'motive fo'rce of the main winding.-

References Cited in the file of this patent UNITED STATES PATENTS

Images