Low reluctance transformer core

A low reluctance transformer core utilizing first and second members, each having a leg element tapered in opposite directions. The tapered leg elements have continuous tapered interfaces designed to cooperatively mate with a wedging action forming a low reluctance leg.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to an electromagnetic device and 
specifically to a low voltage transformer relay. 
2. Description of the Prior Art 
Electromagnetic devices such as the magnetic remote control switch 
described in U.S. Pat. No. 3,461,354 to Bollmeier may be used to control 
high voltage, high current electrical loads by remotely located low 
voltage switches. This type of remote switching device is generically 
called a low voltage transformer relay. 
One of the principle advantages of such low voltage transformer relays is 
the ability to control the electrical load by a multiplicity of low 
voltage switches located in various locations. For example, if a low 
voltage transformer relay is used to control a lighting load within a 
room, one or more low voltage switches located within the room as well as 
one or more remotely located low voltage switches may be used to control 
the load. Such a configuration allows one to extinguish all of the lights 
within a building from a single remote location having a low voltage 
circuit to each transformer relay. 
There is a continuing need, however, to reduce the fabrication costs and 
improve the electrical and mechanical performance of such low voltage 
transformer relays. 
SUMMARY OF THE INVENTION 
A ferromagnetic core has a source of operating flux for establishing a 
magnetic field in the ferromagnetic core. The ferromagnetic core has a 
first member having first and second leg elements tapered in opposite 
directions and a second member having first and second leg elements 
tapered in opposite directions. The tapered leg elements have continuous 
tapered interfaces adapted to cooperatively mate with a wedging action 
forming low reluctance first and second legs. 
The ferromagnetic core is constructed with the first and second leg 
elements of the first and second members having a coefficient of friction 
with respect to each other and with the continuous tapered interfaces of 
the first and second leg elements of the first member and the second 
member forming a taper angle. Preferably the value of the tangent of the 
taper angle is not more than the value of the coefficient of friction. In 
a preferred embodiment the taper angle is greater than zero and not more 
than thirty-five degrees, and still preferably the taper angle is 
approximately fifteen degrees. 
The first leg of the ferromagnetic core may form the core for a primary 
winding while the second leg may form the core for a secondary winding. 
The primary winding may be adapted to be connected to a power source and 
the secondary winding may be connected to a rectifying switch where the 
rectifying switch may selectively control the direction of induced current 
in the winding for selectively establishing an operating flux. 
As a consequence of the tapered leg geometry, the flux flowing between the 
individual elements of the first and second members is presented with an 
area much larger than the core leg cross-section. The tapered leg geometry 
also produces a wedging action when the first and second members are 
brought together creating a very small clearance or interface dimension. 
Both of these actions cooperate to lower the reluctance of the first and 
second legs and, in one embodiment, reduce the reluctance to one-half of 
the value of reluctance of a comparable butt or lap joint.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The low voltage transformer relay illustrated in FIG. 1 includes a core 9 
having an upper core member 10 and a lower core member 11, a primary 
winding 50 formed on a spool structure 39, a secondary winding 51 formed 
on a spool structure 44, sources of latching flux 25 and 26, a flux return 
bracket 27 and an armature 28. The sources of the operating flux 12 are 
the primary winding 50 and the secondary winding 51. This operating flux 
is carried by the core 9. Sources of latching flux 25 and 26 are 
positioned between the ferromagnetic core 9 and the flux return bracket 
27, one on either side of gap 13. Preferably the sources of latching flux 
25 and 26 are permanent magnets, such as Plastiform flexible permanent 
magnets available from Minnesota Mining and Manufacturing Company, St. 
Paul, Minn. The source of operating flux and the source of latching flux 
generate magnetic flux conducted through core 9, flux return bracket 27 
and armature 28, which activates a load switch 29, to form a magnetic 
circuit which will latch the armature to one of the pole faces 14 or 15. 
The sources of latching flux (25 and 26) are positioned in the present 
invention and the core 9 is constructed to minimize total magnetic 
reluctance in the low voltage transformer relay. 
To ensure that the reluctance of the ferromagnetic core 9 is low, a low 
reluctance core structure is utilized. As shown in FIG. 2, the 
ferromagnetic core 9 is formed from an upper core member 10 and a lower 
core member 11. The upper core member 10 has a first leg element 40 and a 
second leg element 41, each having one continuous tapered interface 45 and 
46, respectively. Likewise, the lower core member 11 has a first leg 
element 70 and a second leg element 72, each having one continuous tapered 
interface 47 and 48, respectively, complementary to the tapered interfaces 
45 and 46 of upper core member 10. The taper angle .alpha. is preferably 
less than 35.degree., and still preferably is approximately 15.degree.. 
During assembly, the upper and lower core members are inserted into a 
spool structures 39 and 44 having hollow central portions for receiving 
the leg elements (40, 41, 70 and 72) and for receiving the primary winding 
50 and the secondary winding 51. The interior dimension of the hollow 
portion of the spool structure 39 is smaller than the sum of the widths of 
the bases of first leg elements 40 and 70. Similarly, the interior 
dimension of the hollow portion of the spool structure 44 is smaller than 
the sum of the widths of the bases of the second leg elements 41 and 72. 
Insertion into the spool, therefore, forces the tapered faces 45, 46, 47, 
48 into wedging contact. The first leg elements 40 and 70 of the upper and 
lower core members 10 and 11, respectively, together define a first leg; 
and the second leg elements 41 and 72 together define a second leg. This 
wedging action reduces the interface dimension L, the space between the 
mated tapered faces 45 and 47 and between the mated tapered faces 46 and 
48, to a minimal value due to force amplification caused by the continuous 
tapered interfaces, and which locks the upper core member 10 and lower 
core member 11 together through frictional forces preventing any 
subsequent loosening and attendant increase in the interface dimension L. 
The taper angle .alpha. should be chosen such that the value of tangent of 
the taper angle .alpha. is not more than the value of the coefficient of 
friction between the continuous tapered interfaces 45 and 47 and between 
the continuous tapered interfaces 46 and 48. In some preferred embodiments 
the value of the taper angle .alpha. is from greater than zero to not more 
than thirty-five degrees. In one preferred embodiment, the taper angle 
.alpha. is approximately fifteen degrees. As a consequence of the geometry 
of this design the flux flowing between the upper and lower core members 
10 and 11 is presented with an area A, along the continuous tapered 
interfaces 45, 46, 70 and 72, much larger than the core leg cross section. 
The wedging action of the spool structures 39 and 44 creates a very small 
clearance or interface dimension L. The effect is to minimize the factor 
L/A to which reluctance is directly proportional. This construction 
reduces the reluctance to one-half of the value of the prior art butt or 
lap joint construction. 
In FIG. 3 the electrical connections to the low voltage transformer relay 
are shown. A primary winding 50 and a secondary winding 51 are wound on a 
spool structures 39 and 44, respectively. During assembly the spools are 
oriented such that the secondary winding 51 surrounds the second leg 
elements 41 and 72 of the core 9, and the primary winding 50 surrounds the 
first leg elements 40 and 70 of the core 9. 
In operation the primary winding 50 is connected to a source of A.C. 
voltage through leads 52 and 53. The A.C. voltage across the primary 
winding 50 induces an A.C. voltage on the secondary winding 51. 
Rectifying switches 54 and 55, are connected to the secondary winding 
through leads 56 and 57 which permits half wave current to flow in the 
secondary winding opposing the primary flux and resulting in operating 
flux appearing in the flux paths 30, 31 (shown in FIG. 1) of the device. 
The rectifying switches 54 and 55 include single pole double throw 
switches of the momentary contact type, and a pair of diodes. The cathode 
of one diode and the anode of the other diode of the pair of diodes 
associated with switch 54 are connected to one terminal 60 of the switch 
54. The opposite ends of each diode are connected to the switched 
terminals of switch 54. The common terminal 61 of the switch 54 is 
connected to the secondary winding lead 57. The second switch 55 is 
connected similarly. In operation, the switches are used to selectively 
connect one of the diodes in series with the secondary winding. In this 
position, an electrical circuit is completed which allows the induced 
voltage in the secondary to establish an unidirectional current in the 
coil and a corresponding magnetic field in the core 9. This is the source 
of operating flux 12 (shown in FIG. 1) to transfer the armature 28. The 
two positions of the switches correspond to the two positions of the 
armature 28. As illustrated in FIG. 3, an arbitrary number of rectifier 
switches 54, 55 may be connected in parallel to control the low voltage 
transformer relay from a number of remote locations. 
The armature 28 carries a pair of electrical contacts which cooperate with 
a pair of stationary contacts to form a load switch 29. When the armature 
28 contacts pole face 15 it carries the contacts thereon into contact with 
the stationary contacts to complete an electrical circuit to a power a 
load. When rectifying switch 54 or 55 is momentarily moved to its off 
position the armature is moved to pole face 14 separating the contacts and 
disconnecting the power to the load.