US2901715A - Variable transformer - Google Patents

Description

Aug. 25, 1959 s, PARKER 2,901,715

VARIABLE TRANSFORMER Original Filed April 30, 1952 Fig. 2

INVENTOR. SAM E. PARKER ATTORNEYS United States Patent VARIABLE TRANSFORMER Sam E. Parker, San Diego, Calif.

Original application April 30, 1952, Serial No. 285,298, now Patent No. 2,762,971, dated September 11, 1956. gigidetsl and this application April 12, 1956, Serial No.

1 Claim. (Cl. 33687) (Granted under Title 35, US. Code (1952), see. 266) This is a divisional application of application Serial No. 285,298, filed April 30, 1952, and issued as Patent No. 2,762,971, on September 11, 1956.

This invention relates to a variable transformer and more particularly to a variable RF transformer utilizing a shielded rotatable secondary winding.

In the prior art there has long existed the problem of providing adequate magnetic shielding between primary and secondary of RF coupling transformers. This has been particularly true of the type in which the secondary winding is rotatably disposed within the primary winding. The prior art shielding consisted of encasing the primary winding with a magnetic shield. The main disadvantages of this prior art expedient lay in the generation of excessive eddy currents, and the inequal and asymmetrical capacity between the secondary and ground for various coupling positions.

It is thus an object of the present invention to provide a shielded RF variable coupling transformer in which the ground capacity effects are constant for any coupling position.

A further object is the provision of an inter-winding transformer shield with a minimum of eddy currents.

According to the invention a secondary winding is insulatedly mounted within a primary Winding on a rotatable shaft. A magnetic shield which is divided into two symmetrical parts resembling two cup-shaped halves of a closed right circular cylinder is mounted on the shaft with the open ends adjacent one another. The cylinder thus substantially surrounds the inner coil or secondary winding. One of the two halves is conductively connected to the shaft, which is grounded. The two parts of the shield are connected to one another by a single conductor at one point only on each shield, minimizing eddy currents in the shield. Since the shield rotates with the inner coil or secondary, capacity to ground will remain constant regardless of the position of the shaft.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. l is a sectional view of a preferred embodiment of the invention; and

Fig. 2 is a prospective view of the shielding of Fig. 1.

Referring now to the drawings, there is shown in Fig. 1 a preferred embodiment of the present invention. The secondary winding 80 is rigidly secured to shaft 82 and positioned inside stationary primary winding 84 which is maintained in place by bracket 85. Connections to the primary are through leads 81 and 83. A slotted shield 86 surrounding the secondary is symmetrical to ground at all times so capacitive coupling to ground is constant, capacitive coupling between the two windings is eliminated, and single-point injection of the induced volt- 2,901,715 Patented Aug. 25, 1959 age is provided. The slots in shield 86 are effective to cut down on losses due to eddy curents. The degree of coupling (M) of the two windings is indicated by the setting of pointer 88 on scale 90 as secondary is rotated by means of knob 92. One end of secondary winding 80 is connected to the coupling circuit by means of lead 98 which is insulated from shaft 82 and shielding 86. From lead 98, the current flows through conductor 102, which is a concentric sleeve affixed to said shaft and insulated therefrom, and is taken off by means of wiper arm 104 as the shaft rotates. The other end of secondary winding 80 is simply connected through lead 103 to shaft 82 which in turn is electrically connected to shielding 86 and to grounded bushing 105. The two sections of shielding 86 are connected through two opposing fingers at point 106.

Fig. 2 shows, in a cutaway perspective view, one section of shielding 86. Eddy current losses are held to a very low value by making the slots continuous almost to the hub and by forming the shielding in two generally similar sections. In practice, both sections of shielding 86 may be grounded by connecting one finger of the insulated section with an adjacent finger of the grounded section. The single point of contact does not materially reduce the efficiency of the shielding.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

What is claimed is:

A variable transformer comprising a first hollow cylindrical form, a first coil wound on said first form, a second hollow cylindrical form disposed within said first form, a second coil wound on said second form, a shaft rotatably mounted in said first form normal to the longitudinal axis thereof, said 2nd form mounted on said shaft with the longitudinal axis thereof normal thereto, a sleeve of insulation surrounding said shaft and extending from one side of said second form through said first form, a conductive sleeve surrounding said insulation sleeve, 21 contact brush mounted outside said first form in sliding electrical contact with said conductive sleeve, an electrostatic cylindrical shield symmetrically disposed around said second coil and within said first form, said shield comprising a first part conductively mounted on said shaft between one side of said 2nd coil and said first form, and a second part insulatedly mounted on said shaft between the other side of said 2nd coil and said first form, said shield being concentric and rotatable with said shaft, and said first and second parts comprising a bisected closed right circular cylinder, the plane of bisection of said right circular cylinder being normal to the longitudinal axis thereof, a plurality of slots in said shield parallel to said shaft, said slots having extensions forming radial slots in the ends of said shield, one end of said second coil conductively connected to said shaft and the other end of said second coil conductively connected to said conductive sleeve and a single conductor electrically connecting said two parts of said shield for minimizing eddy currents therein.

References Cited in the file of this patent UNITED STATES PATENTS 1,422,429 Filkins July 11, 1922 1,500,827 Langley July 8, 1924 1,624,473 Clement Apr. 12, 1927 1,651,658 Young Dec. 6, 1927 2,138,209 Seeley Nov. 29, 1938

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