Tesla Polyphase Patents held to Cover Polyphase Synchronous Motors

On February 20 Judge Seaman, of the Circuit Court of the United States of the Eastern District of Wisconsin, handed down an opinion holding that the fundamental Tesla patents, issued May 1, 1888, cover the rotary converter and synchronous motor operated by polyphase currents. The defendants to the suit contended that Tesla's invention is the rotating field mode of operation and this is limited to a self-starting motor so operated; that the defendants' synchronous motor with direct-current excitation is not within the invention, being simply a reversed two-phase alternating-current dynamo of the prior art used as a generator; and that the claims in the Tesla patents which introduced "direct-current excitation producing synchronous operation" must be limited to such use after the motor is started by the Tesla means and method and brought up to speed. The court held that if the invention is to be so limited the defendant escapes infringement; on the other hand, if not so restricted, it is equally clear that the claims of the patents are broad enough to reach the defendants' means and method of operation.

The court stated that the Tesla discovery was of means and methods in the polyphase rotating field motor to produce the "whirling field of force," and employment thereof was obtainable, with or without direct-current excitation of the armature or secondary element. The fact that its use produced a self-starting motor without the aid of such excitation cannot alone serve to limit the invention to that effect. The theory advanced of the obliteration of the alternating-current magnetization by that of the direct current of the secondary element of the motor, the court held, was not material, as it is unquestionable that the Tesla invention is brought into the operation of the motor and the structure is within the patent description.

As to the Bradley patent, upon which stress was laid in the argument as showing a polyphase machine stated to be adapted for use "as a motor when fed or actuated by alternating currents," this is subsequent in date of patent issue to the Tesla patents, though the original application bears date prior to that of Tesla. The testimony of Mr. Bradley and his attorney cannot affect the interpretation which must be given the disclosure of the invention in the original application, and if such application can be treated as a disclosure of the prior art, it fails to reach the Tesla conception. Its deficiencies in that regard were pointed out in previous suits involving the Tesla patents.

The conclusions of the court are that the defendants' synchronous motor structure and operation are within the generic invention covered by the claims of two of the fundamental Tesla patents; that the invention is utilized in the defendants' motor operation, notwithstanding the introduction of direct-current excitation to make it synchronous in lieu of the full realization of the revolving field effect for self-starting; that such supplemental aid does not escape infringement with or without advantage in such association; and that, in any view the combination patents referred to are not limited to after effect of direct-current excitation, but apply to the defendants' combination.

The parties of the suit were the Westinghouse Electric & Manufacturing Company and the National Electric Company.