A two-speed salient-pole synchronous motor

The design features and performance characteristics of salient-pole synchronous motors are well known and have been thoroughly covered in the technical press. The synchronous motor has been handicapped in the past because it is inherently a single-speed machine, and a change in speed could be obtained only by a change in the frequency of the power supply. Changing the frequency, however, is not practical in most applications. The special pole described in this paper which allows two-speed operation of a synchronous motor to be obtained at high efficiency is a new feature
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. The same principle applied to a generator enables two frequencies to be obtained at the same speed or the same frequency at two different speeds. All that is necessary to change the speed (or frequency) is a pole-changing switch for the stator winding and a reversing switch for the rotor winding. A 5000/2500-h. p., 600/300-rev. per min., two-speed synchronous motor was built without having first constructed a model of any kind. This motor proved to be entirely satisfactory and its characteristics obtained by test agreed very closely with the calculated characteristics. At either speed the two-speed synchronous motor functions exactly as the ordinary synchronous motor. There is nothing special or complicated about its construction, it does not require any more attention than the ordinary synchronous motor, and its expense of maintenance is just the same. The first cost of such a motor is only slightly more than that of the ordinary synchronous motor whose rating is equal to the low-speed rating of the two-speed motor. Therefore, the two-speed synchronous motor is a practical machine and it should open a new field for synchronous motor application.