Historical research guide · 0382 · 1888
Electro-Magnetic Motor (US Patent 382,279)
Nikola Tesla
In one minute
Nikola Tesla filed this patent in November 1887 and it issued on 1 May 1888, in the middle of the run of applications that gave the world the alternating-current motor. The machine he describes has no commutator and no brushes touching the spinning part. Two independent coil circuits wrap a ring of soft iron, and alternating currents that peak a quarter-turn apart are fed to them from a generator wound the same way. The two currents add to one pair of magnetic poles whose position walks steadily around the ring — a magnetic field that rotates although nothing mechanical is rotating, built purely out of the timing of two currents. Tesla’s new step in this patent is what he puts inside the ring: a plain iron disc carrying closed loops of wire that connect to nothing at all. The travelling field induces currents in those loops, the induced currents raise their own poles, and the reaction between the two sets of poles drags the disc around after the field.
Connection
This is the founding document of the rotating field — the architecture behind every later device on this site that works by spinning a field instead of a part, from the rotating-superconductor experiments of chapter 11 to the accelerated spin and vibration of charged systems in chapter 8. It is also the clearest early demonstration of chapter 10’s theme: the controllable quantity is phase, and shifting the phase of two currents moves a real physical field.
From the source
A motor whose field magnet is wound with two or more independent energizing circuits, connected to the corresponding generating circuits of an alternating-current generator, has the poles of its field magnet progressively shifted around the ring by the timing of the two currents alone, with no commutator on either machine.Specification, opening paragraphs; Claims 1 and 2
Rotation need not come from direct attraction between magnetised parts: the shifting of the poles can instead be used to set up currents in a closed conductor placed inside the field, and the rotation then results from the reaction of those induced currents upon the field.Specification, ‘I have discovered that advantageous results may be secured…’
Because the currents are induced in the closed armature coils in the same manner at every instant, the poles of the disc follow the poles of the annular field magnet continuously and hold the same relative position, which gives an even and continuous action.Specification, description of Figures 1 and 2
Running unloaded, the armature turns almost synchronously with the rotating poles and very little current appears in the closed coils; adding load slows the disc slightly, which augments the induced currents and increases the rotary effort in proportion, so the machine regulates its own pull.Specification, ‘When these motors are not loaded, but running free…’
Reversing the connections of one of the two energizing circuits reverses the direction in which the poles shift and reverses the motor almost instantaneously; at that moment the resistance of the motor falls sharply and much more current passes, which Tesla attributes to the variation in self-induction of the energizing circuits.Specification, ‘An advantage and characteristic feature…’
Because no commutators are required on either the generators or the motors, the system as a whole is simple, reliable, economical to build and maintain, safe to handle, and involves little loss.Specification, closing paragraph before the claims
The way in
https://patents.google.com/patent/US382279/enHow to cite it
Nikola Tesla (1888) Electro-Magnetic Motor (US Patent 382,279) US382279