Tesla Induction Motor

Build and run this on a low-voltage AC source, never on the mains. Enclose the rotor before powering up — an unbalanced rotor at speed throws parts.

Tesla claims two or more independent circuits carrying alternating currents producing a rotating field. There is no commutator, no brush and no connection to the rotating part.

Open a small DC motor and find the split commutator and brushes. Run it in the dark: it sparks. That sparking is wear, radio interference and an ignition source.

Wind two opposed pairs at right angles around a laminated ring. Each pair is one phase, and their axes must be exactly 90° apart.

Build the core from insulated sheets. A solid core carries its own induced eddy currents and turns your motor into a heater.

Drive a single pair with AC. The field pulses back and forth along one axis. A rotor placed in it will hum and sit still — pulsing is not rotating.

Feed the other pair through a capacitor so its current lags or leads. You now have two fields at right angles, out of step in TIME — that is the whole invention.

Put a small compass needle in the bore at low frequency. It turns steadily rather than shaking — the field vector is sweeping round the circle.

Suspend an aluminium or copper disc in the bore. It rotates, with no windings, no magnets and no connections — this is the Arago disc effect doing useful work.

Set copper bars in slots around a laminated cylinder and short them together with end rings. It is a cage, not a winding, and it has no beginning or end.

The rotor always turns slightly slower than the field. That difference is called slip, and it is not a defect — without it there is no relative motion and no induced current.

Apply a brake. The rotor falls further behind, induced currents rise, and torque rises to meet the load. The motor self-regulates with no controller.

Exchange the leads of one phase. The field now rotates the other way and so does the motor. Direction is set by wiring order alone.

Let it run under load, then examine the rotor. Nothing has worn, because nothing touched it. This is why induction motors last decades in pumps and fans.

The patent. US 381,968, "Electro-magnetic motor", granted 1 May 1888 to Nikola Tesla. He described the polyphase system in a lecture to the American Institute of Electrical Engineers days later, and George Westinghouse licensed the patents that year. The Italian physicist Galileo Ferraris demonstrated a rotating magnetic field independently in 1885 and published in March 1888, two months before Tesla's grant — the two arrived at the same principle separately, and Ferraris deserves naming.

Why time-shifted currents rotate a field. Two coil pairs at right angles, carrying currents 90° out of phase, produce two sinusoidal fields in quadrature. Add them as vectors and the resultant has constant magnitude and steadily rotating direction — it sweeps the circle once per electrical cycle. Nothing mechanical rotates to make this happen; the rotation is a consequence of geometry plus timing. It is the same mathematics as circular polarisation in optics.

Slip is what makes it work. The cage rotor has no supply of its own. It gets current by induction, which requires relative motion between the field and the bars — so the rotor MUST lag the field. At exactly synchronous speed there would be no relative motion, no induced current, no torque. Loading the shaft increases slip, which increases induced current, which increases torque: the machine matches its own load automatically. That elegant self-regulation is why induction motors need no control gear to run.

Why this decided the War of the Currents. Edison's DC could not be transformed to high voltage, so it could not be transmitted far. AC could — Stanley's transformer of 1886 saw to that — but until 1888 there was no good AC MOTOR, and a power system with no motors is only a lighting system. Tesla's motor closed the gap, and with generation, transmission and motive power all working on alternating current, the argument was effectively over. Roughly half of all electricity generated today is consumed by induction motors, most of them still squirrel-cage machines built to this patent's principle.