Electrification & Early Modern (1870–1920)Electromagnetism & Power Generation

Tesla Polyphase AC Induction Motor

US 381,968

Rotating Magnetic Field and Polyphase Alternating Current Induction Machine

Inventor(s)Nikola Tesla

Grant Date1888-05-01

Filing Date1887-10-12

LocationNew York, New York

Tesla's 1888 method for turning a motor with two or more alternating currents that differ in phase. Stationary stator coils produce a magnetic field that walks around the air gap; a closed rotor follows it by induction. No commutator, no brushes.

USPTO PDF

Engineering Analysis & Physical Principles

How It Works: Step-by-Step Mechanical & Physical Breakdown

In 1887 a factory motor meant a DC machine with a split-ring commutator and carbon brushes. The brushes sparked, wore out, and confined useful DC transmission to about a mile. Tesla's answer was to leave the field coils still and let two (or three) alternating currents, shifted in phase, make the magnetic field itself walk around the stator. A closed rotor follows that field by induction.

The Core Breakthrough Mechanism

Two currents 90° apart in perpendicular coils on an iron ring give a net field $\vec{B}_{net}(t) = B_0[\cos(\omega t)\hat{i}+\sin(\omega t)\hat{j}]$ of constant magnitude that rotates at $n_s = 120f/P$ . That traveling field cuts closed copper on the rotor, induces current, and the rotor is dragged along a slip behind the field.

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Detailed Component Architecture

1Stationary Polyphase Stator Coils

Perpendicular pairs of coils energized by out-of-phase AC currents.

Circuit 1 carries $I_1(t) = I_0 \cos(\omega t)$ through horizontal poles; Circuit 2 carries $I_2(t) = I_0 \sin(\omega t)$ through vertical poles. The net magnetic field vector is $\vec{B}_{net}(t) = B_0 [\cos(\omega t)\hat{i} + \sin(\omega t)\hat{j}]$ , having constant magnitude $|\vec{B}_{net}| = B_0$ and rotating at angular velocity $\omega = 2\pi f$ .

19th-C. Term: Energizing-circuits differing in phaseModern: Polyphase AC stator windings

2Closed-Circuit Laminated Rotor (Armature)

A cylinder of laminated electrical steel containing closed copper conductors.

The rotor has no electrical connection to any power source. As the stator's B-field rotates, it cuts the rotor bars with relative speed (slip $s = (n_s - n_r)/n_s$ ). By Faraday's law of induction ( $\mathcal{E} = -d\Phi/dt$ ), this induces large AC currents in the rotor bars, generating Lorentz force torque ( $\vec{F} = I \vec{L} \times \vec{B}$ ).

19th-C. Term: Armature with closed coilsModern: Squirrel-cage induction rotor

3Brushless Laminated Core Design

Thin insulated sheets of silicon steel stacked together.

Laminating the iron stator and rotor cores interrupts closed circulating eddy currents inside the bulk iron, reducing hysteresis and eddy-current losses ( $P_{eddy} \propto f^2 B_{max}^2 d_{lam}^2$ ) and allowing high operational efficiency (>90%).

19th-C. Term: Laminated soft-iron ringModern: Laminated stator core stack

Governing Physical Equations & Principles

Rotating Magnetic Field Vector Synthesis

\vec{B}_{net}(t) = B_0 \cos(\omega t)\hat{i} + B_0 \sin(\omega t)\hat{j} \implies |\vec{B}_{net}| = B_0

Two sinusoidal magnetic fields in space quadrature and time quadrature sum vectorially to produce a single rotating vector of constant magnitude.

Faraday-Lenz Electromagnetic Induction

\mathcal{E}_{rotor} = -N \frac{d\Phi_B}{dt} = -N \frac{d}{dt} \int \vec{B}_{rot} \cdot d\vec{A}

The time-varying magnetic flux through the closed rotor loops generates an electromotive force (EMF) that drives induced currents without electrical contacts.

Asynchronous Rotor Slip & Induction Torque

T_{em} = \frac{3 V_{th}^2 R_r'/s}{\omega_s [(R_{th} + R_r'/s)^2 + (X_{th} + X_r')^2]}

Induction motors operate with a small slip s between synchronous field speed and mechanical rotor speed; maximum torque (breakdown torque) occurs at critical slip.

Why It Still Matters

Most of the electrical energy that becomes shaft work still goes through a three-phase induction machine: pumps, compressors, factory lines, locomotive traction. The name on a modern EV inverter is marketing; the physics is still $n_s = 120f/P$ and a squirrel cage chasing a rotating field.

Legal Claims Decoder (3 Numbered Claims)

Compare dense legalistic claims directly with decoded plain-English functional specifications.

Claim #1Independent Master Claim

Verbatim Historical Legal Text

“The method of operating electro-magnetic motors herein described, which consists in producing a progressive shifting of the magnetic poles of the motor by directing through independent energizing-circuits alternating currents differing in phase, substantially as set forth.”

Plain English Engineering Translation

The master claim covering the method of turning any electric motor by using two or more alternating currents with shifted phases to create a rotating magnetic field.

Key Protected Innovations:

Polyphase AC rotating magnetic fieldPhase-shifted alternating currentsProgressive magnetic shifting without mechanical switching

The Historical Bottleneck

Edison's Pearl Street station (1882) sold 110-volt DC. $I^2R$ loss made that voltage useless beyond about a mile, so every neighborhood needed its own dynamo. Transformers could raise AC for long lines, but factories still wanted a motor that started under load and did not eat its own brushes. Until Tesla, AC was a lighting trick.

Why Prior Art Failed

•DC commutators sparked, needed constant turning, and failed in dusty mills.
•Single-phase AC machines had no starting torque; they had to be spun up by hand.
•Gaulard–Gibbs and Zipernowsky–Déri–Bláthy transformers served lamps, not shafts.
•Niagara's 1880s hydraulic plans had no electrical load except arc lights.

The Breakthrough Insight

“Tesla later said the idea arrived in Budapest in 1882, walking and reciting Faust: two stationary coils, currents in quadrature, a field that rotates in empty iron. Ferraris in Turin published a similar rotating-field observation in 1888; Tesla had already filed. Priority fights followed, but Westinghouse bought Tesla's stack, not Ferraris's paper.”

Patent Wars & Legal Litigations

Vs. Thomas Edison and General Electric (War of the Currents)Infringement Challenge

Rival Claim & Defense:

Edison's camp argued high-voltage AC would kill customers. They funded public animal electrocutions and backed the first electric chair (1890) as a demonstration of AC danger.

Litigation Conflict:

Westinghouse licensed Tesla's polyphase patents in 1888 (cash, stock, and a per-horsepower royalty). The 1893 Chicago fair ran on Westinghouse AC. In 1895 the Niagara Adams plant sent two-phase power to Buffalo. GE, after merging with Thomson-Houston, had to take AC licenses to stay in the transmission business.

Final Resolution & Judicial Outcome:

By 1900 new urban plants were AC. Edison lost the system fight and left the day-to-day running of GE. DC lingered in elevator and traction pockets into the late 20th century.

After the Grant

Tesla left motor design for radio-frequency and wireless-power work. The induction machine became a GE and Westinghouse commodity. His name returned to consumer products a century later; the stator math did not need the branding.

Civilizational Impact

Once a factory could hang an induction motor on a 60 Hz (or 50 Hz) feeder, the steam-shaft alley died. The same polyphase grammar still sets the frequency of every interconnected grid.

Historical Fact

During Westinghouse's 1890s cash crisis Tesla released the per-horsepower royalty. The often-quoted '$12 million torn up' figure is a later estimate, not a cancelled invoice, but the waiver was real and it kept the AC plant program alive.

Further Context

US 381,968 is one of a cluster Tesla filed in October 1887. The companion generator and distribution patents are why Westinghouse could bid Niagara as a system, not a motor.
Galileo Ferraris demonstrated a two-phase rotating field in Turin in 1885 and published in 1888. He did not file in the United States. Historians now treat the physics as independently seen; the industrial system is Tesla–Westinghouse.
Early Niagara generators were two-phase. Utilities later standardized on three-phase because three wires carry more power for the copper. Tesla's claims already covered more than two phases.