Chapter 15
His motor. Their car. One hundred and twenty years apart.
The last automobile Nikola Tesla ever rode in was probably a taxi. He died in January 1943 in a New York hotel room, and in the final years of his life he walked or took cabs. He had no money for a car. He had no heirs.
Sixty years later, two engineers named the company they were building after him. Martin Eberhard and Marc Tarpenning, founding Tesla Motors in July 2003, did not choose the name as a marketing gimmick. They chose it because the motor at the center of their design was his motor — the three-phase AC induction motor that Tesla had patented in 1888, the machine he had first seen in his mind in a Budapest park twenty-one years before that. They wanted the name to mean something. It did.
To understand what Tesla Motors accomplished, you need to understand the thing they were replacing. Electric vehicles are not a new idea. They predate the gasoline automobile.
In 1900, electric vehicles outnumbered gasoline cars on American roads. Baker Electric, Detroit Electric, and Columbia Electric were serious commercial ventures. The problems that ended this first era of EVs were real and remained real for a century: limited range, heavy batteries, slow recharge, and the fundamental limitation of the DC motor. Those motors wore, sparked, and required maintenance. Their lead-acid batteries were heavy, corrosive, and degraded quickly.
The General Motors EV1 (1996–1999) used an AC induction motor and was technically credible: 0–60 in 8 seconds, 80–100 mile range on lead-acid, later 160 miles on NiMH. GM produced about 1,100 and leased them exclusively in California. They refused to sell them, recalled every one, and crushed them in 2003. The EV1 was a compliance car, built to satisfy California's Zero Emission Vehicle mandate, not because GM wanted to sell it.
The technical foundation came from a small California company called AC Propulsion. Alan Cocconi had worked on the GM Impact (which became the EV1) and left to found AC Propulsion in 1992, focused specifically on the AC induction drivetrain. By 2003 his company had produced a prototype called the tzero: a fiberglass two-seater running a three-phase AC induction motor powered by a pack of standard 18650 lithium-ion cells — the same cylindrical cells used in laptop computers.
The tzero ran 0–60 in 3.6 seconds and had a range of 300 miles.
The Tesla Roadster's drivetrain is an application of every principle in Chapters 5 and 6, running in reverse order from what Tesla originally envisioned: instead of AC power grid → AC motor, it is DC battery → inverter → AC motor.
A battery pack produces DC. Tesla's AC induction motor requires three-phase AC. The device that converts one to the other is an inverter, and it is the direct descendant of the VFD (variable frequency drive) described in Chapter 5.
The Roadster's inverter takes the battery pack's nominal 375 V DC and synthesizes three-phase AC at variable frequency, using IGBT power transistors switching at 10–20 kHz. The motor's windings average the switching and respond to the effective AC waveform.
From Chapter 5, synchronous speed depends on frequency:
At f = 10 Hz (2-pole motor): Ns = 600 RPM. At f = 60 Hz: Ns = 3,600 RPM. At f = 200 Hz: Ns = 12,000 RPM. The driver's throttle commands a torque request; the inverter controller solves for the frequency and voltage that produces that torque at the current motor speed, 1,000 times per second.
This is the sensation that defines the Tesla driving experience — the push into the seat from a dead stop, instantaneously, with no buildup, no gear change, no hesitation.
The reason is the torque-slip relationship from Chapter 5. An AC induction motor produces torque proportional to slip. At zero motor speed, slip is 100%. The motor produces near-maximum torque. As the motor accelerates, slip decreases and torque decreases — but the car is already moving fast by that point.
| Drivetrain | Efficiency Chain | Net Wheels Efficiency |
|---|---|---|
| Gasoline V8 | Engine 25–35% × transmission 95% × drivetrain 97% | ~23–32% |
| Tesla Roadster | Inverter 97% × motor 93–95% × gear 97% | ~88–92% |
The electric drivetrain delivers 3–4× more of its stored energy to the wheels. This is why a Tesla achieves the equivalent of 100+ MPGe despite carrying a 450 kg battery pack: the drivetrain losses are so much lower that the efficiency advantage dominates.
When the driver lifts off the throttle, the inverter reverses its operation. The motor becomes a generator: wheels drive the motor, motor produces current, inverter routes it back into the battery. This is Lenz's Law (Chapter 2) doing its job: the induced current opposes the change — here, the deceleration of the vehicle — and in opposing it, generates electrical energy.
In stop-and-go city driving, regenerative braking recovers 15–25% of the energy that would otherwise be wasted as heat in brake pads. EVs are more efficient in city driving than on the highway — the opposite of gasoline cars — because every braking event is an energy-recovery event.
The Tesla Roadster went on sale in 2008 at $109,000. Built on a modified Lotus Elise chassis, it ran 0–60 in 3.9 seconds with 245 miles of range. It was the first production car to use lithium-ion batteries, and the first production EV that anyone wanted to own for reasons other than environmental guilt.
The Model S, introduced June 2012 and starting at $57,400, was a full-size luxury sedan competing with the BMW 7-series on every dimension except the engine. Consumer Reports gave it the highest score in the history of their automobile testing program. Motor Trend named it Car of the Year. It was not "good for an electric car." It was good.
| Model | Year | HP | 0–60 |
|---|---|---|---|
| Roadster | 2008 | 248 | 3.9 s |
| Model S P85 | 2013 | 416 | 4.2 s |
| Model S P85D (dual motor) | 2014 | 691 | 3.1 s |
| Model S P100D | 2016 | 762 | 2.5 s |
| Model S Plaid | 2021 | 1,020 | 1.99 s |
The Model S Plaid's 1.99-second 0–60 time made it, at the time of its introduction, the fastest-accelerating production car ever built by any manufacturer at any price. It cost $129,990. A Bugatti Chiron — 1,500 HP, $3 million — does 0–60 in 2.3 seconds.
Beginning with the Model 3 (2017), Tesla shifted the primary traction motor from AC induction to interior permanent magnet (IPM) design. An IPM motor has permanent magnets embedded in the rotor, creating constant magnetic flux without rotor current — no I²R losses in the rotor. At steady state, the rotor simply follows the rotating stator field. Efficiency: 95–97%, compared to 90–94% for an equivalent induction motor.
The trade-off: permanent magnets create drag when the motor is unpowered. Tesla's solution in dual-motor cars is clean — AC induction on the front axle, IPM on the rear. At highway cruising, the front motor is disconnected and freewheels. It has no permanent magnets fighting to align. The rear IPM carries the full load efficiently. When acceleration is demanded, the front motor re-engages in milliseconds.
The circle closes here.
Nikola Tesla invented the AC induction motor because the brushed DC motor was inadequate: inefficient, maintenance-intensive, spark-prone, and unable to run on AC without a commutator. His insight — the rotating magnetic field produced by phase-shifted currents in orthogonal coils — made it possible to build a motor with no brushes, no commutator, no wearing contact between stationary and rotating parts.
A century later, a company named for him used that motor as the centerpiece of a product that did not merely improve on what existed — it made what existed look obsolete. The AC induction motor, powered by DC batteries through a solid-state inverter that synthesizes AC at variable frequency, produces full torque from zero RPM, requires no transmission, enables regenerative braking, and delivers 90% of its stored energy to the wheels.
Every electric car currently manufactured by every automotive company in the world uses either an AC induction motor or a permanent magnet AC motor. The DC motor, which dominated electric vehicle design for a century, is gone from the drivetrain of any serious electric vehicle. Tesla the man killed it once with the power grid. Tesla the company killed it again in the car.
He didn't live to see it. The taxi carried him home from dinner at Delmonico's for the last time in 1937, when his broken ribs had not healed and he was too frail to walk far. He was 81. He died six years later in Room 3327.
The motors are spinning.
This demonstrates exactly what the Tesla car's inverter does, at bench scale. A VFD synthesizes three-phase AC from a single-phase supply, varies frequency, and drives an induction motor at variable speed with full torque available from 0 RPM.
s = (Ns − Nr)/Ns.The VFD synthesizes AC at whatever frequency you command. Motor speed follows frequency. Torque is available at any frequency including near-zero. Load regulation is automatic via slip. This is the Tesla Model S powertrain at 1/2000th the power — same physics, same control architecture, same motor topology.