Tesla Roadster and motor evolution

Tesla's motors are a compact history of EV engineering: each generation fixed the constraint that limited the one before. The constraints are always the same four: torque per unit of rotor volume, how fast the rotor can spin, how much heat can be removed, and how efficiently the inverter switches.

2008 Roadster: proving the idea

The first Roadster was built on a Lotus Elise-derived chassis with a rear-mounted three-phase AC induction motor of about 185 kW. Its 53 kWh pack held 6,831 18650 cells, the laptop-battery format, which gave 0-60 mph in 3.7-3.9 s and 244 miles of EPA range. A two-speed gearbox was planned and then abandoned for a single fixed reduction: an electric motor makes full torque from standstill and keeps making power over a far wider speed range than a piston engine, so a second gear added mass and failure points for little gain.

Why induction? The rotor is a stack of steel laminations with conducting bars shorted by end rings, and no magnets. The stator's field rotates slightly faster than the rotor (the slip), induces currents in the bars, and those currents make torque. No magnets means no rare-earth supply risk and no demagnetization at high temperature. It also means no field when the inverter is off, so an unpowered induction motor freewheels with almost no drag and cannot generate a dangerous voltage after a fault at high speed.

Model S (2012): induction, scaled up

The Model S kept the recipe and scaled it: a liquid-cooled induction motor with copper rotor bars, which have lower resistance than the usual cast aluminium and so make less rotor heat, fed by an 85 kWh pack of 7,104 18650 cells in 16 modules. Dual-motor versions arrived in 2014 with a second induction motor at the front.

Model 3 and Model Y: magnets arrive

The Model 3 (deliveries from July 2017) introduced a rear IPM-SynRM: permanent magnets buried in V-shaped pockets inside the rotor laminations, plus air-filled flux barriers that make the rotor magnetically uneven. Torque comes from the magnets pulling on the stator field and from reluctance torque, the steel trying to align its easy flux path with that field. At highway cruise the motor runs at light load, where an induction machine spends current magnetizing its rotor and heating its bars; magnets provide that field for free, so range improves. Dual-motor cars kept an induction motor at the front, because a part-time motor that freewheels without drag suits induction. The Model 3 also moved its inverter to silicon carbide MOSFETs, which lose far less than silicon IGBTs, especially at partial load.

Plaid (2021): speed is power

Mechanical power is torque times angular speed, P = T × ω. Torque is set by rotor size and current, so the cheapest way to get more power from the same size is to spin faster. The Model S Plaid uses three motors (one front, two rear) with carbon-fiber-sleeved permanent-magnet rotors that run to about 20,000 rpm, for 1,020 hp. The sleeve matters because hoop stress grows with the square of speed: at 20,000 rpm the surface of a rotor 140 mm across moves at about 147 m/s. Carbon fiber is far stronger for its weight than steel, non-magnetic and much less conductive than a metal sleeve, so it holds the rotor together without adding much eddy-current loss.

Cybertruck (2023): the electrical system moves

The Cybertruck changed the architecture around the motors: an 800 V-class high-voltage system, which halves the current for the same power; a 48 V low-voltage system instead of 12 V, a quarter of the current for the same power and so a lighter harness; and steer-by-wire, with no mechanical link from the steering wheel to the rack.

The second-generation Roadster: claims, not specifications

Tesla unveiled a second-generation Roadster in November 2017 and announced these figures: 0-60 mph in 1.9 s, a 200 kWh battery, 620 miles of range, a top speed above 250 mph and three motors, later joined by a “SpaceX package” of cold-gas thrusters. Every one of them is an announced claim, not a shipped specification. The car was promised for 2020 and has been delayed repeatedly, so check Tesla's site for its current status before quoting any of these numbers. The claims are still worth testing with physics. Reaching 60 mph in 1.9 s averages about 1.44 g, at or beyond the grip limit of road tires, and the thrusters were pitched as a way past that limit. At 160-200 Wh/kg for a whole pack, 200 kWh would weigh roughly 1,000-1,250 kg, heavy for a roadster.

Beyond Tesla: where motors are going

• Hairpin windings: rectangular copper bars bent like hairpins, pushed into the slots and welded, fill around 70 % of the slot versus roughly 40-50 % for round wire, cutting copper loss and conducting heat out better.
• Oil cooling: oil sprayed on the end windings and pumped through the rotor shaft removes heat where it is made, which raises continuous power.
• Axial-flux motors: disc-shaped rotors and stators, short and torque-dense; YASA, bought by Mercedes-Benz in 2021, is the best-known maker.
• Rare-earth-free magnets: Tesla said at its 2023 Investor Day that its next permanent-magnet motor would use no rare-earth elements, while BMW's fifth-generation eDrive uses an electrically excited rotor instead of magnets. Treat unshipped designs as claims.
• Integrated drive units: motor, inverter and reducer in one housing sharing one coolant loop.
• 800 V with SiC: Hyundai's E-GMP cars (Ioniq 5, 2021) use it to charge faster through thinner cables.

Step through the timeline from the 2008 Roadster to today. The second-generation Roadster sits above the line as a dashed diamond because it is a set of announced claims, not a shipped car.

Then compare machine types. Hold the speed at 15,000 rpm and switch motors: the induction curve has already lost power, while the carbon-sleeved PM is still at its peak.