Elon Musk named his car company Tesla in honour of Nikola Tesla, a Serbian-American best known for his contributions to the design of modern alternating current electric systems. Tesla (the man) developed the AC induction motor design that Tesla (the company) used in their first roadster, their current (pun!) Model S and Model X.
It comes as a shock (pun!) then that the Model 3 uses a permanent magnet motor. This is generating (pun – last one I promise!) some controversy.
Induction vs Permanent Magnet
Both these motors consist of a stator (the stationary winding) which surrounds a rotor (the part that spins like a crankshaft). The electricity flowing through the stator acts on the rotor thanks to the magic of electromagnetism. This won’t work if the rotor is electromagnetically inert – there’s no flux to “grab onto”. Induction motors generate their flux using electric power. Permanent magnets generate their flux with the magnets – no additional power required.
Why permanent magnet motors?
By not requiring any power to create their flux, permanent magnet motors provide better peak efficiency. Permanent magnet motors tend to be smaller and lighter than induction motors.
Why induction motors?
The simple answer is that the roadster was already in development when Musk bought the company and it was already being developed with an induction motor.
Induction motors offer good part load efficiency and can be made very large to provide lots of power and torque, since you can up the rotor flux by applying more power. Induction motors tend to be a better choice at 200kW or more.
Induction motors also coast better since you can simply de-power the rotor flux.
So why put a permanent magnet motor in Model 3?
Tesla set out to convince the world that electric cars were not just fast, but brutally fast. Induction motors accomplished this readily in the Models S and X, but at the expense of energy efficiency, resulting in the need for large, heavy and expensive battery packs.
Model 3 is Tesla’s effort to produce a mainstream car. It is still respectably fast and substantially cheaper. Every pound saved reducing the size of the battery pack means less structure elsewhere – lighter frame, lighter suspension, lighter brakes etc.
The Model 3 single motor output is 192kW, just below the 200kW threshold at which induction motors are superior. That’s 257hp equivalent for you gearheads out there – sufficient for a small sedan, particularly when it comes with the “instant on” feeling shared by all electric cars.
Speculation
In a previous article I speculated about when Tesla will develop a four motor car. This would break up the load (Principle 1 – Segmentation) and allow for four smaller and more efficient permanent magnet motors. It would also allow for mixing and matching motor types (Principle 3 – Local Quality) – perhaps induction motors at the back for big power and permanent magnet motors at the front for cruising with high efficiency.
There’s even research being done in hybrid motors that combine the advantages of both induction motors and permanent magnet motors (Principle 5 – Merging). This might be the end game for automotive traction motors in the future.
