Tesla has upgraded the front drive unit in its new 2019 Tesla Model S and Model X to a permanent magnet motor like in Model 3. According to an inside source, Electrek is reporting the new permanent magnet motor picks up 4% efficiency compared to the old induction motor (ref).

Normally, at a constant highway cruise speed, one would expect a one-to-one improvement in range with motor efficiency. In other words, a 4% improvement in motor efficiency would yield a 4% increase in range. How has Tesla turned a 4% improvement in efficiency into a 10% improvement in range?

It's not a simple one-to-one payoff. It’s a bit more complicated.

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tesla motor efficiency slides

Here’s a detailed comparison of EPA range with the new permanent magnet (PM) motor versus the old induction motor (IM). Note that while combined range and MPGe improved around 10%, city MPGe improved a whopping 14%, though highway range only increased 5%. This all makes sense since highway driving is closer to a constant speed and one would expect the payoff on efficiency to be close to one-to-one.  The 4% improvement in motor efficiency improved highway range by 5%.

tesla motor efficiency slides

Why is the city improvement so much higher than highway?

There are two reasons. First, as one would expect, regeneration efficiency is most pronounced in the city driving and has a compounding effect. A 4% improvement in regeneration efficiency increases range by 8%: a two-to-one payoff.

Second, a single point efficiency comparison of motor efficiency is misleading. If the whole motor map is compared, there is a compounding effect because the PM motor has larger efficiency islands.

tesla motor efficiency slides

The induction motor has a lower efficiency AND smaller efficiency islands.

tesla motor efficiency slides

Does it all add up?

Does the improved motor efficiency and improved regeneration efficiency during braking explain the whole range improvement?

It’s close, however, other improvements are at play, such as inverter efficiency and gearbox efficiency. According to ARS Technica (ref):

“Along with the new motor design, Tesla says the latest Model S design has silicon carbide power electronics, and improved lubrication, cooling, bearings, and gear design.”

So it’s not just a more efficient motor, but instead, the whole power unit is more efficient: motor, inverter, and gearbox.

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