EV Transmissions “in Layman’s Terms”

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I got this question last night.  “…can you explain to me, in layman’s terms, why gearboxes on electric motors are useless”

Well, I wouldn’t say useless, but it’s the kind of question that gets you distilling all the crap in your head down to the essential bullet points.  Here’s my stab at it.

  • A gas motor’s power curve is a very abrupt bell, where the peak is relatively narrow, ranging between only a few thousand RPM. The purpose of a transmission is to utilize that powerband over the much broader RPM range of the drive wheel.  An electric motor has no such narrow power band. The power increases throughout the entire RPM range. There’s no practical RPM range that is optimal that the transmission would need to distribute.
  • It’s true that a transmission can help deliver torque from a given RPM range to the drive wheels, such as with a small motor trying to both pull from a stop, and also pull at near-maximum vehicle speed. So in that case the transmission helps reduce the load on the little motor at low speeds, and increases it’s effectiveness at high speeds.  Thus, it seems a 2-speed, or at most a 3-speed transmission would be optimal.*
  • However, the same thing can be accomplished by increasing the size (power capacity and delivery) of the motor. A bigger motor delivers more torque at low speeds and can withstand higher loads due to more thermal mass than the small motor.  So you have two choices. Use a small motor with a transmission to keep it from blowing up, or use a bigger motor with no transmission. The bigger motor solution is more efficient because there’s no parasitic loss.

… some of my best work, that.

(Torque/Power curve via EV Drives)

*Interesting side note.  Every effort you make in increasing a gas motor’s power output effectively raises the level of the highest peak in the power band.  It also narrows it.  So you push it higher, but you make the usable RPM range smaller.  This is precisely why, as, for example, as they squeak more power out of in-line 4 cylinder motors, you see the transmissions get more gears.  You have a narrower usable range, you need more options for putting that band where you need it.  Again, six gears for an electric motor?  Total overkill, because it simply doesn’t have that narrow a power band.  Or any power band, for that matter…

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4 responses to “EV Transmissions “in Layman’s Terms”

  1. I’m not sure I follow about the bigger motor being more efficient. Yes, the transmission will have some parasitic drag on the drivetrain. But what about the big motor sucking down more power to provide that torque? Would you say it’s because the transmission drag is constant whereas the the extra power requirement of the big motor is only during times of high torque demand?

    • You’re confusing efficiency with power consumption. A transmission looses what, 20% in parasitic loss? A bigger motor does not have parasitic loss, so whatever you feed it, it delivers about 95% of – whether there’s a tranny or not. If it’s “sucking down” 10kw, and giving you 9.5kw it’s more efficient than a smaller motor using 10kw and only giving you 8kw. Does that make sense?

      • I think I see what you’re saying, but I am still not sure- is it true that an electric motor is so efficient across its entire RPM/load range? A stalled electric motor has 0% efficiency. When does it transition to 95%? Does a large electric motor starting a vehicle without a transmission achieve 95% efficiency as soon as it starts to rotate, or does it have, say 75% efficiency at 100RPM and full load as it starts?

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