1 - 3 of 3 Posts

#### trucker21r

·
##### Registered
Joined
·
2 Posts
Discussion Starter · ·
I’ve googled searched and haven’t been able to answer my own questions.

Does an electric motor at 2k rpms use the same amount of watts? Volts? Amps? as a motor running at 14k rpm?

Or...

Does a Tesla cruising at 2k rpm have further range than cruising at 14k rpm?

It’s my understanding that Tesla has a 1 gear system and the electric motor turns 14k rpm, give or take. I seen Porsche has come out with a 2 speed.

What I’m trying to answer for myself, if I mate an electric motor (a Tesla motor for example) to a manual transmission and only turn the electric motor, say 2k rpm @ 65-75 mph cruising speed, will I have greater range than a 1 gear with the motor turning 14k rpm?

Thank you for reading this. I appreciate any input.

#### remy_martian

·
##### Registered
Joined
·
3,098 Posts
You have to look at the efficiency curves of the motor if you're going to nitpick range.

For your purposes, the power used is the same at a given vehicle speed. Power needed from the motor equals power lost due to rolling resistance of tires, losses in the powertrain, and aero drag.

What a transmission does is trade torque for speed where the ratio of the two can be considered constant. So a 1:2 low gear gives you half vehicle speed, double torque. At that constant speed from the previous paragraph, assuming gear losses are the same in the transmission, motor power is the same, more or less. Watts, or volts x amps...same. Low or high gear, same...assuming the flat torque curves motors have (to a point....).

The Porsche's (actually Rimac's) powertrain will improve acceleration in the lower gear, pay the time price of changing gears, then accelerate more slowly (lower torque) to a higher top speed than low gear gave.

We're assuming for simplicity here (there are some very pedantic people on here) that torque remains constant throughout the speed range to keep your understanding simple.

Multispeed means more weight, more complexity, higher losses. In the end, a single speed does the job in most road uses, and is more efficient for transferring power, which is why almost everybody uses it.

My little Bolt shreds the tires (at times when I don't want it to...on dry pavement, pulling into traffic, for example), and will bust any speed limit in the US. Why would I pine for it to have a two-speed? A change of 20 degrees outside makes more difference in range than any transmission choice ever would.

There's no place in the USA (used to be Montana) where a two speed electric, or laser-based headlights are needed. On a track, maybe a different story...notably to facilitate a pissing match on the Nurburg-ring by Mr Ego and a challenger.

#### remy_martian

·
##### Registered
Joined
·
3,098 Posts
Note that the Tesla turns 14k at 155MPH and more like 6k or 7k on the highway. Just like the Bolt does. Just like the Leaf does...

1 - 3 of 3 Posts