How do I get the most mileage from an electric motor with gears?

Question

Suppose that I have vehicle that has electric motor and a six speed gear box.
If I'm driving at 40 mph, would I get the most distance by:
A) putting the vehicle in the highest gear and keeping the RPM count low, or
B) setting the gear so that the RPM count would be the closest to the motor's maximum efficiency.

Note that for an Internal Combustion Engine (ICE) you would get the most distance by using the highest gear and keeping the RPM count as low as possible. (Option A)

Electric motors are different in that the are most efficient when they are running at 75% - 90% of their maximum RPM count, so I think that the answer should be Option B.

Is that correct? Can anyone explain why one option would be better than the other for an electric motor?

Answer 1

It will vary. Typically in industry, a motor running at a 20 percent load will will certainly draw more current than no load, but a lot less than a motor loaded at 50% or at burnout of 100%, but is also doing more work- but how to measure that in a vehicle? Factors of wind resistance/drag, rolling friction, weather conditions will impact that sometimes severely. Ideally the least "loading" of the motor is best with the 20% loading considered to be the target point for best efficiency versus energy consumption.



Ideally, top efficiency for each gear would be within the range you are considering for option B. This gives you the option of running the motor at optimum at slower vehicle speeds as well. The transmission does also allow for the use of a smaller motor than would be required if you were to drive the wheels with direct connection to the motor. It is a matter of mechanical advantage or torque multiplication when the motor is run through gears. This aids in extending your range considerably as well as maximum current draw occurs when the motor armature is not in motion. a 100 Amp motor can easily draw well over 200 Amps before the armature overcomes it's static iniertia. Even more if the motor has stalled. A transmission gives you torque multiplication.



Best option is actually a CVT that way the motor itself can be set up to run at optimal efficiency from zero to 60 MPH for example.



For example, the lowly starter motor of your car will move the vehicle off the railroad tracks if you bypassed the clutch safety switch. In the process though, you will be drawing 150 to 250 Amps from the battery on average when you do that, but it will not get you very far nor very fast.



However a starter generator for a B-24 will get you moving down the road at about 45 MPH drawing drawing less current even corrected for the 30 volts of the starter generator instead of 12 volts of your car. 150 amps at 12 volts versus 45 amps at 30- this latter is drawing less total power, and doing more work in the process.

Answer 2

We commonly say that an electric motor has full torque at 0 RPM while an internal combustion engine (ICE) must be running at about 3000 RPM to achieve its full torque potential. Having to always come back to the "sweet spot" in RPM for an ICE is the reason that the ICE requires a gearbox. But an automatic transmission can have over 200 moving parts and further takes away from the already low efficiency of an ICE.



It is the engine efficiency that is most significant when trying to get the highest fuel mileage.

"The [Tesla] Roadster's motor efficiency, battery-to-wheel, is 92% on average... For comparison, internal combustion engines have a tank-to-wheel efficiency of about 15%." 1 An electric motor can be 6 times more efficient without gears as an ICE using them.



But each electric motor is different. Speed and torque requirements may dictate some gearing for economic reasons. In these situations there may be no set rule. It will depend upon the characteristics of the motor, load and what is required. It is most economical to not run the motor at all but as efficiency is a measure of useful work out over power in without the job there is no efficiency.



EV conversions frequently leave the transmission in place as an easy way to connect the drive train and get a mechanical reverse. In this case the vehicle may simply be left operating in the top gear which would have a 1:1 ratio or one gear down.

Answer 3

get rid of the 305 and installation a 350--guess the mileage improves and so does the overall performance. you could throw each and all the stuff on the 305 and could no longer strengthen using fact it would not have the capability to pull it.

Answer 4

electric motors have linear torque and are not suited to gearboxes, at low revs they would not have the power to use high gearing