right electric engine for your EV, and efficiency

AC, DC, amps, volts and kilowatt. It's all discussed in here
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nazar
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right electric engine for your EV, and efficiency

Post by nazar » Thu, 25 Oct 2012, 03:48

woody said this to me in another thread, which sparked me to do research in the area of engines.
woody wrote:
B) Petrol cars change gears at high RPM as the efficiency of the motor is better at lower RPM (also quieter, longer life) electric motors are more efficient at higher speed usually


The below two quotes were from this link http://www1.eere.energy.gov/manufacturi ... 097517.pdf
what i found interesting was that it was important to match the motor to the car. i understand that there are mainly 9 & 11 inch, and some electric motors are built for torque instead of speed.
from the link just above wrote: Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range of 5 to 10 hp; peak efficiency is at 7.5 hp
mathematical equations from the above link (to work out) is on page 3


here is another link i found really interesting about efficiency and motors
http://www.itrc.org/reports/vfd/r06004.pdf
but it is too technical for me to work out.

the question i had originally, is what is the maximum efficiency of an electric engine - in relation to changing gears to go faster.
for example, from the above quotes and links - i would assume that a 10,000 RPM electric engine would use the least amount of battery power at 7,500 RPM.
or have i got the wrong end of the stick here

i would appreciate people's thoughts here - as i would like to get the most out of batteries.
there is an awful lot to learn about EV's :)

Stiive
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right electric engine for your EV, and efficiency

Post by Stiive » Thu, 25 Oct 2012, 14:47

Depends what motor your looking at using.

Most AC types of motors (esp induction) have poor efficiency at light torque loads.
Considering to maintain a certain velocity with a defined drag you need a certain power, if your motor is not sitting above ~40% of its rated torque, decreasing the speed of the motor and increasing the torque to produce the same power (Power=torque.angluar velocity) will allow you to cruise more efficiently

OTOH, when using near full load torque, higher RPMs are better.

IMO a gearbox is beneficial in an AC EV if you know how to use it.
Rgds,
Stiive

nazar
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right electric engine for your EV, and efficiency

Post by nazar » Thu, 25 Oct 2012, 15:14

Stiive wrote: Depends what motor your looking at using.

Most AC types of motors (esp induction) have poor efficiency at light torque loads.
Considering to maintain a certain velocity with a defined drag you need a certain power, if your motor is not sitting above ~40% of its rated torque, decreasing the speed of the motor and increasing the torque to produce the same power (Power=torque.angluar velocity) will allow you to cruise more efficiently

OTOH, when using near full load torque, higher RPMs are better.

IMO a gearbox is beneficial in an AC EV if you know how to use it.
so a heavy car (like a duel cab ute or 4wd) would be better to have a AC motor?? i am assuming the biggest drag/load is the weight of the car
there is an awful lot to learn about EV's :)

Stiive
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right electric engine for your EV, and efficiency

Post by Stiive » Thu, 25 Oct 2012, 15:43

nazar wrote: so a heavy car (like a duel cab ute or 4wd) would be better to have a AC motor?? i am assuming the biggest drag/load is the weight of the car


The biggest load during cruise is aerodynamic drag which a dual cab sucks at.
The weight only contributes to slow acceleration.

Yes a heavy car benefits more from an AC system as you have more kinetic energy to recover, and have wasted more energy accelerating the thing compared to a lighter vehicle.

A heavy un-aerodynamic vehicle such as a dual cab however would benefit less from a transmission as you will be using a significant portion of the motors capability anyway just to keep it at speed (Perhaps 20kw at 100kph, rather than 10-12kw for an aerodynamic car).
Rgds,
Stiive

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