# Most efficient RPM



## rankhornjp (Nov 26, 2007)

I tried to search for this answer but the word "rpm" is too short to be searched.


SO, Is there a rpm range that is most efficient for most DC motors? I know it will vary a little from car to car, but I'm trying to plan out my gearing and need to know an educated WAG of a range.

Looking to run 144v, cruising speed of 45MPH(95% stop and go traffic), 20-25mile range, probably a 9in motor, single speed gear box (reason I asked the question).


Thanks


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## major (Apr 4, 2008)

rankhornjp said:


> SO, Is there a rpm range that is most efficient for most DC motors? I know it will vary a little from car to car, but I'm trying to plan out my gearing and need to know an educated WAG of a range.
> 
> Looking to run 144v, cruising speed of 45MPH(95% stop and go traffic), 20-25mile range, probably a 9in motor, single speed gear box (reason I asked the question).


Purely from motor efficiency, there will likely be only a few percent difference over a broad range, say 1500 to 4000 RPM. Now the motor may self-cool better towards the higher end of that range. I'd shoot for maybe 3000 RPM at 45 mph and if you then have prolonged travel at lower speeds or hill climbing at lower speed, down shift to keep the RPM up to keep motor current lower and motor cooling sufficient.


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## lowcrawler (Jun 27, 2011)

You can get graphs for many of the motors that show effficency.

A popular 9 inch motor is the Netgain Warp 9 and you can see it's information on this graph:


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## major (Apr 4, 2008)

lowcrawler said:


> You can get graphs for many of the motors that show effficency.
> 
> A popular 9 inch motor is the Netgain Warp 9 and you can see it's information on this graph:


Yes, but this graph shows the efficiency over a range of load with a fixed voltage applied, 72V in this case. I assumed that rankhornjp was inquiring at what RPM to run the motor when there is the means to adjust the motor voltage via the controller and also vary the load with the gear selection. However if you could get similar motor curves for 36V, 96V, 120V and 144V, you would not find significant changes in motor efficiency over the load range of interest.

Also, one shouldn't be overly concerned about the drop off in efficiency to the far left on the curve. This represents loads approaching zero. And at zero load, efficiency is zero. And even though efficiency may be lower at really low loads, the losses are also low. But for prolonged operation at a very low load, you could increase motor efficiency by up-shifting which would lower motor RPM and increase motor current.

To maximize the electric drive train efficiency where you have a motor speed controller and a shifting transmission, you should have a battery current motor. For the driving conditions (grade and speed and load), the gear selection which yields the lowest battery current is the most efficient.

I maintain that it is also wise to have a motor current meter so you do not inadvertently overload the motor by a gear selection. And a motor tachometer so you don't overspeed the motor.


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## rankhornjp (Nov 26, 2007)

major said:


> Purely from motor efficiency, there will likely be only a few percent difference over a broad range, say 1500 to 4000 RPM. Now the motor may self-cool better towards the higher end of that range. I'd shoot for maybe 3000 RPM at 45 mph and if you then have prolonged travel at lower speeds or hill climbing at lower speed, down shift to keep the RPM up to keep motor current lower and motor cooling sufficient.


I'm shooting for a single speed gear box (make it easier for the wife). Looking at the graph, is 5000rpm the highest speed to turn that motor safely? If so, I was thinking about gearing for 55MPH @ 4800rpm which would be an overall ratio of 7.84:1. Then you would have:

46MPH @ 4000RPM
40MPH @ 3500RPM
34MPH @ 3000RPM
28MPH @ 2500RPM
23MPH @ 2000RPM

Most of my driving will be between 30-45MPH


Thoughts?


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## dladd (Jun 1, 2011)

according to Netgain, you should design your car so that their motor will be operated around 2500 to 3500 rpm's, and keep extended use below 4000 rpm or so. This is based largely on cooling, I believe.


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## Ziggythewiz (May 16, 2010)

When you say most of your driving is at 30-45, do you mean most of it is at ~37.5 or is most of it at ~45?

All my driving is 30-45 (my limit in 2nd), but I spend 90% of that time at 45. You really want to optimize it where you'll be most of the time, especially considering optimizing around 30 means your top possible would be low 50's.


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## rankhornjp (Nov 26, 2007)

dladd said:


> according to Netgain, you should design your car so that their motor will be operated around 2500 to 3500 rpm's, and keep extended use below 4000 rpm or so. This is based largely on cooling, I believe.


 Thanks for that information. I will make adjustments.


Ziggythewiz said:


> When you say most of your driving is at 30-45, do you mean most of it is at ~37.5 or is most of it at ~45?
> 
> All my driving is 30-45 (my limit in 2nd), but I spend 90% of that time at 45. You really want to optimize it where you'll be most of the time, especially considering optimizing around 30 means your top possible would be low 50's.


That's a hard question. From home to work is about 6 miles one way, I usually stay around 45Mph for that trip. However, I do some driving around town (lunch, daycare, errands) that are 6-10miles per day and the speed limit in town is 35MPH, so I probably average 25-30MPH on those trips.


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## Ziggythewiz (May 16, 2010)

Might you ever want to go above 60? (The answer is always yes)


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## dladd (Jun 1, 2011)

you know, in my car third gear can be used from 0-55ish mph. This works fine around town for my wife who doesn't like to shift (she claims she doesn't know how... I don't believe her, but that's another story  ).

But when I drive it I use 2nd,3rd and 4th gear. 

You can design around a single gear, but you may want to consider also keeping the trans for those times you want a little more at the bottom or top end.


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