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Which gear gives lowest amp draw?

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Paul9 View Drop Down
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    Posted: 02 September 2016 at 11:00am
Hi people,

I have received advice from a number of sources recently that electric motors are more efficient at higher rpms than at low rpms. I assume that the term "more efficient" means that motors draw less amps at higher rpms than at low rpms. If I can draw less amps then I can increase my range?

Having driven ICE's for decades, I have always used the highest gear suitable for my speed as it gives lower rpms which means the ICE is working less hard and therefore uses less fuel. The advice I have received re electric motors appears to be the reverse ie. if, at a given speed, I can be in 3rd gear or 4th gear, I should try to be in 3rd? Is this correct?

I do not doubt the validity of the advice I have received I am just trying to understand why this advice is correct? Is there an explanation which a layman such as myself could understand? Have I interpreted the advice I have received correctly?

Thanks in advance,
Paul
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Johny View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 02 September 2016 at 11:03am
Check your battery pack current Paul. If it's lower for a given gear, speed and incline then it's a more efficient speed/gear for the motor.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richo Quote  Post ReplyReply Direct Link To This Post Posted: 02 September 2016 at 12:46pm
Well these is a "sweet spot" for efficiency - and it literally is a SPOT!
And in general it does tend to be at a higher RPM than a petrol car.
There are also other factors petrol cars use lower RPM - Wear life.
With so few parts to wear out on an electric motor higher RPM doesn't affect the wear life as significantly as an ICE.

At lower RPM the controller will be trying to Limit current during each PWM cycle.
So as the RPM is around where the motor back EMF matches the battery voltage the controller will be doing less V/I conversion.
But as the RPM increases the losses will increase in the laminations - assuming it's of that motor type.

At low torque the efficiency band is quite broad.
So the difference between 3rd and 4th wont be significant (2-3% perhaps)
At higher speeds 3rd may not give achieve the required RPM (motor max rpm) anyway so 4th maybe the only option in some cases.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richo Quote  Post ReplyReply Direct Link To This Post Posted: 02 September 2016 at 12:51pm
Originally posted by Paul9 Paul9 wrote:

if, at a given speed, I can be in 3rd gear or 4th gear, I should try to be in 3rd? Is this correct?


In short I'd say yes try 3rd.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 02 September 2016 at 1:20pm
Thanks Johny and Richo,

I assume Richo that the figures, ranging from 77+ to 92+, on the graph you supplied, are percentages? ie 77% to 92% efficiency?

Thanks again,
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 4Springs Quote  Post ReplyReply Direct Link To This Post Posted: 02 September 2016 at 7:43pm
Power = volts x amps.
To run the motor faster requires higher voltage. So if the power stays the same and the volts go up the amps must go down.
So if you need the same power in 3rd or 4th then the motor will be given more volts and less amps in 3rd (faster rpm).

BUT - just because the motor is using less amps doesn't mean it is going to give you more range. In this example you are using the same amount of power in either gear, and so you'll get the same range.

To understand this you need to think of the volts, amps and power at the battery (before the controller) and the volts, amps and power at the motor (after the controller). The power will be the same at the battery and at the motor (ignoring losses). At the battery the voltage does not change (well, not much) - only the amps go up and down.
For instance if your controller gives the motor 75V at 100A then you are using 7.5kW of power. If your battery is 150V then it still has to give up 7.5kW. 7.5kW/150V = 50A at the battery.

Since the battery voltage stays the same, then if you can see the amps drawn at the battery you are pretty much looking at the power consumption. This is what Johny said above. So the amps at the motor does decrease with increasing rpm, but it isn't all that important. More important is amps at the battery, since that is an indication of the power that the motor is using, and thus how efficient it is.
If you see less amps at the battery in 3rd going along the same stretch of road than in 4th then yes, 3rd is more efficient. In practice I find that that this is very hard to tell!

Another thing to consider is that when an air-cooled motor runs faster it gets more air through it. So there will be better cooling at faster RPM.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Adverse Effects Quote  Post ReplyReply Direct Link To This Post Posted: 02 September 2016 at 10:40pm
not to mention the losses are less at higher volts and lower amps
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Post Options Post Options   Thanks (0) Thanks(0)   Quote poprock Quote  Post ReplyReply Direct Link To This Post Posted: 03 September 2016 at 6:56am
And that is how I melted a brush holder. Deceived myself by thinking that lugging the motor around town at low revs was more economical,but it ran much hotter. Now I have the original heater three speed fan for motor cooling
" A little foolishness now and then is relished by the wisest men "
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 03 September 2016 at 7:44am
Thanks to 4Springs, Adverse Affects and poprock!

I think I am getting the hang of it! Though trying to get any concept into my head is like trying to chip away at a brick wall with a spoon!

Cheers
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jonescg Quote  Post ReplyReply Direct Link To This Post Posted: 03 September 2016 at 9:32pm
Yeah, it's tempting to think the battery is the only one suffering, but the motor is the one doing all the work. Power in = power out * efficiency. So When you rip on the throttle of a machine geared really tall, the battery will deliver very modest amps at full voltage. However the controller is delivering to the motor full amps at very low voltage. So while the battery doesn't experience the hit, the motor does. At high speeds both the battery and the motor are suffering.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richo Quote  Post ReplyReply Direct Link To This Post Posted: 05 September 2016 at 12:47pm
Originally posted by Paul9 Paul9 wrote:

the figures, ranging from 77+ to 92+, on the graph are percentages?


Yes they are % of efficiency.
They are from tests of a 2004 Prius hybrid.
http://www.osti.gov/scitech/biblio/890029

Interestingly it is optimised for high efficiency at lower RPM which is where it is used the most.
So NOT a particularly good example for this thread or for series DC motors.
But it does show the efficiency "spot".
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Post Options Post Options   Thanks (0) Thanks(0)   Quote antiscab Quote  Post ReplyReply Direct Link To This Post Posted: 06 September 2016 at 2:28pm
I'd love to put my car on a dyno to map out an efficiency map.
Matt
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