Motor efficiency Vs Amps used

AC, DC, amps, volts and kilowatt. It's all discussed in here
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Motor efficiency Vs Amps used

Post by EV2Go » Fri, 20 Feb 2009, 20:25

After playing around with the idea of twin motors I started looking at the power draw of the optional motors. I have often seen this great debate about why AC is better than DC based on efficiency. Typical AC is 5-10% more efficient than DC, but my question is does efficiency really matter a rats bum?

I put together a bit of a comparison picture comparing the smaller DC motors vs the larger DC motors (forgot memory stick with jpg on it today) and although the larger motors don’t have as high as efficiency as some of the smaller motors they seem to draw far less amps.

Which makes me wonder if all things are equal (all NetGain motors tested at a constant 72v) and one motor uses less amps than another does this mean it will travel further on the same battery pack even though it makes more power?

Surely I am missing something from the equation...

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Post by Johny » Fri, 20 Feb 2009, 20:48

No, I don't think you missed anything major. The efficiency of DC motors appears to very a lot.
This EV calculator always confused me because the range changed when you changed only the motor.
http://www.evconvert.com/tools/evcalc/#
Then I started to read about brush construction and brush timing...
There is also the issue that DC motors are flogged in EVs. Smaller motors would spend more time in saturation - wasting power.
This is all based on reading - anyone have proof?


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Post by Electrocycle » Fri, 20 Feb 2009, 21:10

there are a lot of factors, but resistive loss is where electrical power is converted to heat.

Heat generation is I²R (current in amps squared, times resistance.

So, for any given motor resistance (larger motors are usually lower) the power wasted as heat rises with the square of the current draw.

Therefore you're better off running the motor at higher voltage and rpm, getting the same kilowatts out as you would at lower rpm and more current (but wasting more power as heat and running the motor hotter)
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Post by Rattrap » Fri, 20 Feb 2009, 23:21

Electrocycle wrote:Therefore you're better off running the motor at higher voltage and rpm, getting the same kilowatts out as you would at lower rpm and more current (but wasting more power as heat and running the motor hotter)


so is this where AC systems get their 5 -10% greater effency due to their higher operating voltages?

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Post by woody » Sat, 21 Feb 2009, 00:01

I don't think that makes a huge difference. Brushes maybe a small bit too.

I think a lot of the efficiency comes from the controller avoiding inefficiencies like magnetic oversaturation by controlling the voltage directly.

I.E. the AC motor is near peak efficiency across most of its operating range, whereas the DC generates a lot of heat when you flog it.

You can probably set up your AC controller to flog your motor too (Danfoss torque boost?)

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Post by EV2Go » Sat, 21 Feb 2009, 01:03

Ok I don't want to start an efficiency debate because it is totally irrelevant to my point... I re did my graph

I have two main comparisons here, one at a given RPM and the other at given torque outputs

First looking at the Impulse it uses 449 Amps to produce 70 ft lbs, the Warp 11 uses 290 Amps, and the Warp 13 uses 250. Now they are all running at 72 volts but the Warp 13 produces the same torque at nearly half the Amps of the Impulse 9 even though the Warp 13 has bad efficiency (albeit at different revs).

End of the day if I use less Amps to produce the same torque at the same voltage, do I go further on less power being consumed? This seems to be too good to be true and we all know what they say about that.

The Warp 11 makes 135 ft lb for the same Amps that the Impulse 9 makes 70ft lbs. Although the bigger motor doesn’t rev as hard because it makes more torque I can get away with taller diff gears. But if 70 ft lbs is enough torque to get it moving it can be done at more economical Amps.

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Post by antiscab » Sat, 21 Feb 2009, 05:30

Hi EV2GO,

when doing efficiency calculations, always compare power mech and power elec, no power elec and torque.

you will notice that although the the bigger motors take fewer amps to produce the same torque, they also need a higher voltage.
the result, at a fixed voltage (72v) the rpm for the same torque is lower.
torque * rpm = power

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Post by antiscab » Sat, 21 Feb 2009, 05:37

from an efficiency perspective, the 72v graphs that warfield electric make for their motors arent particularly useful, since they arent pushing their motors anywhere near their potential.

because at the tested values, the brush timing on their motors (this is more true for their bigger motors than their smaller ones) is over-advanced, their efficiency actually improves with increased amps, before declining.

this is one of the reasons i prefer the adc motors, because they give you performance curves for the voltage at which you intend to run the motor at (though still nowhere near enough current).

the optimal brush timing advance is a function on motor amps and motor rpm (more of either means greater adavance, optimal advance at 0 rpm is 0 degrees)
the degree of mis-advance the commutator will take before arcing is dependant upon the motor voltage (higher voltage means less tolerance).

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Motor efficiency Vs Amps used

Post by coulomb » Sat, 21 Feb 2009, 17:53

EV2Go wrote:First looking at the Impulse it uses 449 Amps to produce 70 ft lbs, the Warp 11 uses 290 Amps, and the Warp 13 uses 250. Now they are all running at 72 volts but the Warp 13 produces the same torque at nearly half the Amps of the Impulse 9 even though the Warp 13 has bad efficiency (albeit at different revs).

End of the day if I use less Amps to produce the same torque at the same voltage, do I go further on less power being consumed? This seems to be too good to be true and we all know what they say about that.
As has been pointed out, although you get the same torque at lower amps, you are getting less power. Think of it as a sort of reduction gear on the larger motors; the larger the motor, the larger the reduction. So on the larger motors, you will need an overall (gearbox+diff or just diff) smaller reduction ("taller" ratio) to get the same speed range.

So while you get the same torque at lower amps with the bigger motors at the motor shaft, at the wheels you won't be getting the same torque at less amps at the wheel. So figure out what overall ratio you need to be comparable in terms of speed range (this may be subjective; try to figure out what you would like to use). Then you need to compare equal power points, because of the strange way that DC motors change torque with speed.

70 units of torque on the small motor at 2754 units of speed equals 193,000 units of power (not watts, but it doesn't matter what the units are for this exercise). On the larger motor, we need to find a torque speed product about the same, but at the maximum speed it's only about 156,000. Interestingly, at 72v, the larger motor can't produce as much power as the small one.

So let's try again at 40 ft-lbs of torque; the power is 40 x 3370 = 135,000, drawing 319A. The larger motor at 1385 rpm develops 100 ft-lb of torque; that's 100 * 1385 = 140,000 units of power, at almost the same current (320A). So as expected, the larger motor is a little more efficient. Note at this power point, the larger motor is at 1385/3370x100% = 41% of the speed, so the extra torque of the motor (320 ft-lbs) has to be multiplied by 0.41 (you need a 0.41:1 overdrive to get the same speed as the smaller motor) so that's 100 * .41 = 41 ft-lbs, compared with 40 for the smaller motor.

So that's why you have to compare at equal power points; torque at the motor is meaningless until you know the overall ratio; it's torque at the wheels that you want. Torque can always be traded for speed by adjusting the overall ratio. So it's the product of speed and torque that is important for a given power from the battery; of course, speed multiplied by torque is mechanical power.

Edit: just typos
Last edited by coulomb on Sat, 21 Feb 2009, 19:50, edited 1 time in total.

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Post by Electrocycle » Sat, 21 Feb 2009, 17:57

yeah, you need to forget about looking at torque on its own (same as with IC engines!)

The important part is the power, so even though a larger motor will make more torque for the same amps, it'll be doing lower rpm, and possibly making less power than a smaller motor, depending on its efficiency.

What you need to do is work out the gearing you can realistically use, then the voltage you'll run it at, then you can work out which motor(s) will cover the necessary rpm range. That'll narrow the options down enough that you can then pick the motor based on efficiency (maximum power out for the power you put in), price, size, power, etc.

It's no use running a small motor if you can't get a short enough gear ratio to let it run at decent rpm, and there's no point running a massive motor if it's much heavier and you won't be able to supply enough current to use its capabilities.

In the table above, at the 70lbs-ft point:

     Power in   Power out   Efficiency
9"   32.3kw     27.4kw      85%
11" 21kw       18.3kw      87%
13" 18kw       15.1kw      84%


So, at 70lbs-ft torque (95Nm) on 72v the 9" is making the most power because it's doing the highest rpm.
The 11" is marginally more efficient, but it needs more voltage to make use of its power. At 72v the rpm is just too low to be useful.
The 13" motor is more of the same.

So, if you can get a decent gear ratio, the 9" motor will work well - but if you have to use a taller ratio the larger motors will make the necessary power at a lower rpm at the expense of extra weight.
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Post by EV2Go » Sat, 21 Feb 2009, 20:36

Thanks guys for the input, most of that went over my head and I will need to read it several times for some of it to sink in.

I think what coulomb said made the most sense and what Andrew added kind of added to it.

Not being all that electrically savy I need the examples broken down beyond basic to grasp the concept, but I think I now get it.

Problem is this only further adds to the confusion...

I took Kearon’s Capri for a drive during the week and too be honest a single Impulse 9 just won’t cut it by a very long shot (already discussed my finding with Kearon).

Unless I can get a “holy sh*t” ride it just isn’t worth doing. I don’t mind throwing a bit more cash at the job to get it done properly, but I don’t want to spend supercar money to get Hyundai performance.

I now doubt that even a single Warp 11 is going to come even close.

I have for about the last 20 years wanted to make a RWD Mini, but I fear by the time I have enough motor power I would never get enough batteries in.

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Post by Electrocycle » Sun, 22 Feb 2009, 00:47

yes, this is what I've kept trying to explain to people!
There was a rather heated discussion a while back that resulted in someone getting annoyed and leaving the forum...

For anyone who has driven high performance ICE cars, getting comparable or better performance out of an electric car is an extremely expensive exercise, and the end result is a car with much less range than you'd like, if you drive it hard.

With high performance ICE cars you'll usually drop your range to 2-300km by fanging it, but with an EV it's going to drop to sub 50km with similar treatment, which is not enough to go for a decent drive in a sports car!

I'm willing to bet that the Tesla won't deliver much range either when driven by a hoon :P
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Post by antiscab » Sun, 22 Feb 2009, 03:02

i think the Top gear guys still got the Tesla to do 100km at circuit racing speeds.

EV2Go - if you want decent power, buy a decent conrtoller.
If you want a h*ly Sh*t ride, get hold of a zilla 2k ehv.
thats the only controller available that can push a warp 11 to its full potentonial (250kw mech, with 170v and 2000A applied to the motor).

then spend $25k on either a collossal TS battery pack (and get awesome range, 150km+) or spend the same amount on a much lighter higher rate and lower weight battery, such as k2 cells, and get less range.

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Post by Kearon » Sun, 22 Feb 2009, 06:24

Hi Matt, A 1k or 2k Zilla is a bit of a given, I realized a little while ago that a Curtis just wasn't going to cut it. I also realize drawing the sort of amps it is going to need is going to reduce the capacity fairly dramatically.

Problems is as Andrew pointed out I don't know if I am really going to come close to satisfying my needs with current technology. I would like to think it could be done but I am starting to have serious doubts.

Edit: Should have mentioned I am at Kearon's place so it's not Kearon posting as you probably guessed.
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Post by Electrocycle » Sun, 22 Feb 2009, 08:03

don't underestimate how hard it is to impress someone who's driven seriously fast cars :P

This is why I built my bike around a purpose, and didn't try to make it fast. Having ridden bikes with 120kw or so for a while, it'd be a bit hard to compete.
Even harder is competing with cars making ~400kw!
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Post by antiscab » Sun, 22 Feb 2009, 10:51

oh 400kw is possible,
that needs a warp 13 with a zilla 2k ehv (510kw with 2000A and 340v applied to the motor, with twin peaks, the second at twice the rpm at as the first due to the dual windings)
of course the battery pack needed gets into the $40k range.

yes high performance EVs cost more than the same in petrol, until you attempt to use the petrol one *every* day.
if you use the petrol one *every* day, then the cost over say 5 years can break even.

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Post by Electrocycle » Sun, 22 Feb 2009, 22:03

yeah, we're usually talking about cars that are used fairly occasionally, but hard.

Even though you can make 400kw, you can't do it for particularly long - which is why electric can be great for drag racing, but not so good for sustained high power use.
How much cooling would the warp 13 need when it has 2000A going through it for any length of time?

The main point I'm trying to make I guess is that with the currently available technology, an EV is a very viable replacement for a "general use" sort of car, and can be done on a fairly reasonable budget.
That's really the EV "niche" at the moment, which is conveniently a very large one!
At the other end of the scale are things like my bike, where the cost is very low, but it's limited to a certain short range, but potentially very frequent use.

From what I've seen so far, attempts to build high performance EVs will be disappointing to the owner, unless they have a lot of cash to spend.
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Post by antiscab » Mon, 23 Feb 2009, 18:12

for a warp 13, 2000A isnt all that far above its one hour rating.
a kostov 11" could also do more than 400kw, but it couldnt do it for more than a 20 sec

but yes i agree, performance does take some big $$.

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Post by EV2Go » Mon, 23 Feb 2009, 20:04

This is the sort of performance I would like but think it may be a little too much to ask out of an EV...

Iain’s (guy taking video) reaction after he looks up at the dyno graph at the end of the run still cracks me up.

http://www.youtube.com/watch?v=hU8G0aVk3vw

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Post by Electrocycle » Tue, 24 Feb 2009, 00:29

ha! That's me with the fire extinguisher :P

I'd forgotten about that :)
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Post by EV2Go » Tue, 24 Feb 2009, 00:34

Been meaning to put it up on Youtube for a while now. Good for a laugh Image

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Post by Richo » Tue, 24 Feb 2009, 02:26

was there a dyno plot you could post up?
So the short answer is NO but the long answer is YES.
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Post by EV2Go » Tue, 24 Feb 2009, 20:09

Here you go...

It was running lean according to their dyno 02 sensor so they shut it down. My wideband 02 sensor in the car said it was fine but I am more inclined to believe theirs.

I replaced the injectors and fuel pump a few weeks later but never got it back on the dyno due to a careless neighbour early one morning soon after on the way to work.

Needless to say it went a crap load harder after I put a lot more boost into it and took it all the way to the 7500rpm redline. Max reading I saw on the Haltech software was 29.3psi

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