# Most efficient use of battery - DC vs AC



## pengyou (Nov 21, 2012)

I have read a number of posts on this forum about the advantages of a DC vs. AC motor. They all talked about the performance, function, etc but I never read anything that discussed efficient use of battery energy. Given a DC and AC motor of approximately the "same size", each motor having the same size battery pack, which motor will be able to get further on the battery pack? I would think that converting DC from a battery to AC to run an AC motor would be inefficient.


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## major (Apr 4, 2008)

pengyou said:


> I have read a number of posts on this forum about the advantages of a DC vs. AC motor. They all talked about the performance, function, etc but I never read anything that discussed efficient use of battery energy. Given a DC and AC motor of approximately the "same size", each motor having the same size battery pack, which motor will be able to get further on the battery pack? I would think that converting DC from a battery to AC to run an AC motor would be inefficient.





> I would think that converting DC from a battery to AC to run an AC motor would be inefficient.


It is a necessary evil which happens either way. The DC motor system does it with the commutator and brushes; the AC motor system does it with the inverter. The system efficiency from the battery is similar. I would give a slight advantage to the AC systems of a couple percent.


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## pengyou (Nov 21, 2012)

major said:


> It is a necessary evil which happens either way. The DC motor system does it with the commutator and brushes; the AC motor system does it with the inverter. The system efficiency from the battery is similar. I would give a slight advantage to the AC systems of a couple percent.


But what about a brushless system?


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## major (Apr 4, 2008)

pengyou said:


> But what about a brushless system?


Brushless is AC. Maybe not sinewave AC, but AC nonetheless. Brushless systems need a DC to AC inverter (often called controller). DC motors use a DC to DC converter (also called a controller).


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## pengyou (Nov 21, 2012)

major said:


> Brushless is AC. Maybe not sinewave AC, but AC nonetheless. Brushless systems need a DC to AC inverter (often called controller). DC motors use a DC to DC converter (also called a controller).


Wow...the plot is getting thicker than the gravy! So a brushless DC motor is actually an AC motor that uses a controller to convert dc to ac?


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## major (Apr 4, 2008)

pengyou said:


> Wow...the plot is getting thicker than the gravy! So a brushless DC motor is actually an AC motor that uses a controller to convert dc to ac?


Yes. Brushless DC is often called BLDC.



> There is nothing that is DC in BLDC motors: these are truly AC motors, so we should really call them BLAC instead.


From: http://liionbms.com/php/motor_drivers.php#AC


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## dougingraham (Jul 26, 2011)

I would give a slight efficiency nod to the AC systems as well. But if you do lots of driving in stop and go traffic conditions or in hilly terrain an AC system can give you additional range due to regenerative energy recovery. This is generally not practical on the DC motors systems but is readily available on AC systems. A range increase of up to 25% would be possible in some rare cases but more typically you get 10% and some people almost nothing. Note that if all your miles are at constant speed on nearly level roads you will gain almost nothing from regen.

This is one of those cases where your mileage really might vary greatly.


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## midaztouch (Mar 28, 2011)

What is the highest efficiency that you've seen from a motor?


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## aeroscott (Jan 5, 2008)

the switched reluctance motor/ controller has the advantage of a square wave (10 times less on/off switching losses then sine wave) . New work is looking into oscillating the amperage( no explanation given on how) with the switching times ( switching at low amp part of the cycle ) . So power losses will be reduced even more . To restate, voltage would be high but amps would low resulting in less power loss. 
Switching power on and off is the source most of the power loss in ac or dc controllers.
SRM (type of ac motor) has cooler running armature according to LTI corp. 
98% claimed combined eff. would depend on size of motor ( bigg
er motors more eff.)


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## dougingraham (Jul 26, 2011)

midaztouch said:


> What is the highest efficiency that you've seen from a motor?


The Swiss company Maxon makes a basket motor that operates at a max of 94% efficiency. It is a brushed DC motor that can do about 10 watts max so of not much value in powering an EV, unless it is very small.

The question needs to be constrained somewhat in order to get a meaningful answer. The answer will be very different depending on the application. A given motor will vary all over the place as to its efficiency depending on load. For example a motor might be 90% efficient at a power level sufficient to run the car down the highway at 60 mph. But that same motor might be only 60% efficient when pushed to 1000 amps during acceleration away from a stoplight. 

The commercial motors we use seem to be in the mid to upper 80% efficiency range at the power levels we use for highway cruising. A few percent difference is not all that important for normal driving since bad driving techniques can easily make a 20% difference in range.

The methods for increasing motor efficiency are known but generally too expensive to do. An example of this would be replacing the copper in your motor with Silver. This would give an immediate increase in efficiency which would be greatest at high currents. This is because Silver has a lower resistance than Copper. It also costs quite a lot more. Yesterdays copper price was $3.30 per lb. Yesterdays silver price was $20.15 per oz or $322.40 per lb. I don't know how much copper is in something like a Netgain WarP9 motor but I could easily imagine 30 lbs. So price of copper in that motor would be $99. Using Silver instead would change that to $9672. That is a lot of additional cost for a 5% decrease in resistance. And it would make little difference at the highway speed example. It would make the most difference when pulling extreme currents like in a drag race where currents of thousands of amps are common for the whole race.

I hope that helps. I know it doesn't answer your question.


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## major (Apr 4, 2008)

midaztouch said:


> What is the highest efficiency that you've seen from a motor?


Highest efficiency that I've seen? You realize that motor efficiency is very difficult to actually see? You need instruments for the voltage and current input and for the RPM and, the hard part (read expensive), the shaft torque, all on a dynamometer. So for me? And I've done more motor testing than most people; I think about 90% 

Since you're interested, I suggest you type _motor efficiency map_ into google and then click on images. You'll see hundreds of fine examples including such as the Prius motor. Study that for awhile


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## midaztouch (Mar 28, 2011)

major said:


> Highest efficiency that I've seen? You realize that motor efficiency is very difficult to actually see? You need instruments for the voltage and current input and for the RPM and, the hard part (read expensive), the shaft torque, all on a dynamometer. So for me? And I've done more motor testing than most people; I think about 90%
> 
> Since you're interested, I suggest you type _motor efficiency map_ into google and then click on images. You'll see hundreds of fine examples including such as the Prius motor. Study that for awhile


@Major

I'm glad you are part of this forum! Thank you and I've check those maps before. 


Even though this might seem a little strange, let me through some numbers at you. You've seen a lot of motor charts. What would you think, if a motor gave you numbers like this? 

*72v = over 9000 rpms, 130amps max, 47 lbs of torque and 15hp continuous...motor is 44lbs
*

*correction 150 amps max*.

Keep it Clean and Green
Midaz


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## Karter2 (Nov 17, 2011)

midaztouch said:


> What would you think, if a motor gave you numbers like this?
> 
> *72v = over 9000 rpms, 130amps max, 47 lbs of torque and 15hp continuous...motor is 44lbs*


 I would think you have a number error somewhere !


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## midaztouch (Mar 28, 2011)

Karter2 said:


> I would think you have a number error somewhere !


Karter2


*correction 150 amps max*.

Keep it Clean and Green
Midaz


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## Duncan (Dec 8, 2008)

midaztouch said:


> Karter2
> 
> 
> Assume the numbers as correct.
> ...


The numbers can't be correct
72v x 130 amps MAX = 9360 watts = 12.48Hp - MAX

Which is less than 15 Hp continuous


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## midaztouch (Mar 28, 2011)

Duncan said:


> The numbers can't be correct
> 72v x 130 amps MAX = 9360 watts = 12.48Hp - MAX
> 
> Which is less than 15 Hp continuous


Average 15hp. What concerns me is the speed of the motor, 9000 rpm!

*correction 150 amps max*.


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## major (Apr 4, 2008)

midaztouch said:


> Average 15hp. What concerns me is the speed of the motor, 9000 rpm!


How can average power output be greater than the maximum power input  And what's wrong with 9000 RPM? I've run AC motors well above that. They make high speed motors, and low speed motors, and all types


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## midaztouch (Mar 28, 2011)

major said:


> How can average power output be greater than the maximum power input  And what's wrong with 9000 RPM? I've run AC motors well above that. They make high speed motors, and low speed motors, and all types


The figures are not solid. The university started some tests but the team leader cancelled the testing program. Im not a student and They wouldn't give me the charts. It's seems impossible to get a bench dyno in Japan for an average person. You have to be an OEM or part of a university program. I must put my motor in my bike then have a dyno ...but first I need batteries.

My motor is a brushed DC motor that I handmade. That's why I was surprised the rpm was high and I was told the motor was cool during the tests. I compared it to other motors that are in the same class as mine.
1) The rpms stand out for me from the uni test.
2) From my test, the motor ran for 43 mins on a 12v @36 ah lead acid battery.

* correction= 150 amps max* 

It seems a little unusual from my knowledge....


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## Karter2 (Nov 17, 2011)

Your numbers still do not make sense.
And even if the numbers were correct, a "max" current, does not equate to a "continuous" motor output !
If you hand made this motor, surely you knew what speed you designed it to run at ?


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## Duncan (Dec 8, 2008)

Hi Midas

_It's seems impossible to get a bench dyno in Japan for an average person. You have to be an OEM or part of a university program.

_If you are serious about doing some motor testing make yourself a bench dyno - its not difficult
You need

Accurate speed measurements - optical tacho
Accurate torque measurements - strain gauge and a level - calibrate using weights

Some method of applying a load_ - _belt on an old flywheel, another motor, best would be a water brake


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## midaztouch (Mar 28, 2011)

Karter2 said:


> Your numbers still do not make sense.
> And even if the numbers were correct, a "max" current, does not equate to a "continuous" motor output !
> If you hand made this motor, surely you knew what speed you designed it to run at ?


The students that started the testing were new to the program. It looked like they did their best and knew what they were doing. When I got home, I check some numbers and I thought the test was faulty also. 

I sent my guy an email asking if the numbers were correct. His reply was, he thinks that what they did was correct.... That's why I posted here. Just wanted some expert opinions and more information.

Well, if the numbers are good or bad, I just want to put my handmade motor in my bike! I need an EV grin!

Here a pic of my motor. What do you think of my winding technique?

Keep it Clean and Green
Midaz


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## gunnarhs (Apr 24, 2012)

midaztouch said:


> The students that started the testing were new to the program. It looked like they did their best and knew what they were doing. When I got home, I check some numbers and I thought the test was faulty also.
> 
> I sent my guy an email asking if the numbers were correct. His reply was, he thinks that what they did was correct.... That's why I posted here. Just wanted some expert opinions and more information.
> 
> Well, if the numbers are good or bad, I just want to put my handmade motor in my bike! I need an EV grin!


Ok, the number would need some explanation but giving us these specs this could be true in a different context.
I love students measuring with a (often badly configured dyno) and understanding roughly the data 
(have got them measuring the dyno efficiency instead of the motors)
But being a student once I feel obligated to defend them as much ass possible 
So let us try to make the data look reasonable (yes this is a important part of a research program)

1) Total (continuous) Power of Motor is 15 Hp (15*745W = 11 KW)

2) During continuous power at base speed(?) 72V and 150A are measured
At this point your motor is not at 9000 rpm for sure. 
72V is most likely nominal Voltage
P = U*I = 72*150 = 10,8 kW

3) Maximal (?) torque is 47 foot-lbs (61 Nm) . 
Assuming maximal torque is just below base speed (during nominal continuous operation, (72V(?) and going from 11 KW) 
using P = T*2*pi*Motorfrequency -> 
MotorFrequency = (11.000 / (2*3,14* 61) = 29 Hz ( 1740 RPM)
So base speed would be near 1800 rpm

4) I have doubts about the 9000 rpm (150 Hz) if 3) is right. 
Would assume more like 5-6000 rpm max at least for 72V.
When applying load you will not exceed 6000 rpm for sure
But it is probably possible to over-voltage with no load to achieve a pointless max RPM (dangerous for DC-series though)



midaztouch said:


> Here a pic of my motor. What do you think of my winding technique?
> Midaz


Looks nice!!


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## Duncan (Dec 8, 2008)

_Ok, the number would need some explanation but giving us these specs this could be true in a different context.
I love students measuring with a (often badly configured dyno) and understanding roughly the data 
(have got them measuring the dyno efficiency instead of the motors)
But being a student once I feel obligated to defend them as much ass possible 

__(have got them measuring the dyno efficiency instead of the motors)_

This I don't understand - you measure torque and rpm with a dyno - 
there is no "Dyno efficiency"


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## gunnarhs (Apr 24, 2012)

Duncan said:


> This I don't understand - you measure torque and rpm with a dyno -
> there is no "Dyno efficiency"


What kind of dynos have you been working with?
(I have OEM experience in this by the way)

This thread discusses battery/motor efficiency not the whole vehicle.

Why would you use a dyno to measure motor-rpm?????
You usually do that with the attached sensors...

Motor torque can be measured with a dyno but
usually you are measuring the output Power in general 
(which is P = Torque*RPM by the way) and the total efficiency of the electric system.

Meaning on one side you are putting in energy from Battery/controller which drives the motor, the motor is then fixed to a generator (PM-machine usually) which generates electric power and this output power is measured.
(you can also use a mechanical output system but problems there are higher friction losses which you have to calculate to 
you use that usually when you measure the complete system not the motor only. Meaning with a vehicle you let it drive on a belt which drives a generator or is driven by motor depending on tests)

So the efficiency of the electric system is Power(dyno-out)/Power(Battery).
Normally you know the exact efficiency of the dyno at a speed -interval so
Motor/controller-Efficiency = Total Electric Efficiency/DynoEfficiency.
Then you can finegrain that with further tests giving you either motor or controller efficiency.

The problem with using a generator is that the generator efficiency is only reliable in a certain spectrum. 
So when changing rapidly motor speeds, you need to know the total dynamic efficiency spectrum of the generetaor (not easy) or you have to apply a dynamic gear-ratio which keeps the generator in its reliable speed limit.

The case I was referring to was that the student had the motor (rated) efficiency data and calculated the generators efficiency (which was known). The point was measuring the motors efficiency during different speed.


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## Duncan (Dec 8, 2008)

gunnarhs said:


> What kind of dynos have you been working with?
> (I have OEM experience in this by the way)
> 
> This thread discusses battery/motor efficiency not the whole vehicle.
> ...



I was in charge of engine test at Cummins DEP in the UK for 7 years, 
I developed the drive systems used on the dynos for another five Cummins plants

A "Dynamometer" measures rpm and torque - both fairly simple things to measure
Talking about "dyno efficiency" is like talking about "multimeter efficiency"

Using a motor like a dynamo and then "estimating" the power is a really half assed way of doing things

It would be relatively easy to mount your "generator" on a set of bearings and add a reaction rod to *measure* (not guess) torque 

When you are measuring a complete vehicle - the biggest issue is normally tire distortion
We used a four meter diameter roller in the basement so that the vehicle tire was driving on a nearly flat surface

I have used; water brakes, friction brakes, eddy current dynos, ac motor dynos with power levels up to 4000Hp

All of them have mounted the "brake" on coaxial bearings and used a reaction rod to measure torque 

Most dynos used in industry are water brakes - you could measure the power absorbed by measuring water flow and temperature - but you don't

The AC motor dynos (I installed them at Cummins CMEP in Indiana) could have been used by measuring the power output (which we used to run the rest of the plant)
But that would have been monstrously inaccurate and simply so much more difficult to do than measuring torque as usual

Mount your motor on bearings - then you not only get a simple accurate measure of torque but it is dead easy to use another reaction rod and some weights to calibrate it

I have used inertial "dynos" for small engine work - these are the only dynos that do not use a reaction bar to measure torque
The problem with these is that by their very nature you can't do constant speed testing - very useful for some things but not much cop if your customer is actually using the power to do something

AC motor dynos were twice the price of eddy current dynos and we had to jump through hoops with the power company - and those were only 300Kw units!


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## gunnarhs (Apr 24, 2012)

Duncan said:


> I was in charge of engine test at Cummins DEP in the UK for 7 years,
> "I developed the drive systems used on the dynos for another five Cummins plants"


Ok I was working two years at MAN (fuel cell bus project) and 4 years (halftime) at German Army. My project in both case was throwing out old measurement systems and installing new and connecting them to a centralized software-controlled lab. I have a BS in Electric-engineering and additional degree in Software-Engineering.



> A "Dynamometer" measures rpm and torque - both fairly simple things to measure
> Talking about "dyno efficiency" is like talking about "multimeter efficiency"


Hmm sounds more like old style "Break HP measurement" to me.
You have to apply a dynamic load somehow when making a real-world test. 
One way is to use a mechanical "absorber", the another an electric.
In my case the electric is a generator and when using such you have to know the generators efficiency.
The advantage of the electric is that you can include acceleration scenarios without much additional effort.



> "Using a motor like a dynamo and then "estimating" the power is a really half assed way of doing things"


No you measure the electric power, it is not being estimated. and you measure the rpm exactly. The torque is calculated from the values.
The advantage over the mechanical arrangement is more flexibility when going through a driving cycle with different load conditions.



> "It would be relatively easy to mount your "generator" on a set of bearings and add a reaction rod to *measure* (not guess) torque"


You can do that , but then you would do it directly on the motor-shaft, no use for the generator in this case. I can see how you can to do a constant torque measurement but not a drive situation with variable torque/speed without quite a effort and friction losses.



> "When you are measuring a complete vehicle - the biggest issue is normally tire distortion
> We used a four meter diameter roller in the basement so that the vehicle tire was driving on a nearly flat surface"


And this roller was driven by....? An electric motor I suppose
How did you get torque/speed and efficiency values from this test?



> "All of them have mounted the "brake" on coaxial bearings and used a reaction rod to measure torque"


I see how it can be used to measure constant (break) torque/power but if you try to simulate a driving uphill/downhill-scenario you will have a lot of friction losses which would be hard to measure.



> "Most dynos used in industry are water brakes - you could measure the power absorbed by measuring water flow and temperature - but you don't"


Actually this is in use when measuring water-pump-systems but I have not seen this in automobile industry for quite a long time (20 years or so).
In my case at the army we did not have water-brakes, but did measure the total volume of water pumped during a period of time. 
This was not a test for efficiency-calculation (pump efficiency was not exact) but a fine way to compare the workload our Inverter with the Simovert from Siemens when attached to same motor/pump.



> The AC motor dynos (I installed them at Cummins CMEP in Indiana) could have been used by measuring the power output (which we used to run the rest of the plant)
> But that would have been monstrously inaccurate and simply so much more difficult to do than measuring torque as usual"


The accuracy depends on the type of motor but this is not any problem at all.
You do not have more losses in an accurate configured electric dyno than in a mechanical system. 
If this was a problem, why are most OEM then using electric dynamometers today?



> Mount your motor on bearings - then you not only get a simple accurate measure of torque but it is dead easy to use another reaction rod and some weights to calibrate it


When finding out optimal "starting torque/speed" for a known load situation or finding the breakdown torque this would be better than the electrical test.
But when going through a driving circle which includes acceleration and decceleration, hill-driving and so on the electrical system is more flexible.



> I have used inertial "dynos" for small engine work - these are the only dynos that do not use a reaction bar to measure torque
> The problem with these is that by their very nature you can't do constant speed testing - very useful for some things but not much cop if your customer is actually using the power to do something


I agree, this inertia tests which include driving from start to a certain rpm with some fixed load are useless in most cases.



> AC motor dynos were twice the price of eddy current dynos and we had to jump through hoops with the power company - and those were only 300Kw units!


Yes they are much more expensive but today test-requirements at OEM require transient tests (fuel consumption and emission - criteria) so they have to use them as well for their normal engines as electric.


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## Duncan (Dec 8, 2008)

Well if you know how to do all this

Why are you using some dumb students to do your testing!

In my experience - 30 + years - nobody uses generators as dynos - they do have to use AC motors as brakes when doing transients but if you actually look at those units you will find coaxial bearings and reaction rods - proper dynos!! 

I have been to over thirty engine plants and test centers
NONE of them used anything but proper dynos

_The accuracy depends on the type of motor but this is not any problem at all.
You do not have more losses in an accurate configured electric dyno than in a mechanical system. 

_*Utter cobblers one is measuring something directly - the other is inferring it from a secondary measurement*_

If this was a problem, why are most OEM then using electric dynamomete_rs today?

They are using AC motors as loads in the dynamometers BUT the measurement is from the reaction rod and load cell


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## major (Apr 4, 2008)

Anymore I imagine most places use a torque sensor on the output shaft of the motor being tested. That works steady state or dynamically for any load device. Something like this: http://www.sensorsportal.com/HTML/DIGEST/february_05/Torque_sensor.htm


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## gunnarhs (Apr 24, 2012)

Duncan said:


> Well if you know how to do all this
> Why are you using some dumb students to do your testing!


No one is dumb here, I was just referring to a similar experience like in this thread when looking at the data from the testbench in this thread.
This was not my testing by the way.. I was just helping out.



> In my experience - 30 + years - nobody uses generators as dynos - they do have to use AC motors as brakes when doing transients but if you actually look at those units you will find coaxial bearings and reaction rods - proper dynos!!


You do not seem to have much knowledge about electric motors, they are both motors-loads and generators usually.
Your experience seems to me something like 30 years old. I agree with you that when doing static (break torque/HP tests) it is better to use mechanical brakes than electric. But this is only one part of the test which are done today. Therefore when doing the statical tests you have of course mechanical breaks. But you do not use them (today) for transient tests which is the topic of this thread.


> I have been to over thirty engine plants and test centers
> NONE of them used anything but proper dynos


I do not understand why you then do not classify AC-Motor (loads) as dynamometers. In my case there are no "proper dynos", you just use what gives the best results.
Maybe my English is that bad but when I look up "Dynamometers"
http://en.wikipedia.org/wiki/Dynamometer#Electric_motor.2Fgenerator_dynamometer
Here electric-motors/generators are listed up as Dynamometres



> Utter cobblers one is measuring something directly - the other is inferring it from a secondary measurement
> They are using AC motors as loads in the dynamometers BUT the measurement is from the reaction rod and load cell


Yes the we agree on the static measurement. You still have to explain to me how you do the dynamic measurement (like this what the thread discusses)
with a mechanical apparatus which does not give heavy friction lossess.
What kind of load cell do you use?


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## gunnarhs (Apr 24, 2012)

major said:


> Anymore I imagine most places use a torque sensor on the output shaft of the motor being tested. That works steady state or dynamically for any load device. Something like this: http://www.sensorsportal.com/HTML/DIGEST/february_05/Torque_sensor.htm





gunnarhs said:


> It still remains that during the (dynamic) tests you have to apply different loads dynamically to the motor.
> And I will stay to that electric-motor/generator dynamometer test gives the best results when all is taken into account.
> Until you guys find something better


Ok I made phonecall today to my friends / former coworkers regarding this piezoelectric torque sensors.
They have been using them on testbench since 2004 for "smaller" motors and for not to high rpm-motors.
(I was there only till December 1999 so obviously I am outdated  the automobile industry is moving so fast these days)
Seems like these are the only torque-sensors reliable for dynamic measurements.

For power-consumption and driving- load - cycle procedure is the same as it was, at least in the EV-department.
Measuring Power in from battery and Power out from electric dynamometer.

But these piezo-torque-sensors give more exact values than calculating torque from power, when not accelerating fast/driving very high rpm.
(During high motor rpm, the ratio of the motor/generator coupling has to been changed, it is possible to put the torque sensor then to the "generator" side.)
The acceleration problem still remains


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## Arlo (Dec 27, 2009)

gunnarhs.

Duncan is right. What he is getting at is you can not be accurate enough when measuring the output of a AC motor or generator for the known power out put... 

What we are talking about here is hooking your motor you are Dyno testing... (IE the motor you are putting power into) up to another motor or generator and then using the second motor or generator as a generator as a load. Yes that part works. BUT you need to measure the output in a manner that is accurate and the voltage amperage and rpm output is NOT accurate.
What is accurate is measuring how much twisting force is applied to the Driven motor (generator) and calculating the HP from the twist and the rpm. To do this you mount the driven motor (generator) on some bearings to allow it to pivot then use a strain gauge or something that can measure the twisting force applies to the driven motor (generator) so imagine this. A motor you feed power into spinning a generator and the generator has a torque wrench mounted on it to measure the twisting force because the generator is mounted in a way it can pivot or spin freely the torque wrench is what stops it and can measure the twisting force (torque) you take that number with rpm and you know the HP (or Watts) out of the first motor then you look at HP (or watts) into the first motor and calculate efficiency.


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## gunnarhs (Apr 24, 2012)

Arlo said:


> gunnarhs.
> 
> Duncan is right. What he is getting at is you can not be accurate enough when measuring the output of a AC motor or generator for the known power out put...
> 
> ...


Ok if I understand you correctly you are going through a set of rpm applying a torque-measurement on each rpm value. This is static load/measurement.
It is impossible to use a strain gauge for dynamic torque measurement during acceleration, it does not work (not at least the ones I have tested) .

The situation I am looking at is the dynamic part not the static. And the problem is how to measure (or calculate) the applied torque during acceleration/decceleration with as little error as possible.

The output - measurement is Power = Voltage X Current. This gives a quite good efficiency info compared to the input info but the problem is that (momentan) torque ripples are not detected. So the calculated torque curve becomes smoother than it is.

So when for example simulating a driving cycle for EV including acceleration and regen by applying a AC motor/generator load (assisted by a simple inverter) you can measure the power flowing in both directions.
In the electric system the acceleration generates an increase in current but this gives not a very exact value as you and others have pointed out.

It is not easy to do that with a mechanical setup, but you may prove me wrong if you can show me an apparatus that works for that case without significant friction losses.
I can say that hydraulic and strain load cells do not work under dynamic stress but I have no experience with other load cells like the piezo-cell for example which seems to work under dynamic stress but is (as I have been told) not capable of measuring acceleration.


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## gunnarhs (Apr 24, 2012)

And here is the lovely equipment used for the torque-estimation calculated by power-measurement, they claim +-2% accuracy.
http://www.zes.com/download/products/zes_flyer_drehmoment_e.pdf
The example here does it directly on a motor with or without load.
Unfortunately, I do not have a LMG500 in my garage...


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## Duncan (Dec 8, 2008)

gunnarhs said:


> You do not seem to have much knowledge about electric motors, they are both motors-loads and generators usually.
> Your experience seems to me something like 30 years old. I agree with you that when doing static (break torque/HP tests) it is better to use mechanical brakes than electric. But this is only one part of the test which are done today.
> Therefore when doing the statical tests you have of course mechanical breaks. But you do not use them (today) for transient tests which is the topic of this thread.
> 
> ...


Electric motors can (and do) provide the brake load,
What they don't do is give accurate, repeatable, consistent measures of torque - which can be related back to a metrology standard
And that is where they die - if you can't go back to your universal standards legislators (and customers) get a bit antsy
(Torque and power can be "calculated" but this is clumsy and to subject to other variables like temperature) 

The load cell on the reaction rod has NO (zero) frictional losses and gives effectively instantaneous feedback

The type of brake is irrelevant - If you are doing this type of testing there are a huge number of things to be taken into account from the rotational inertia's of the various parts to the torsional stiffness of all of the couplings

Eddy current brakes are used for almost all testing except the (small number) tests that require you to actually accelerate the engine
An Eddy current brake can change loads in the blink of an eye

My experience is not completely current - I last used an industrial dyno six years ago (2007) - but most of the test cells I designed/modified are still in use
The Darlington Engine Plant is making >300 engines/day
CMEP is over 600 engines/day
CDC is over 500 engines/day

Don't know how many engines are being made at Wuhan (China) - those cells were only interim and should be being replaced soon


Just seen your comment about dynamic testing – that is where you absolutely have to know the rotational inertia's and torsional stiffness’s involved - the actual measured torque is NOT the torque produced by the motor/engine 
Neither is the electrical power absorbed the same as the power produced (except as a total integral)


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## gunnarhs (Apr 24, 2012)

Duncan said:


> Electric motors can (and do) provide the brake load,
> What they don't do is give accurate, repeatable, consistent measures of torque - which can be related back to a metrology standard
> And that is where they die - if you can't go back to your universal standards legislators (and customers) get a bit antsy
> (Torque and power can be "calculated" but this is clumsy and to subject to other variables like temperature)


I would not be so sure about that. I can tell you that for the dynamic test we got more realistic values than the data from the motor/inverter producers. This was verified by our biggest customer which was at this time Siemens. What I am talking of in this case was the actual motor efficiency during the driving cycle. Their data and our data from the elder testbench showed 10-15% better result than was verified later both on the electrical-testbench and then in the fuel-bus itself.
To give a number average efficiency was given 85%. It really was about 75%.
I gave a shit about the customers data, but two of them came actually to try to prove their point, without success.
We had 5 potential customers and from their specs Siemens AC-system was the best and a competitors DC system the worst. It ended up to be vica verse (it did not change who was chosen though but it was a kick in the ass).
So when not looking at point-value but average value electric dynos are not so bad



Duncan said:


> The load cell on the reaction rod has NO (zero) frictional losses and gives effectively instantaneous feedback


I buy that for the static test but not the dynamic case unless you have been using another type of load cell.



> If you are doing this type of testing there are a huge number of things to be taken into account from the rotational inertia's of the various parts to the torsional stiffness of all of the couplings


That is what I would then call dyno-efficiency(?)  
Inertia is of course taken into account, you have to calculate both rotors and the external load (if present)




> Just seen your comment about dynamic testing – that is where you absolutely have to know the rotational inertia's and torsional stiffness’s involved - the actual measured torque is NOT the torque produced by the motor/engine


Yes this is especially the case during acceleration (and I am told the modern piezoelectric torque-sensor does not help with that). 
So you have to calculate/measure the load acceleration additionally.
One method is of course a (separate)inertia test (often useless if with standard load condition)




> Neither is the electrical power absorbed the same as the power produced (except as a total integral)


Here I would disagree but you somewhere corrected it (?) if I understand total integral correctly(?) 
You get different momentan values from the Power product (torque-sensor x angular rotation) than the electrical (average value ) Power = U X I
but over the total test the average power should be the same (and the total energy used losses not taken into account)


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## Arlo (Dec 27, 2009)

gunnarhs said:


> Ok if I understand you correctly you are going through a set of rpm applying a torque-measurement on each rpm value. This is static load/measurement.
> It is impossible to use a strain gauge for dynamic torque measurement during acceleration, it does not work (not at least the ones I have tested) .
> 
> The situation I am looking at is the dynamic part not the static. And the problem is how to measure (or calculate) the applied torque during acceleration/decceleration with as little error as possible.
> ...


No using the electrical out put is not an accurate way of measuring the power. What you are forgetting is all the losses add up for 1 and for 2 the losses are not repeatable. What I mean and what Duncan is trying to tell you the generator will have losses in the bearings, the windings, the eddy currents, the connectors, the rectifier, etc. And even if you think you have the losses figured out they will change drastically with little changes in temperature and with load/current flow. 

What we are suggesting is a load it doesn't matter if its a motor or a water brake or a disc brake as long as what is causing drag (the load) is able to pivot freely and the strain gauge torque wrench or what ever fancy equipment you use is stopping the rotation and measuring how hard it is trying to rotate you can accurately measure the torque and calculate the HP and its repeatable!


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## Arlo (Dec 27, 2009)

gunnarhs said:


> Duncan
> [I said:
> 
> 
> ...


Duncan is right. You have to understand the set up. I will try to explain.
Imagine the generator is on bearings so it can spin in circles as the motor drives it. But then the torque arm is mounted to it and a strain gauge attached to the torque arm. The bearings the generator can pivot on are not spinning but provide no resistance so as the rpm of the generator changes it can still provide drag (a load) to the drive motor without spinning because of the torque arm placed against the strain gauge. 
See picture 3 in this thread. http://endless-sphere.com/forums/viewtopic.php?f=30&t=46210&hilit=dyno


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## gunnarhs (Apr 24, 2012)

Arlo said:


> What we are suggesting is a load it doesn't matter if its a motor or a water brake or a disc brake as long as what is causing drag (the load) is able to pivot freely and the strain gauge torque wrench or what ever fancy equipment you use is stopping the rotation and measuring how hard it is trying to rotate you can accurately measure the torque and calculate the HP and its repeatable!


What I am saying is that under dynamic conditions and especially under acceleration you will have the similar problems with the mechanical parts .
(temperature, friction do also affect them.) 
The other thing is that you will have problems measuring the changing torque conditions in the same testphase as the static load (meaning putting a variable load during a stationary condition) but this is essential when going through a driving cycle.
In a electric system there is no problem to measure the real work power versus the eddy current with a good electric measurement. 
In fact it is no extra effort when you have to measure regen effects considering EV.
You can nothing do about the losses in the bearings though (the same problem when putting a mechanical load or electrical load).


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## Arlo (Dec 27, 2009)

gunnarhs said:


> What I am saying is that under dynamic conditions and especially under acceleration you will have the similar problems with the mechanical parts .
> (temperature, friction do also affect them.)
> The other thing is that you will have problems measuring the changing torque conditions in the same testphase as the static load (meaning putting a variable load during a stationary condition) but this is essential when going through a driving cycle.
> In a electric system there is no problem to measure the real work power versus the eddy current with a good electric measurement.
> ...


 No I am explaining it in laymen's terms you would not use a torque wrench but use a computer measuring the strain gauge and recording it 1000s of times a second so measuring thought a variable rpm/load its no problem and very accurate.

And no temperature and friction will not effect this system. Because its just simply something to cause drag we don't care how well the drag gets rid of its heat or energy we are measuring the twisting force applied to the whole system. No matter what it stays VERY accurate!


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## gunnarhs (Apr 24, 2012)

Arlo said:


> See picture 3 in this thread. http://endleNicss-sphere.com/forums/viewtopic.php?f=30&t=46210&hilit=dyno


Nice setup but how are you measuring the torque at specific RPM without breaking the motor?
(which is the difference between a static and a dynamic test)


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## Arlo (Dec 27, 2009)

gunnarhs said:


> Nice setup but how are you measuring the torque at specific RPM without breaking the motor?


The motor shaft is allowed to spin as we are saying but when you regen from the motor it becomes a generator and this applies a drag or brake but its shaft can still spin the torque arm against the scale measures the torque and you calculate the HP or watts with the torque and rpm which can all be don with computers which is the professional way Duncan is trying to explain to you.


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## gunnarhs (Apr 24, 2012)

Arlo said:


> The motor shaft is allowed to spin as we are saying but when you regen from the motor it becomes a generator and this applies a drag or brake but its shaft can still spin the torque arm against the scale measures the torque and you calculate the HP or watts with the torque and rpm which can all be don with computers which is the professional way Duncan is trying to explain to you.


Ok but the motor shaft is not spinning like it did before if I understand you correctly?
Meaning in my case I would put an additional voltage-command to my motor, resulting the rpm to increase say from 2000 RPM to 3000 RPM. 
I would like to know 
1) How much the torque was increased during this command
2) How much power was wasted
3) How much of the wasted power was effective (real).

The other case I would put an additional electric (or magnetic) resistance in the generator to simulate an additional resistance
I would like to keep the speed constant so I would like to know 1)-3) for this case.

This sounds also professional I hope I know that with my electric calculation I get this values with about +-2% accuracy.
But I do not think that I can get this result from your setup directly?


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## Duncan (Dec 8, 2008)

gunnarhs said:


> I know that with my electric calculation I get this values with about +-2% accuracy.
> But I do not think that I can get this result from your setup directly?


+/-2% would have got me fired!

Most engine development engineers would sell their grannies for 1% improvement,

If you just want a rough number then you can use the electrical output, if you want to "measure" it you need to measure the torque,

Once you have the torque (as Alro says measured several thousand time a second) then you can start to calculate the actual engine torque,
Its not just the inertia's you need the torsional stiffness's as well

All dyno's use this method.

We also had "Free Running" test cells where the engine was run with no load - we could detect a "weak" cylinder on these by measuring the variability in speed over a complete cycle (2 revs) 
We would actually measure over a long period (15sec) and analyze the speed signal 

In these cells we would do sound analysis - surprisingly we could detect a miss set tappet

I visited Toyota's Kentucky Engine Plant - one of the things that made me jealous was they used free running cells and their engines were so damn quiet that any faults were instantly obvious,
My Cummins 6BTAA engines were not that quiet


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## gunnarhs (Apr 24, 2012)

Duncan said:


> +/-2% would have got me fired!
> 
> Most engine development engineers would sell their grannies for 1% improvement,


Ok we are not talking about electric motors any more I suppose  ?
Obviously not when reading further.
Well I have no clue about normal engines.

But the problem with electric motors is usually a huge efficiency drop during a normal driving cycle considering an EV. As gone through in another thread the average efficiency is about 75-80 % compared to 82-90% in the specs. 
So you are not looking for 1% losses here. As I have mentioned before I have got the original data from motors tested with dynamometers.
So in my opinion something has been tested or calculated wrong.

In the link provided by Arlo I see this text:
"Efficiency number are much lower in Drive Calc, than what I calculate based on the reaction torque and motor speed."

The thread ends without finding an adequate explanation for the difference but it is surely more than 2% . 

As I say not the first time I see that. So what could be the explanation?
(I know at least that I am always in the 2% difference region  ?


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## Arlo (Dec 27, 2009)

gunnarhs said:


> Ok we are not talking about electric motors any more I suppose  ?
> Obviously not when reading further.
> Well I have no clue about normal engines.
> 
> ...


The link I provided was just for a picture to help you understand how to do it the right way. The link is a fellow ES member who is just a hobbyist doing the best he can with DIY on a budget. 
As for your other question what Duncan and I are pointing out is using load cells to mesures 1000s of times a second and you can have a computer do all the work including increasing the load to a set amount to watch the motor accelerate in a controlled manner meaning it can control the load as well so the rpm can increase as it wants... IE from 2000-3000 rpm. 
Internal combustion or electric motors this is the best way and the ONLY ACURATE WAY to get the job done right. I have seen this as well at a engineering facility for very advanced electric motors which I can not talk much more about.


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## Duncan (Dec 8, 2008)

Hi gunnarths

I assure you engineers developing electric motors would be willing to sacrifice both grandmothers for a 1% improvement in efficiency

When you are hunting for losses in a system you need the most robust measurement systems you can get,

Something that is +/-2% is OK for hunting the real silly ones but very quickly you will need better resolution

Besides its actually EASIER to do it correctly


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## gunnarhs (Apr 24, 2012)

Arlo said:


> The link I provided was just for a picture to help you understand how to do it the right way. The link is a fellow ES member who is just a hobbyist doing the best he can with DIY on a budget.
> As for your other question what Duncan and I are pointing out is using load cells to mesures 1000s of times a second and you can have a computer do all the work including increasing the load to a set amount to watch the motor accelerate in a controlled manner meaning it can control the load as well so the rpm can increase as it wants... IE from 2000-3000 rpm.
> Internal combustion or electric motors this is the best way and the ONLY ACURATE WAY to get the job done right. I have seen this as well at a engineering facility for very advanced electric motors which I can not talk much more about.


Ok Arlo and Duncan thanks for the patience explaining this torque tests, it took me a while to grasp this with the reaction torque attached to the motor itself (I was wrongly assuming it as an mechanical brake on the rotor).
As mentioned I have no experience with other motors than electric an I have never performed prototype-tests on a newly built electric motor.
The only bench-test I have executed myself are efficiency tests (electric) of already mature electric motors and I have made endurance tests to motors attached to water pumps (workload tests). I have seen the some of the these motors put into a break test (mechanical and electrical (Eddy current), to find values like breakdown-torque etc.
But the efficiency tests were made with professional equipment (probably the best you can get) and gave results which were then verified with later performance test in the vehicle. I can guarantee you that the energy - consumption and the efficiency of the electric system was quite exact as I was able to measure all (electric) input and output channels of the electric system. The same goes for the RPM -measurement.

The one disadvantage was that the Torque-calculation gave smooth torque (which could differ up to -2%) from the actual momentan torque value.
The other that when applying the generator/motor load (electric dyno in my case) you had to be careful driving the generator near its optimal specs.

The motors tested were from Siemens, Lenze and Danfoss and they provided us with detailed data both for motor and Inverter which came from their testbench (I have no doubt that it was a professional one)
In this specs (as in the motors) mechanical losses, copper losses and iron losses where specified. Also inertia of the rotors.

When putting them to our bench we got the same efficiency values in the nominal spectrum but when driving them outside that (lower RPM and higher RPM, for example the difference was up to 15%. They claimend it had to do with the load situation which made a difference but not that much as we were near full load.

When 2 of them (Siemens and Danfoss) came to us and looked at the exact testresult they made their own tests outside the normal bench-procedure. Doing that they got similar data as we did.
In terms of Max-Power / Torque the Siemens motors did outperform their own specs by far. but not in terms of efficiency.

The other providers did not comment much. Lenze claimed its new fieldoriented control was still under development, 
I do not remember who the forth provider was.
Only the DC guys which were quite happy with the results as only their equipment was holding what was promised in terms of efficiency.

Since then I have been some skeptical about this (mechanical) Dynamometer-measurements. Data from OEM regarding fuel usage coming from chassis tests and testdrives is usually 25% "better" than real world test with the same driving cycle which does not increase confidence.

But ofcourse I reconsider if proven wrong. I would today use a piezoelectric torque-measurement device for torque-measurement which would back up the electric torque calculation and compare the logs (this was not available when I was at MAN) , especially during acceleration/load increasing.
This is the device Major was hinting at in this thread.

Would you trust this device to measure real (momentan) torque.
Please let me know your opinion...


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## gunnarhs (Apr 24, 2012)

Duncan said:


> Hi gunnarths
> 
> I assure you engineers developing electric motors would be willing to sacrifice both grandmothers for a 1% improvement in efficiency


For me it is no problem to improve the cycle-efficiency of 1% when using own inverter for control (meaning improving it from 70 to 75% for an induction motor even). This is in my opinion the easiest way to gain efficiency. It is very hard to maintain 80% (only Tesla seems to be doing that today)
Impossible for me to improve max efficiency of 90% of the motor itself unless replacing copper with silver 
In case of a PMSM an improvement of 1% means usually saving of 2% in cooling power (need constant liquid cooling). So 1% is really 3% there



Duncan said:


> When you are hunting for losses in a system you need the most robust measurement systems you can get,
> 
> Something that is +/-2% is OK for hunting the real silly ones but very quickly you will need better resolution
> 
> Besides its actually EASIER to do it correctly


The +/-2 is actally in (momentan) Torque and momentan Power values to be exact (during ripple) , but regarding the total efficiency over complete workload I think was quite spot on. 
But as you say I would have to compare it with a right mechanical setup to prove that 100%.


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## pengyou (Nov 21, 2012)

....good example of a highjacked thread....


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## gunnarhs (Apr 24, 2012)

pengyou said:


> ....good example of a highjacked thread....


I am sorry, I will stop posting...


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## midaztouch (Mar 28, 2011)

gunnarhs said:


> I am sorry, I will stop posting...


Gunnarhs

Keep posting! The conversation between you and Duncan is interesting! It's helping me to learn more! 

Enjoy

Midaz


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## kennybobby (Aug 10, 2012)

*Did you not get an answer on page 1?*



pengyou said:


> ....good example of a highjacked thread....


Did you have more questions? As you said the gravy gets thicker-- but sometimes you can learn quite a bit from the experiences of others as one question leads to another.

Concerning dyno testing of electric motors-- it is much more meaningful to plot torque or current on the x-axis and speed on the y-axis, and not the other way as done with combustion engines. The reason is that for a given voltage the speed of a motor is determined by the mechanical load (torque) it is working against. Increase the load and the speed drops, remove load and the speed will increase. 

A dyno puts a known mechanical torque load on the shaft and you measure the resulting mechanical shaft speed (rpm, and voltage and current from the power supply and the motor controller output). From that data you can know everything about that motor's performance and calculate all the efficiency factors you desire...


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