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AC vs Series DC efficiency

52K views 82 replies 20 participants last post by  gunnarhs 
#1 ·
Hi,

I know, it is discussed before, but how do you think, how much difference is between AC 3 phase induction motor and DC series motor efficiency?

Example DC series motors: Kostov K9'' 144V / Netgain Impulse9 144V
Example AC motors: Siemens 1PV5135-4WS28 / ABB 3GAA 131 316
These motors have different power ratings, but main question is about efficiency (enery loss).

I've heard that generally DC motors have ~60-70% efficiency, while AC motors have ~85-95%.

But I have doubt about that because:
* DC motor brushes have no such big friction, to give 20% difference?!
* both motors have strator fields, that use electricity.
* assuming that I would use same voltage for AC or DC (~144V).
 
#57 ·
There is also the classic Tesla page, where an engineer explains why Tesla chose induction motors over PM motors:

http://www.teslamotors.com/blog/induction-versus-dc-brushless-motors

It doesn't go into great detail, but it lists eddy current and hysteresis losses as reasons that you'd like to have low field for low power demand situations.

Note that the article was written in early 2007, so some statements like only 3 car models use induction motors are no longer be true.
 
#58 ·
1. Siemens does not make all setups for highest efficiency but also for cost and other factors. Inverters for AC motors are a fairly recent thing and the cost is largely related to their functionality. Properly vector supported controls allow MUCH higher efficiencies than your graph. Not to mention that a proper automotive drive train is a very different story in itself.

2. There were so few car makers with AC systems on roads (all of them relied on knowledge of the gentleman making the AEM motors) either fully or partially, because inverters were non existent and problematic until very recently. It means they were not available.

3. As the industry was trying to kill off the full electric car or let it happen on its own, AC motors, which are basically the best fit, took a back seat to PM motors (rare earths did not appear to be an issue at the time) due to going the hybrid route, often meaning constant RPMs, which in principle allows PM motors some mild advantages. For Full electric, the induction motor, when all factors are included, is the best choice.

4. Lack of their presence was down mainly to inverter issues and the simpler tech (brushed DC, etc) being more readily available and also relatively cheap due to already existing economies of scale. Much like the cost of existing 18650 lithium cells was already low due to economies of scale from consumer electronics field, which lead Tesla and others to use them, even though they are very unpractical. If they had prismatic cells at same cost from get go, they would never touch the cylindricals or would have them in much greater sizes, of 100Ah cells or higher...

Some issues are technological and some economic in nature. Economics play a crucial role. They are after all what keeps you from having 500miles worth of lithium in your cars right now...
 
#61 · (Edited)
I think PMDC motors and ACIMs are fairly similar in efficiency and one or the other may be better or worse under various conditions. Also, you can get motors rated at different efficiencies, especially ACIMs which come in premium efficiency types which may approach 95% in larger sizes as used in EVs. The PM types may be better in terms of power/weight.

But this thread is about AC vs DC series efficiency. Since many DIY conversions use series DC, we should discuss that. I found some performance curves here:
http://www.go-ev.com/motors-warp.html





Notice that these motors peak at about 85% efficiency (only 72% in series!), and only put out about 30 HP and 140 lb-ft torque maximum at 72 volts. I'm sure they can be overdriven to 144V or higher for short periods of acceleration, but as the losses go by the square of current I would imagine these motors are giving less than 50% efficiency under those conditions. If these graphs are typical, then I wonder why anyone would use a series DC motor, unless they are afraid of high voltage or more complex controls. But for efficiency, maximum range, and lower Wh/mile, they don't seem very good. :rolleyes:

However, I found that the Warp 9 has 90-93% efficiency. I don't know why there is such a large difference. Here is a chart for the newer 2010+ Warp 9 (the older models are about 80-88%):

 
#64 ·
It's really not that complicated. PM motors have higher peak efficiency because they run at a higher power factor (power factor is NOT the same as efficiency, but a low power factor slightly reduces efficiency, all other things equal). Well controlled induction motors have a higher average efficiency since they can control field strength independently at any speed.
Ruckus prefers the higher peak efficiency motors, and people who drive electric vehicles prefer the higher average efficiency.
 
#69 ·
A proper automotive AC system (inverter and motor) will trump any other system in efficiency in a full electric vehicle. Across the complete operating range, it will defeat a PM, which has a high nominal peak efficiency, which you will not gain from in the actual operating range.

Again, the reason you saw few of them in automotive industry was the late arrival of proper inverters. Think of early 1990ies of actual vehicular inverters coming to market. It is normal for industries to try to use legacy, in terms of economies of scale. Something on market for some time usually already has some economies of scale. Like cylindrical lithium cells.

So even it has less or no sense, it is viable if it has a starting advantage, due to already being available and relatively affordable.

Even Chinese, which govern rare earths, do not wand them to be the centerfold of electric drive trains.
 
#75 ·
It seems that the synchronous motor was in the 2010 model Nissan Leaf:
http://livingleaf.info/2010/11/nissan-leaf-electric-motor-and-transmission/
http://www.nissanusa.com/ev/media/pdf/specs/FeaturesAndSpecs.pdf

But I think it is a permanent magnet AC motor:
http://www.johnwmorehead.com/nissan-leaf-ev-and-what-is-a-permanent-magnet-ac-motor/

The 2013 Leaf will use inductive charging:
http://www.engadget.com/2011/12/05/nissan-leaf-to-get-inductive-charging-lose-its-stem-in-2013-vi/

Toyota may be shifting from PM to ACIM:
http://www.greencarreports.com/news...-ditch-rare-earth-metals-from-electric-motors

Synchronous motors usually use permanent magnets or a wound rotor supplied with DC through slip rings. There is a reluctance type synchronous motor which is suitable for EV traction use, but it is not self-starting and loses torque if it goes out of sync. Hysteresis type synchronous motors are self-starting but are only for small sub-fractional HP motors such as electric clocks and turntable motors which need to run in sync with the power line frequency. ACIMs have higher torque and other characteristics which make it much better for EV traction use:
http://en.wikipedia.org/wiki/Synchronous_motor
http://www.engineersedge.com/motors/synchronous_motor.htm
http://ecee.colorado.edu/~ecen4517/materials/SynchronousMach.pdf
http://www.allaboutcircuits.com/vol_2/chpt_13/2.html

The only real advantage I can see for a synchronous motor in an EV is that it can easily be used as a generator.
http://www.pdhonline.org/courses/e171/DOE-HDBK-1011-v3.pdf
http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/h1011v3.pdf
http://www.youtube.com/watch?v=pIbSMpHQ9a8
 
#77 ·
Don't think anyone uses wound rotor synchronous motors in EVs. Yes, you can do field weakening, but an induction machine is a better option, and doesn't need brushes. The vast majority of synchronous machines are utility scale generators. They work great for that, since real power output can be manipulated independent of reactive power output.
 
#79 ·
AC motors win out for full electric vehicles. You will not see a leaf or Prius winning a consumption competition until on AC.

The Monaco ECO Rally this year has only AC driven cars in top spots.

http://www.acm.mc/documents/6/EN-24-Classement_Consommation.pdf

In actual real world use, across the range, AC is more efficient. And there are
ways to make them even more efficient but the cost of each percentage point of hugher efficiency is simply not worth it and is better spent on battery tech..
 
#82 ·
Well...

I think we should point out what are the "AC vs Series DC efficiency" (as Title states) differences for OFF-SHELF components and not what OEM are using, due to the fact that OEM have $$$ to spend on R&D and we are buying that available to us as a off-shelf components.

Please correct me if I am wrong... Txs. :)
 
#83 ·
OK, now I want to add some stuff

1) Don't look at the numbers, look at efficiency through the driving circle
(AC induction motors for example have high efficiency only around specific RPM)
2) Don't forget to count in forced cooling. Most Permanent magnet motors only have 90% + efficiency if they are cooled with liquid. Meaning you are loosing up to 10% of your battery energy in the cooling system for the motor.
3) Controllers and motors do not drive continuously with 1000A +. Meaning that when you are reaching 100 kW performance, you have to use higher voltage (most DC equipment does not support high voltages).
4) It is important that you can adapt (with software usually) the control-box /motor combo to your existing drivetrain (torque/speed) limits.
5) Power requirement What driving experience are you looking for?
(drag (brute force), driving in city (high regen), highway driving (constant speed), racing at high speed. )
6) Cost, maybe the most important for DIY.
7) Complexity of the system

I am myself prefer AC-induction but based on experience for DIY the
Series DC is in my opinion the best compromise for DIY under 50 kW (30 kW continuous)
 
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