AC vs Series DC efficiency - DIY Electric Car Forums

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#1
03-17-2011, 02:00 AM
 YohnnyLuksh Junior Member Join Date: Jan 2011 Location: Latvia, Riga Posts: 18
AC vs Series DC efficiency

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).

#2
03-17-2011, 03:16 AM
 steven4601 Senior Member Join Date: Nov 2010 Location: the Netherlands, Overijssel Posts: 510
Re: AC vs Series DC efficiency

Various parameters come to play.

First is resistance. I^2R is very dominant.
Second is field angle vs rpm. Efficient torque production through a wide RPM range is difficult.

And somewhere the last issue is friction. The air resistance of a vehicle is far great than brush or bearing friction.

Hope this helps.

If you are technically and financially ready for it, go AC.
(Not being negative about DC but its not as 'green' as AC )
#3
03-17-2011, 05:14 AM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 7,336
Re: AC vs Series DC efficiency

Quote:
 Originally Posted by YohnnyLuksh 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?
Hi Yohn,

Generally speaking for the size and power range of EV motors, I'd give AC about 2 or 3% advantage over DC. And then maybe a percent or so is given back in the AC controller versus the DC counterpart.

Towards the higher power end of the EV range, AC drives will run higher voltage systems like 300V batteries. There are some AC drives available now in the lower to medium power range rated for about 100V batteries.

That's the way I see it

major

#4
03-17-2011, 05:24 AM
 Yabert Senior Member Join Date: Feb 2010 Location: Québec, Canada Posts: 1,484
Re: AC vs Series DC efficiency

Seach for motor performance curve. You can see than the K9 motor seem have 84% peak efficiency and Impulse seem have 86% peak efficiency. This peak efficiency is only reachable in a tight range of the motor rpm.

Another really good point for DC motor is the high efficiency of the controller at 98-99% (sometime near 99.5%) compare to 90-95% efficency for AC controller.

Exemple of motor / controller efficiency:

DC: 86% x 0.99 = 85.1%
AC: 90% x 0.95 = 85.5%
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#5
03-17-2011, 05:55 AM
 Tesseract Senior Member Join Date: Sep 2008 Location: Tampa, FL USA Posts: 3,199
Re: AC vs Series DC efficiency

Quote:
 Originally Posted by steven4601 Various parameters come to play. First is resistance. I^2R is very dominant.
Yes, and that is true for either AC or DC motors. The problem with a lot of AC vs. DC comparisons is that DC motor efficiency is often given for intermittent duty loads while AC motor efficiency is often given for continuous duty loads. Unless you can get efficiency ratings for both motors at the so-called "S2-60" duty then I wouldn't read too much into them; you aren't comparing apples to apples.

Quote:
 Originally Posted by steven4601 Second is field angle vs rpm. Efficient torque production through a wide RPM range is difficult.
Not really. If you amend that statement to say "Efficient torque production at low RPMs is difficult" then I'll agree with you. This applies to either type of motor, btw.

Quote:
 Originally Posted by steven4601 And somewhere the last issue is friction. The air resistance of a vehicle is far great than brush or bearing friction.
For larger motors, sure, windage and friction are trivial causes of inefficiency. Of course, the rotor of the ACIM can be made much "smoother" than the rotor (armature) in a DC motor, so that gives it an advantage at higher RPMs (e.g. - above 6000).

Quote:
 Originally Posted by steven4601 If you are technically and financially ready for it, go AC. (Not being negative about DC but its not as 'green' as AC )
Statements like this, however, infuriate me. How do you figure that AC is "more green" than DC? That's not even a scientifically quantifiable statement! It costs a lot more to make an AC inverter and they use silicon much less efficiently (at best: ~70%) than a DC motor controller (~100%) so they are less "green" right from the start. And even if the AC controller + motor combo is more efficient, how long will it take to pay back the difference in price between the two systems? None of the AC crowd ever thinks about this, they just hide behind the "known fact" that AC motors (only) are more efficient and they can do regen so they must be superior.

Ok, maybe, but please run some numbers first. In fact I'll do it for you. Let's assume that you have two motor + controller systems, DC and AC, that deliver the same average power of 20kW with the same average efficiency (AC motors *can* be more efficient than DC motors. but DC controllers are always more efficient than AC inverters for the same power rating). Let's be extra generous and say that the AC system can recapture an average 25% of the energy with Regenerative Braking (5-10% is more typical).

In other words, the AC system uses 15kW while the DC system uses 20kW to do equivalent amounts of work. Wow... 5kW sounds like a lot. Well, at an average price of \$0.10/kWh here in the US that means we save \$0.50 in electricity per hour of operation.

How much does the AC system cost vs. the DC system? How long will it take to pay that back? Here's a representative of each:

Curtis 1238 AC system

There are numerous combinations of DC controller + motor, but ones of equivalent size to the above would cost around \$2000 (e.g. - Alltrax 7245 controller + D&D ES-31B motor).

A price difference of \$2500 divided by \$0.50 means you have to drive for more than 5k hours (an estimated 250k miles) to recover the difference in cost between the two systems.

That Regenerative Braking is a real benefit when driving in hilly areas is a good reason to go with AC, but claiming it is so much more efficient it will save you money or that it is more "green" is total nonsense.
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#6
03-17-2011, 06:21 AM
 steven4601 Senior Member Join Date: Nov 2010 Location: the Netherlands, Overijssel Posts: 510
Re: AC vs Series DC efficiency

Quote:
 Originally Posted by Tesseract Statements like this, however, infuriate me. How do you figure that AC is "more green" than DC?
I knew I was pissing into the wind with posting something like that
Green is a very vague statement, I wouldn't take that too serious.

Maybe I was in an unscientific way trying to promote the use of AC.

Where did you get the figure (at best: ~70%) from? Wild statements like that....

Last edited by steven4601; 03-17-2011 at 06:31 AM.
#7
03-17-2011, 07:56 AM
 etischer Senior Member Join Date: Jun 2008 Location: San Francisco bay area Posts: 997
Re: AC vs Series DC efficiency

Quote:
 Originally Posted by Tesseract In other words, the AC system uses 15kW while the DC system uses 20kW to do equivalent amounts of work. Wow... 5kW sounds like a lot. Well, at an average price of \$0.10/kWh here in the US that means we save \$0.50 in electricity per hour of operation. A price difference of \$2500 divided by \$0.50 means you have to drive for more than 5k hours (an estimated 250k miles) to recover the difference in cost between the two systems.
Adding an additional 25% battery capacity to my car would cost me \$3000 bucks and add 175 pounds to my car. Not to mention the space needed to make them fit!
#8
03-17-2011, 08:05 AM
 YohnnyLuksh Junior Member Join Date: Jan 2011 Location: Latvia, Riga Posts: 18
Re: AC vs Series DC efficiency

Quote:
 Where did you get the figure (at best: ~70%) from? Wild statements like that....
This told one expirienced EV converter, but he couldn't say exactly where are so big losses are generating, so I wanted to know some thoughts / facts from you guys, in this forum.

Quote:
 That regenerative braking is a real benefit when driving in hilly areas is a good reason to go with AC, but claiming it is so much more efficient it will save you money or that it is more "green" is total nonsense.
Yeah, regen is one good extra for AC systems, but as you regularry fill little amount in batteries, does'nt this shorten their lives (LiFePO4)? (use more cycles doing so?) Also regen seems to be difficult to set for everyday use, because of need for coasting. So you must adjust regen somehow with brake pedal, etc.?

So maybe we can do conslusion: AC vs DC systems have almoust the same efficiency, if not counting regen, and higher voltage usage possibility?
Interesting, that DC system in motor actually looks like AC system (because of brushes alternating field)? So only real difference is where this AC is generated - in controller (inverter) or in commutator!

Ehh, I want to build controller myself, and it seems that go for DC and OpenRevolt will be best to do. This AC thing is very inviting, but I am afraid that I could stuck there with controller / inverter build.
#9
03-17-2011, 08:08 AM
 Tesseract Senior Member Join Date: Sep 2008 Location: Tampa, FL USA Posts: 3,199
Re: AC vs Series DC efficiency

Quote:
 Originally Posted by steven4601 Where did you get the figure (at best: ~70%) from? Wild statements like that....
Because the switches in the inverter must be modulated at all times - they can't ever go to 100% on. The highest average modulation depth is 70.7%. There are some exceptions to that (shifting the neutral point around, etc.) but, in general, you have to turn all of the switches off in an inverter for some meaningful percentage of time in order to create sine wave currents in the motor. No way around that, and not so wild after all, eh?

Quote:
 Originally Posted by etischer Adding an additional 25% battery capacity to my car would cost me \$3000 bucks and add 175 pounds to my car. Not to mention the space needed to make them fit!
This is assuming that having equivalent range for the two technologies is the main criterion for comparison - a different, and altogether much more difficult argument to make as it depends on terrain, driving style and the ratio of time spent accelerating, braking and at a constant speed.

And, anyway, 25% return from regen is exceptionally generous in the first place and depends on a most favorable confluence of the above factors. Anything less than 10% variation in range is probably unnoticeable by the average driver unless they drive a very precise route every day and are running right at the edge of capacity.

Like I said in my original response, you can make a rational case for AC over DC, but not by invoking economics or "greeness".
__________________
Chief Electron Herder for Evnetics, LLC.

Into EV's? Check out ChargedEVs Magazine
#10
03-17-2011, 08:47 AM
 Qer Senior Member Join Date: May 2008 Location: Sweden Posts: 1,760
Re: AC vs Series DC efficiency

Quote:
 Originally Posted by YohnnyLuksh and higher voltage usage possibility?
On which side?

Motor? Depends on the motor. Yes, most DC motors run below 200 Volt, but so does some AC-motors. The voltage is also pretty irrelevant since what will determine the performance of the car is how much torque and power you get. Torque is proportional to the current, but that is PER MOTOR! If you take two different types of motors and run them at the same current you might very well get totally different torque. Power is power, power in equals power out + losses. So a higher possible motor voltage is just that; higher possible motor voltage.

Battery? All controllers smart enough to have a setting for it can limit motor voltage to a suitable max motor voltage no matter what battery voltage your pack has (ok, it can't increase the voltage, just decrease). You can run for example a Soliton at 300+ Volt pack and only allow up to 170 Volt over the motor. The controller also convert power to power so by having a high pack voltage means your pack doesn't have to handle full motor current since Umotor*Imotor=Ubattery*Ibattery. If Umotor<Ubattery it means that Ibattery<Imotor.

I've said it before and I say it again. You must compare complete systems, you can't just compare AC versus DC and ignore everything else. Before you put your system in a vehicle with a suitable pack it's just a very theoretical discussion without much bearing in real life.
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Quote:
 Originally Posted by toddshotrods I like the Soliton's hi-tech build and ability to deliver whopping doses of current until someone screams "Uncle!"

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