# AC vs DC



## ToyXCAB89 (Oct 6, 2012)

I am trying to decide if I should build an AC or DC system. I understand that AC is more efficient. So I understand that it depends on driving habit etc. If I put an AC system in one car and drive it and put a DC system in the same car with the same 144 volt system. What MIGHT the different be. 40 miles vs 80 miles, 40 vs 100 miles??? 

89 Toyota Xcab truck


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## Duncan (Dec 8, 2008)

ToyXCAB89 said:


> I am trying to decide if I should build an AC or DC system. I understand that AC is more efficient. So I understand that it depends on driving habit etc. If I put an AC system in one car and drive it and put a DC system in the same car with the same 144 volt system. What MIGHT the different be. 40 miles vs 80 miles, 40 vs 100 miles???
> 
> 89 Toyota Xcab truck


There is little or no efficiency difference AC/DC,
However AC will enable you to use re-gen braking

If you are in a hilly area that could give up to 10% advantage in range,
If its not so hilly then it depends on your driving style, 
If you drive like a hooligan than re-gen can give about 10%
If you drive nicely then the advantage is much less - 3%??

So you are talking 40 miles v 44 miles

AC is definitely the way of the future, unfortunately just now it is either wimpy or expensive or both

Currently an AC system will cost a lot more than a DC system - so for optimum range get DC and spend the money saved on batteries


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## Qer (May 7, 2008)

I concur.

As far as I've seen there can often be a minor improvement of efficiency in AC motors compared to DC motors, BUT an AC inverter is less efficient than a DC controller. In the end a complete AC system might still be more efficient than a complete DC system, but it's minimal (and it can even be the opposite).

My advice; Look at the whole system! Compare, for example a WarP 9" and a fitting controller with a Curtis 1238(?) AC kit (preferably you compare a few more systems than just two too) and then do a list of cons and pros. Power, price, weight, regen, water sealed, required pack voltage etc etc etc. Then sit down and look at what system that will fit your budget and which is most likely to be a pleasant drive.

AC versus DC is, in the end, a pretty minor detail when the car's up and running. You'll notice it less than, say, gas versus diesel. Especially if the terrain is flat so you won't really benefit from regen...


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## eldis (Sep 3, 2013)

I keep hearing this statement "regen will not increase your range much, expect few percent, not worth the trouble, just put more batteries, etc."
I guess that somewhere outside of cities, on those beautiful roads in US, stretching flat hundreds of miles it must be true. But here in Geneva, where you have a traffic light or a roundabout every 100 meters top, always start-stop driving conditions.. I'm really looking forward to finish our AC system EV to get my own numbers. Also why is DC system more efficient? I expect that you use two sets of high-low IGBT's, so you always have losses in two IGBT's at the same time - which should be pretty comparable in efficiency to how AC system operates.

If I accelerate to 50km/h on a flat terrain, then put neutral in my ICE car, it will move for quite some time before losing speed. That is ~1 ton of metal on wheels - considerable amount of energy to be recuperated when red light is on and you have to stop.


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## PhantomPholly (Aug 20, 2008)

eldis said:


> I keep hearing this statement "regen will not increase your range much, expect few percent, not worth the trouble, just put more batteries, etc."
> I guess that somewhere outside of cities, on those beautiful roads in US, stretching flat hundreds of miles it must be true. But here in Geneva, where you have a traffic light or a roundabout every 100 meters top, always start-stop driving conditions.. I'm really looking forward to finish our AC system EV to get my own numbers. Also why is DC system more efficient? I expect that you use two sets of high-low IGBT's, so you always have losses in two IGBT's at the same time - which should be pretty comparable in efficiency to how AC system operates.
> 
> If I accelerate to 50km/h on a flat terrain, then put neutral in my ICE car, it will move for quite some time before losing speed. That is ~1 ton of metal on wheels - considerable amount of energy to be recuperated when red light is on and you have to stop.


Mileage may vary. There are losses in regen. The guys who have actually built EVs here are pretty knowledgeable. They are reporting real-world results. The rule of thumb today is that the added expense of an AC regen system runs more than the cost of additional batteries with respect to extending range.

You might have different results in your area, but don't be disappointed if it is less than hoped for.


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## sunworksco (Sep 8, 2008)

The stop and go city traffic in your area can use the advantage of regen and your brakes will last much longer with regen. There is less maintainence with brushless motors, as well.


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## Qer (May 7, 2008)

eldis said:


> Also why is DC system more efficient? I expect that you use two sets of high-low IGBT's, so you always have losses in two IGBT's at the same time - which should be pretty comparable in efficiency to how AC system operates.


Nope. The simplest form of a DC controller has only one transistor. Usually when you have more than one is because the current is too high to handle by one transistor. That's for example why Soliton Jr only has one transistor but Soliton 1 two. Since an AC inverter has to be able to generate both positive and negative voltage (or have twice the pack voltage, which has it's own drawbacks) it needs a minimum of two transistors. Per phase.

Also, due to the wave forms in AC, where the average voltage is substantially lower than pack voltage, you need a lot more silicon to handle the same power (I don't remember exactly how much more, but Jeffrey has mentioned this a few times in these discussions, just search the forum) so the parts for an AC inverter will also always be noticeably more expensive. And complex.

In the end it's, once more, a question of what needs you have. You can't even just pick an "AC-system" and think that that will solve everything, and if you don't need regen it's an awful lot of money for something that you don't use. So you have to compare systems, where "AC" versus "DC" is just one of several parameters...


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## eldis (Sep 3, 2013)

Thanks. I really appreciate the knowledge provided by all of you guys. Actually this forum inspired me to do my own conversion (I don't want to be a person that criticizes everything while never doing it himself).

I'm quite aware of current situation with AC controllers, and even though there is not an universal solution, we are working on something that could be "close enough", at least we hope so. I wrote a post about our system in the controller section here. Any comments are welcomed, as everything is work in progress.


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## ToyXCAB89 (Oct 6, 2012)

Thank you for your in put! I live in Hawaii I does AC systems are better for hill climbing. We do have some steep hill (such as H3) and is this a factor or can I just throw it in low gear and I will be okay. I read somewhere another EV has to pull over to the side to let ours pass him because he was going to slow.


Duncan said:


> There is little or no efficiency difference AC/DC,
> However AC will enable you to use re-gen braking
> 
> If you are in a hilly area that could give up to 10% advantage in range,
> ...


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## Tesseract (Sep 27, 2008)

eldis said:


> I keep hearing this statement "regen will not increase your range much, expect few percent, not worth the trouble, just put more batteries, etc."


Well, the reason regen doesn't boost range all that much in the real world is that it can only capture a portion of the energy which was used to accelerate the vehicle or to go up an incline. If you don't repeatedly floor the throttle leaving every stop light then hammer the brakes at the next one and/or where you live is fairly flat and/or you do a lot of highway driving then regen won't do much for you. Frankly, the benefits of regen seem more psychological than practical.



eldis said:


> Also why is DC system more efficient?


Short answer: only one switch is in series with the pack voltage in a DC controller plus the switch can be operated from 0 to 100% duty cycle. In a 3ph. inverter, each of the 6 switches must turn off for some portion of each cycle to synthesize an AC voltage, and the switches must be rated for 30-40% more current to deliver the same RMS current as the DC controller because the peak current is 1.414x the RMS current and the peak can persist for many milliseconds when synthesizing a low frequency (like when starting from a stop). Also, there are always two switches in series with each phase winding so twice the conduction and switching losses.


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## eldis (Sep 3, 2013)

Tesseract said:


> Frankly, the benefits of regen seem more psychological than practical.


Fair enough. At least the regen comes for free  (not counting the development time). We are really close to move from small motors to high voltage tests with our sensorless FOC, so I'm really looking forward to that.


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## major (Apr 4, 2008)

eldis said:


> I really appreciate the knowledge provided by all of you guys.





eldis said:


> .....We are really close to move from small motors to high voltage tests with our *sensorless* FOC, so I'm really looking forward to that.


Save yourselves the trouble and use the sensor


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## sunworksco (Sep 8, 2008)

You need a sensor to achieve low speed starts, unless you are drag racing.


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## Duncan (Dec 8, 2008)

ToyXCAB89 said:


> Thank you for your in put! I live in Hawaii I does AC systems are better for hill climbing. We do have some steep hill (such as H3) and is this a factor or can I just throw it in low gear and I will be okay. I read somewhere another EV has to pull over to the side to let ours pass him because he was going to slow.



Hi
This is more about the amount of power that a system can sustain,
a 1000kg car doing 100kph up a 10% grade will require
28m/sec x 10% x 1000Kg x G = 28,000 watts - 
That is the extra power needed just to lift the car against gravity

So you need a continuous power over 40Kw, most of the DC motors used should be able to handle that - some may need some extra air flow from an external blower

This is where the AC does score a bit as most AC systems have less of a gap between maximum power and continuous power whereas the DC systems can often deliver gobs of power but only for a few seconds

You need to compare the systems - costs, max power, continuous power


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## ToyXCAB89 (Oct 6, 2012)

Duncan said:


> Hi
> This is more about the amount of power that a system can sustain,
> a 1000kg car doing 100kph up a 10% grade will require
> 28m/sec x 10% x 1000Kg x G = 28,000 watts -
> ...


Soooooooooo.... would all of these work okay Netgain War P9 series wound, Advanced DC 4001A series Wound. Has everyone has good experience in gards to range?, is one better than the other? Would a larger motor give you better range(unlike a bigger ICE eats more gas) would it be more effecienct or be a better hill climber?


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## eldis (Sep 3, 2013)

sunworksco said:


> You need a sensor to achieve low speed starts, unless you are drag racing.


Actually, it seems that you don't have to. But it require a lot more sophisticated algorithm than those standard zero crossing BLDC based on a HobbyKing design. Especially since we are more interested in induction motors. Basically you need to be able to predict the behavior of the motor (have it's model), then you can get pretty good estimation of angle and torque even at low speeds. 

You can check TI InstaSPIN FOC algorithm:
"Zero speed start-up feature provides 100%+ torque at start and closed loop control with FAST providing angle in less than one electrical cycle". People using it already seems to have some very good experience with full load starts (water pumps for example).

You can of course always replace software encoder with a hardware one (our Siemens 1PA6133 has it). But most of the induction motors does not, so the idea is to make it universal for others, not to rely on a single AC motor type per controller, tunable only at the factory level.


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## Duncan (Dec 8, 2008)

ToyXCAB89 said:


> Soooooooooo.... would all of these work okay Netgain War P9 series wound, Advanced DC 4001A series Wound. Has everyone has good experience in gards to range?, is one better than the other? Would a larger motor give you better range(unlike a bigger ICE eats more gas) would it be more effecienct or be a better hill climber?



Bigger v smaller - bigger is more metal, can handle more power
BUT heavier,
How heavy is your project going to be (estimate)
How long is the hill
What is the grade
What is the target speed

Once you have those four bits of information run the numbers and see how much continuous power you need and for how long

Then look for a motor,
I am using a motor from an old forklift - it will blast my car up any hill I have seen and it cost $100


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## major (Apr 4, 2008)

eldis said:


> Actually, it seems that you don't have to. But it require a lot more sophisticated algorithm than those standard zero crossing BLDC based on a HobbyKing design. Especially since we are more interested in induction motors. Basically you need to be able to predict the behavior of the motor (have it's model), then you can get pretty good estimation of angle and torque even at low speeds.
> 
> You can check TI InstaSPIN FOC algorithm:
> "Zero speed start-up feature provides 100%+ torque at start and closed loop control with FAST providing angle in less than one electrical cycle". People using it already seems to have some very good experience with full load starts (water pumps for example).
> ...


Encoders, resolvers, and position/velocity sensors are a PITA. They are costly and a source for problems. Yet every OEM EV marker uses one on their AC drives. Why is that? You think they don't know about FOC? Even experimenters and DIYer end up with feedback on their AC.

But go ahead and find out for yourself. You said:


eldis said:


> I really appreciate the knowledge provided by all of you guys.


 Yet you know better


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## dougingraham (Jul 26, 2011)

eldis said:


> We are really close to move from small motors to high voltage tests with our sensorless FOC, so I'm really looking forward to that.


Sensorless works poorly in a car. Works fine in a boat or airplane. There are a couple of things wrong with it in a car. One, when starting from 0 rpm you have a 50% chance of the motor starting in the wrong direction. This backwards impulse is a little disturbing for vehicle occupants. The other issue is low starting torque. Neither of these matter if you are going to have the motor idle and use a clutch or torque converter to isolate the motor/flywheel from the drive train. But that is not the normal approach. The OEM's all seem to have settled on a single final drive ratio coupling the motor to the wheels. My suggestion would be to keep the sensor and use it to start the motor from the zero RPM condition and as a sanity check for the sensorless software.


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## eldis (Sep 3, 2013)

major said:


> Encoders, resolvers, and position/velocity sensors are a PITA. They are costly and a source for problems. Yet every OEM EV marker uses one on their AC drives. Why is that? You think they don't know about FOC? Even experimenters and DIYer end up with feedback on their AC.
> 
> But go ahead and find out for yourself. You said: Yet you know better


I am sorry, Major. I don't want to sound like if I know better. In fact I don't, that's why I'm trying to confront my approach with your experience, to find out what problems does that presents. Also I don't want this to evolve into a philosophical conversation, just facts, opinions and experience. I know that EV makers are using sensors, and as I said, our system supports them, and our motor has them. It's just about trying to test if the computing power of DSPs and algorithms didn't advance enough for this. InstaSPIN solution from TI is on the market for around one year, too little to be implemented by many manufacturers. Some of their customers claim that they really get full torque at low RPM, and we are going to evaluate that in the coming week(s). 

Do you agree that the biggest problem of AC systems right now (except for higher cost) is the necessity to have a matched controller and motor? Basically limiting you to get the combo from someone. I've registered several topics here "which inverter for motor XYZ?", and answers were usually that this motor is not supported (too high voltage of industrial standard ones, not supported by controllers on the market, etc.). If one could get a reliable model of such motor, using automatic identification procedure on the user side, having an integrated booster that gives you up to 500V from a reasonable battery pack - would that solve those problems, or do you think it is not possible/feasible? Any other problem you can think of?


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## major (Apr 4, 2008)

The AC approach requires a higher degree of sophistication with the application engineering involved with motors and drives than does the DC approach, or the purchase of a packaged system. There is no easy answer. But if that's your goal, good luck


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## gunnarhs (Apr 24, 2012)

eldis said:


> Fair enough. At least the regen comes for free  (not counting the development time). We are really close to move from small motors to high voltage tests with our sensorless FOC, so I'm really looking forward to that.


Ok good luck  You will be up for some surprises with the bigger systems.
And careful with sensorless FOC at low speeds. 
(I am still marked after using a car with a standard inverter to do that).


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## Frank (Dec 6, 2008)

One way to estimate if it is worthwhile is to get a friend with a Prius to drive around with you and see how much they regen. If you live in a very hilly area the breaking effect may be more important. DC vehicles have to have good brakes!


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## Frank (Dec 6, 2008)

Obviously it is supposed to be "braking" but auto complete works faster than I do in the morning, lol.


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## sergiu tofanel (Jan 13, 2014)

Frank said:


> One way to estimate if it is worthwhile is to get a friend with a Prius to drive around with you and see how much they regen. If you live in a very hilly area the breaking effect may be more important. DC vehicles have to have good brakes!


Typical driving in a Prius:

15 mile trip, about 3 miles hilly (with braking in a 30mph zone) + 3 miles city stop and go traffic: may get 1.5 " green cars" regenerative energy, which is worth about 75Wh. If this were a full electric, you would get an extra 1/4 mile range, or 1.6%. Of course, your regenerative mileage may vary.

An easier way to get something for nothing would be a solar panel on the flat part of the rooftop. At 15% efficiency, one could get as much as 150Watts. Imagine the car sitting in a sunny parking lot while you are at work or at home. That's potentially 1200Wh to 1500Wh per day, or an extra 5 miles.


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## dougingraham (Jul 26, 2011)

sergiu tofanel said:


> An easier way to get something for nothing would be a solar panel on the flat part of the rooftop. At 15% efficiency, one could get as much as 150Watts. Imagine the car sitting in a sunny parking lot while you are at work or at home. That's potentially 1200Wh to 1500Wh per day, or an extra 5 miles.


This part is way off topic so I apologize for that. This seems like a good idea but I ran the numbers a couple of years ago and the best I was able to come up with was about a 48 year payback on the cost of the panels + electronics. And all I was able to do in the winter was extend my range by about 4 miles. You get about 8 miles in the summer. You are quite a bit better off just spending the money on more cells. The reason for this is that electricity is cheap. Cost to go the 6 miles daily average is 1800 wh. Cost of the electricity for that for me is 19.8 cents. You don't get that every day. Where I am we get 111 sun days per year. So ballpark savings per year would be $22. Optimistic cost of the system would be $1000 so takes 45 years to pay off not counting interest. Even if I am off by a factor of 2 it still isn't worth it.

But it would be cool to do and I am still thinking about it. The coolness factor might completely trump the cost.


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