# Help on choosing motor/controller



## Iyzik (Oct 4, 2012)

Hey guys, first post. I am going to do an EV conversion an a small sports car, I am thinking something like a Mazda RX8 or something cool looking like that but not super expensive. Anyway I am trying mainly to decide between AC and DC, I have two setups in mind but I don't know the big differences. 

DC: Motonergy ME1002 Motor: 38.6hp @ 144v (200a)
Curtis 1231C Controller
1400+1600=3000$

AC: 43hp @ 72v
Curtis 1238 Controller
http://www.evparts.com/products/street-vehicle/motors--dot/ac-motor-controller-kits/mt1962.htm
2900$


So I really don't have enough knowledge to know what is better, to me the AC system looks much better at first. Higher horsepower at half the voltage (half the weight) and probably regenerative braking...


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## Ivansgarage (Sep 3, 2011)

I say ac. Check this post out this is what the big boys wish they had.

http://www.diyelectriccar.com/forums/showthread.php?t=79495


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## Iyzik (Oct 4, 2012)

Ivansgarage said:


> I say ac. Check this post out this is what the big boys wish they had.
> 
> http://www.diyelectriccar.com/forums/showthread.php?t=79495


Yeah that's what I was thinking as well except posts just like that one you linked is what makes me second guess the AC system for ~3000$. It seems like most DC systems are 3-4000$ and AC are usually upwards of like...alot more lol. So I'm not sure how good of a system the one I posted a link to is...
Like what will be the range and top speed vs a DC system of twice the voltage?

Thanks


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## PStechPaul (May 1, 2012)

On the basis of extensive research but limited practical experience, there are several issues to consider. The range ultimately depends on the overall efficiency of the system and the effective kWh of the batteries. For performance, a series wound DC motor has much higher low-end torque, but an AC motor may be more efficient and has less maintenance and less likelihood of catastrophic failure. There are probably many more DC DIY conversions than AC, so you may benefit from a greater depth of experience on this and other forums. 

DC motors tend to be lower voltage but higher current, so there may be less danger of electrical shock, but connections and wiring are heavier and more expensive. The technical challenges are different, and perhaps more complex with a DIY AC system if you use standard industrial components that may need to be modified. An AC system can more easily incorporate dynamic braking and regeneration.

My own preference is AC, and that is really the only type with which I have had actual experience, which has been just a small electric tractor. If you can find some EVs to look at closely and drive, that will also give you valuable information on the two types of motors. Most commercial EVs and hybrids use AC or perhaps BLDC, while older electric cars, golf carts, and industrial vehicles like fork lifts have been mostly series DC.


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## Qer (May 7, 2008)

If you get pizza in the end and it's yummy, who cares how the baker looks?

A controller/inverter is just a power converter, it converts battery power to motor power following the wish of the driver through the throttle. A motor then translates that electrical power to mechanical power and that's about it.

AC or DC pretty much like asking if you should get a car with a V6 or an I6. In most situations it won't matter much compared to other specs like power, smoothness or quality. Imo anyone starting with the question "DC or AC?" is beginning in the wrong end and what you SHOULD ask is "Curtis 1231C or Curtis 1238?" instead.

At first glance it might mean the same thing, but it opens up a wider perspective and more options. When you ask DC or AC you've pretty much boiled down the options to what you see as two main representatives, but since there easily can be bigger difference between, say, two DC-systems than between 2131C and 1238 it's a false objectivity. By asking 1231C or 1238 the whole view point is open for alternatives. Like a system from someone else but Curtis which maybe might be better for you.

So my suggestion is to back up a bit and start looking at the options again but from a different angle, ignoring AC versus DC altogether. And regen? Well, unless you live where it's very hilly or you do a lot of stop and go you probably won't notice any difference. You're pretty unlikely to get more than 5-10% longer range due to losses in transmission, motor and batteries anyway so it might be easier to simply add a few more batteries and/or try to drive careful and energy efficient instead.


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## Iyzik (Oct 4, 2012)

Qer said:


> If you get pizza in the end and it's yummy, who cares how the baker looks?
> 
> A controller/inverter is just a power converter, it converts battery power to motor power following the wish of the driver through the throttle. A motor then translates that electrical power to mechanical power and that's about it.
> 
> ...



Thank you for the very thorough replies! Yes, that is actually kind of how I'm looking at it, I don't really care wether or not it's AC or DC, I mostly just wanted to know what the range of 6x12v lead acids on a Curtis 1238 + AC vs a 12x12v lead acids on a Curtis 1231C + DC.

And btw I do live in a very hilly area, 4/7 days of the week it's a 25 mile up over a fairly decent mountain and down the other side (25 miles). So if were to be able to get ~60 mile range with good top speed on an AC rig, then that's what I would do. Even if I were to have two battery packs (6x12v each).


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## frodus (Apr 12, 2008)

I wouldn't use an AC20 or smaller in a car, Use an AC31 or AC50. The AC20's are good in really small cars or motorcycles, but you might be left wanting more.


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## PThompson509 (Jul 9, 2009)

AC50 is what I would consider to be a minimum level of power for the rx-8. If you need more power, you could wait a bit for the AC75. Go for the highest voltage the controller can handle, too.

Cheers! Peter


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## ricklearned (Mar 3, 2012)

I have an AC31 in my VW and love it.


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## dladd (Jun 1, 2011)

Iyzik said:


> And btw I do live in a very hilly area, 4/7 days of the week it's a 25 mile up over a fairly decent mountain and down the other side (25 miles). So if were to be able to get ~60 mile range with good top speed on an AC rig, then that's what I would do. Even if I were to have two battery packs (6x12v each).


whoa, hit the brakes here. You are not really even in the ballpark I don't think. Even with 12x12v leads you will be lucky to get 30 miles on a nice fresh pack on level ground. 50 miles in the hills? not with lead. At least not if you want them to last more than one year. Fwiw, 12v lead acid batteries are not suited for EV duty, based on what I've read around here anyway.


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## frodus (Apr 12, 2008)

Yeah, I agree (I missed that post).

For a car, and hilly area, you're looking at 250-300wh/mile depending on weight and how fast you go up the hill (If you really dig into the throttle or not).... so 60 miles range requirement, you're going to need at the very least a 15kwh pack... and likely much more like 18kwh.... 

And forget using 72V, you're going to want to use higher voltage... like 96V or the max voltage of the controller/motor combo (whether it's AC or DC doesn't matter). 

If you DID use lead, you're going to need something like 30-40% more battery just to compensate for the fact that it's lead and you can't go past 80%, and it's got a peukert effect working against it.... so lets sat 18kwh * 140%, that's something like 25.2kwh.

25kwh pack and ~25wh/kg energy density of lead acid, that's about 1008kg, or ~2200lbs of lead..... and that's IF you could even fit them into the car. Even if you could fit them in the car, you'd have to completely overhaul your suspension to deal with the extra weight, and it would absolutely crawl during acceleration.... forget using the AC20, it wouldn't move a 5000lb car very well, you'd need something with more power/torque.

Long and short of it, don't use lead acid. Use lithium. For 18kwh, a lithium pack would be maybe 110wh/kg, so 163kg (~360lbs).


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## Iyzik (Oct 4, 2012)

dladd said:


> whoa, hit the brakes here. You are not really even in the ballpark I don't think. Even with 12x12v leads you will be lucky to get 30 miles on a nice fresh pack on level ground. 50 miles in the hills? not with lead. At least not if you want them to last more than one year. Fwiw, 12v lead acid batteries are not suited for EV duty, based on what I've read around here anyway.


Well what would you use? Li-Ion? Aren't those ridiculously expensive not to mention I can't find anywhere to buy them. 

Ok fine I just want to know the difference in range with 72v AC vs 144v DC, forget the hills just assume its flat. I can use it for around town and my truck for commute.


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## frodus (Apr 12, 2008)

Range is determined more by the batteries than by the system you install..... but AC will be more efficient than DC, and thus give you more range for the same amount of batteries.


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## ricklearned (Mar 3, 2012)

Yes, I second that. On a good day I get 25 miles on thirty two 90Ahr Lithiums. I just bought more to give me 72 batteries 2P36S (36 in parallel)
I live in a hilly beach town. No hill is higher than 400 feet, but I love seeing the regen when I get it.


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## dladd (Jun 1, 2011)

Iyzik said:


> Well what would you use? Li-Ion? Aren't those ridiculously expensive not to mention I can't find anywhere to buy them.


you can see in my .sig what I use. LifePo4. Yes they are expensive, my batteries cost about $8k and give me a 50 mile range in actual usage. Probably 40 miles on the freeway with hills.



Iyzik said:


> Ok fine I just want to know the difference in range with 72v AC vs 144v DC, forget the hills just assume its flat. I can use it for around town and my truck for commute.


Given the same batteries, the 144v pack will go about 2x as far as the 72v pack. Really a bit less than twice as far, since you gain a bit (maybe 10%) of efficiency with the AC, and the 72v pack weighs a lot less.

Fwiw, check out skooker's conversion. It's pretty much exactly what you are talking about, an RX8 with 12x12v lead acid batteries. I believe he is seeing a range of less than 10 miles, and is already converting to Li.


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## frodus (Apr 12, 2008)

Iyzik said:


> Well what would you use? Li-Ion? Aren't those ridiculously expensive not to mention I can't find anywhere to buy them.
> 
> Ok fine I just want to know the difference in range with 72v AC vs 144v DC, forget the hills just assume its flat. I can use it for around town and my truck for commute.


Let's compare costs.

For a Lead Acid 25.2kwh pack, at 72V using 360Ah batteries. Trojan has some 6V batteries at 360Ah, they're $299 each, and you'd need 12 for 72V, so that'd cost $3600 and give you maybe 3-400 cycles depending on what you get.

For a Lithium 18kwh pack using 32 200Ah cells at $250 each, you're correct, it'd be more expensive at $8k, but it'd be 1/6 the weight, and give you well over 1000 cycles. You'd replace lead acid at least times before lithium started to degrade.

So $3600 (if that's even possible to fit) * 3 replacements = $10800 for 900 or so useable cycles until they're dead
or
$8000 for 1000-1500 cycles until you start dipping below 80% of their 200Ah capacity (maybe more)


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## PStechPaul (May 1, 2012)

I've also crunched the numbers, and although I still think you might get reasonable cost/performance with lead, LiFePO4 seems to be the only way to get a useful and reliable vehicle. It is easiest to make comparisons by using cost and weight per Wh and Wh/mile. The best published prices I found are about $0.15 for floodies, $0.30 for SLA, and $0.65 for LiFePO4. So for 40 miles at an optimistic 250 Wh/mile you need a solid 10 kWh, which means 20 kWh for lead because of the Peukert factor. So the cost for lead would be $3000 to $6000, and $6500 for LiFePO4. Lead batteries are about 50 lb/kWh and maybe 20 lb/kWh for LiFePO4, so you're looking at 1000 lb versus 200 lb.

These are very rough estimates (and probably optimistic), and the estimates given by frodus may be more realistic. I also crunched some numbers recently that show a reasonable expectation of ROI for an EV, especially if gas prices skyrocket and electric costs remain fairly stable.


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