# AC VS DC and general AC motor questions



## whisperer (Sep 1, 2008)

I'm following the sugestions for newby's so please don't think I am trying to boast? This is my first post.
I have extensive background in both mechanical and electrical skills. I have been absolutely taken with the idea of building a EV and think it's time to move on this idea. I'm considered a ICE guru by many people and have a complete shop and an automotive based business, I'm a hot rodder along with being a maintenance tech in a semi-conductor manufacturing facility.
It seems that AC motors have a bunch of strong advantages over DC systems, but many people are still making the choice of DC over AC. What am I missing? Is this because it is much easier to make the conversions? 
There are several really cool motors out there that I have seen, both AC and DC but why would they be better then a standard industrial motor for the same HP/watts/rpm's? (at least in AC). I know I must be missing something here as the industrial motors can be had for much less money.

I have a couple of different vehicles that I am thinking about using for my first EV conversion. One is a custom radical 914 Porsche that has been a parked project for a while. I think it would make a great conversion as the ICE engine isn't in it yet but the rest is done. The other is a S-10 pick-up that is a "return" from my daughter when I bought her a newer/better vehicle. It's a bare bones 4 cyl truck that runs and drives excellent, but dollar wise isn't worth anything. That being said it's a nice little truck and is clean with low original miles.

Whatever I choose for a donor, when it's done I want 70 MPH minimum top speed and 60 MPH cruise, and a minimum 60 mile range at full hiway speed. 150 mile range would be stellar. You guys are the experts so you tell me? I'm thinking an AC 25-40 horse motor and drive, and I expect to go the Tesla route with a big load of NiMH D cells. I have a few bucks to throw at this thing BUT I have never been one to spend money frivolously. I'd really like to build the controller but might start this thing with somebody else's unit.

OK, I wasn't trying to write a book so give me some feedback?


----------



## smd (Jul 7, 2008)

pickup + 60 mph + 60 mile range at that speed = you need to go the li-ion way. which means big bucks. pickups are not that aerodynamic, nor are they lightweight. 

as for the motor I think the general consensus is AC motors are more efficient and have better control electronics, plus you get regenerative breaking (which isn't a big deal...), and of course- no brushes. on the other hand DC motors and controllers tend to be cheaper etc. but I think this is a biased opinion, as some permanent magnet DC motors are very efficient. AND they are cheap. AND they are light (14KW LEM-200 = ~20lb used in dragsters btw).

I don't really know why people choose pickups as donor cars. The whole idea of an ev is to stay efficient. Which means aerodynamic and light. no heavy drivetrain, lots of power gets lost in the transmission, bearings etc. (friction, torque overcoming mass of moving parts). efficiency isn't what pickups are built for, get a toyota or suzuki hatchback and you'll see the difference. (ie. 40 A currents @ 30mph instead of 300A).

Oh and another myth! "keep the voltage high, amps stay down" now that theoretically is correct, but realistically? bs. 
Keep your car light and aerodynamic, the motor efficient (rpm) and don't accelerate too fast and 84V would be ok in most cases. Less volts means less batteries which in turn means less volume and mass inside the vehicle and of course less $ go to the chineese.. 

best of luck on your conversion anyways 

edit: and be careful when adding weight to your car. Keep in mind that your range depends pretty much on what you had for lunch. so driving around with stuff you don't need in the trunk is not a good idea. This is also true for all the extra equipment people tend to add to their conversions. Power inverters for instance. you're an engineer so you probably know more on that issue than I do...


----------



## whisperer (Sep 1, 2008)

Maybe neither one of these donor cars are a good choice. The 914 has a body kit on it that certainly adds drag rather then eliminates it. I probably won't choose the LI batts because of the cost per amp hour and think the NiMH is a good compromise today. great batts with no memory and the cost is reasonable.
I was thinking the S-10 for a couple of reasons: it's got a 4 x 6 foot bed to put batteries in, and it can run stock sized tires. The 914 has 10" wheels on the back and 8's on the front and would look stupid with narrow wheels because of the body modifications. 
Another thing I had considered is that friend has a stock bodied 914 electric conversion that has been off the road for several years. It has a DC conversion and the batts are bad. I looked at it before as a possibility but ruled it out because that particular old tech conversion is pretty crude. I'm not sure that I would use very much from that conversion and if I'm going to start over then why start with that car (I thought).


----------



## smd (Jul 7, 2008)

Yeah. Efficiency-wise you should think about thin tires like michelin energy or equivalent. also flip-up headlamps are not aerodynamic I guess

My opinion is you should get a second hand hatchback, rebuild some bodywork for extra weight savings and less drag - I mean you certainly got the capacity to do that judging by what you wrote before. Honda civic is one good example you should consider.. or the crx. It's a nice ride. If you want performance - add a second LEM-200 - you can get them for about $1800 in the EU. (fastest electric dragsterst use 4-6-8 of these). A single 84V LEM-200 runs an existing conversion - it can go up to 90 mph using the original transmission. As for the controller - Lemco sells matching ALLTRAX units for about $1200.


----------



## Madmac (Mar 14, 2008)

To state the obvious ....to get the best results you need all of the points, low weight donor, low drag, low roll tyres, high efficiency motor and drive, high capacity low weight batteries.

Looking at battery pack the best by far is lots of small Li-ion 18650 cells. They give better Ah capacity than Lifo for the same volume and weigh less. Down side is concern about safety and needing a good BMS to baby sit them. Good to see someone else looking at them rather than running away.

It is the route I am going using 4900 in a battery pack. Using the 2.2 Amp hour batteries (nominal 3.6 volt) at around $1.20 at the moment for a Korean battery that is by far the cheapest solution, highest capacity per vol and per weight.

With 50 cells in parallel that gives nominal current of 110Amps (rail voltage of 352 Volts nominal. To get long life discharge should stay under 4C ideally not more than 2C, or 220Amps to 440Amps. This does limit it to non high performance. To get the best from the low current an AC system is needed ( the high DC rail also suits that).

Madmac


----------



## smd (Jul 7, 2008)

I have not seen 18650s cheaper than $3, where are you getting those that cheap?


----------



## whisperer (Sep 1, 2008)

I'm looking at NiMH, nickel metal hydride not Li-ion, Lithium ion. I think the LI batts are not worth the serious increase in cost per amp hour as compared to the NiMH. If someone has a cheap source of LI then I'd like to hear about it but I haven't found them anywhere. If I was building a motorcycle conversion I would likely go to the LI.
I'm not much interested in building a FWD conversion such as a Honda. Not that there is anything wrong with those, It just doesn't turn me on personally. I had considered building an electric track T roadster though as I have most of the car in parts hanging on the wall of my shop. That could be really slick, really light and have a significant cool factor.


----------



## buzzforb (Aug 16, 2008)

Madmac,
Where are you getting lion bats for 1.20/bat. I am also doing a conversion in the near future and find that price appealing. What are the ratings on the batteries you speak of? Also, i have been trying to find someone who will tell me their opinion of a particular ac motor i am looking at(Marathon inverter duty - Black Max 30hp). How far do you think i can push this particluar motor with a quality ac controller pushing higher 120hz frequencies. also, would adding a liquid cooling system to this motor also improve its efficiency. FYI I am about to borrow a book from a local library about motors in the hopes of findings these answers if unable to do so on the wiki.


----------



## skullbearer (Jul 9, 2008)

Whisperer, I think the problem here that people are having with using Ni-MH batts is two-fold:

Compared to Li-ion they have a poorer energy storage per size

Ni-MH manufacturing and design has been on the out for years since Li-ion has taken over in small electronics. Ni-MH patent holders haven't done any new development for a long time and the tech really doesn't have a lot of development room.


That being said, by no means is Ni-MH a dead tech, and is always worth considering. Often Ni-MH batts can be built with a much safer high discharge rate than popular Li-ion types, such as the flat batts (laptop batts), however Li-ion cells compete very well depending on construction quality. Still, I believe most of the existing speed records for EV's were set by Ni-MH vehicles with AC motors.

You have a large storage area on the truck, but remember you'd be adding the truck's half-ton capacity in batteries to max out that range, and you'd lose your acceleration at low speeds as a result. You would probably have to buy heavy duty springs and shocks, and possibly do a diff and wheel swap for something heavier duty as well in order to handle carrying a near capacity or capacity load all the time. I don't know about you guys, but my friends have lots of problems with their S-10's really suffering when actually being used as a truck to capacity. On of them shredded 2 diffs and finally got on rebuilt for hauling by a local truck company with quality forged everything in it. His springs too had to be replaced, and the stock shocks barely survived a year of hauling trash for cash.


Anyhoo, that aside, you could probably get some significant range if you match up your motor power with the new weight of your vehicle. If you have any experience with upper mathematics, you can find rolling resistance formulas as they relate to weight (you will need to find the factor of your S-10, or close enough... this can be done by knowing the RR of the vehicle and solving the equation for the RR factor) and then calculate your new RR.

Good news is that your wheel bearings will operate more efficiently under weight, however they won't survive as long.

It would probably be a good idea to keep your tranny and use your engine compartment for the drive and controller, and cram what batts you can in there for weight balancing. That way you can use 1st, 2nd, and 3rd gear so it will require less amps for getting up to speed and save your motor and batts.



If someone wants to point whisperer to a good site for the RR formula, or provide it, I can write out the calculations for you so you can see your approximate range per-batts you add. It will be very rough, but often the curve (it usually flattens out, meaning more batts = less gain than when you had a few) will give you an idea of the uphill cost of batts when it concerns range, and let you decide at what point cost outweighs the benefits.


----------



## Madmac (Mar 14, 2008)

> I'm looking at NiMH, nickel metal hydride not Li-ion, Lithium ion.


Did not spot that..saw the mention of same as Tesla.



> How far do you think i can push this particular motor with a quality ac controller pushing higher 120hz frequencies.


My knowledge of motors is minimal, I am learning as are most on the board. The electronics is my area.


The Li-ion batteries are sourced through a Asian industrial supplier that my company uses for a lot of other items. That price does not include shipping and also import duty if any is applicable.

Madmac


----------



## Madmac (Mar 14, 2008)

> What are the ratings on the batteries you speak of?


18650 cells are available in a range of capacities, the common highest capacity are 2.2, 2.4 and 2.6 Amp Hour. The voltage for most of the discharge is around 3.6 volts. The cut off voltage depends on manufacturer and is between 2.6 and 3.1 volts. Max charge voltage between 4.1 and 4.2.

The weight of an individual cell is between 44 and 46 grams again depending on supplier. For the battery pack I am working on the weight will be 4900 x 46 = 215.6Kgm (473 lbs)

The capacity 
with 2.2 AHr cells
2.2 x 3.6 x 4900 = 38.8 KW Hrs 110 Amp at 352 Volts
2.4 x 3.6 x 4900 = 42.3 KW Hrs 120 Amp at 352 Volts
2.6 x 3.6 x 4900 = 45.8 KW Hrs 130 Amp at 352 volts

Madmac


----------



## whisperer (Sep 1, 2008)

The deal is I live 21 miles out of the city and about 10 miles of that is hill (long downhill on the way, same up coming back at night). I can charge at my regular job or at my shop no problem, and will, but would like to be able to use the EV for normal daily duties like a run to town and back without an "I must" recharge.
Skullbearer: I'm not concerned about using the S-10 as a truck per se. I have 7 trucks including the S-10 and almost any of the others are better suited to haul a serious load. If I'm going to throw a couple of tires or a bag of groceries in the S-10 to bring it home every once in a while that should not be an issue. I just figured that the truck would handle the batt load better. 
Just roughly, by what I saw available, (so don't kill me here) the Li-Ion batts are the very best BUT the cost is about 175% of the NiMH per unit. (almost twice as much). The NiMH have about 80% the capacity and are still light years ahead of lead acid technology. It seems like a good compromise as long as space is availabe in the vehicle.


----------



## Wiredsim (Jul 4, 2008)

How were you thinking about handling charging your NiMh? I recently did some research on using packs of NiMh D or F cells and found them to be competitive. 

I hate to just link to my previous post but I don't have time at the moment to retype it out so here it is.


----------



## whisperer (Sep 1, 2008)

Madmac: 4800 Li-Ion cells? Wow! How are you building your packs? Tubes?


----------



## whisperer (Sep 1, 2008)

Wiredsim: I hadn't even really thought about the charging side yet as I think I need to finalise my vehicle choice first which dictates how much battery pack I can carry. Maybe read that as I know I will need a certain amount to get the capacity I need to suit my parameters BUT depending on the vehicle I will put as much on board as the vehicle will handle. That is the number I will need to have a charging capacity for.


----------



## Madmac (Mar 14, 2008)

Each group of 50 cells is constructed as 2x25 very much like

http://www.engg.ksu.edu/solarcar/about/batterymodules.html

Each of these is then packaged in groups of 7 with a BMS board to create a 25.2 volt pack. 2 metal plates are used to water regulate the temp. Warming before use in winter and cooling during use. 14 of these packs are then wired in series to create the full pack.

Each pack is linked to a control bus to read out voltages and temperatures.

Madmac


----------



## skullbearer (Jul 9, 2008)

Madmac said:


> Each group of 50 cells is constructed as 2x25 very much like
> 
> http://www.engg.ksu.edu/solarcar/about/batterymodules.html
> 
> ...


Fancy! Does it work out better to separate them into the smaller packs? It always made sense to me in terms of ease of swapping, and it also seems intuitive that the more batts you have the more difficult for a BMS to insure equal strain.

On the flipside, doesn't having multiple BMS make the overall size of the pack larger?


----------



## Madmac (Mar 14, 2008)

Splitting it that way is partly down to where the batteries are going in the vehicle. Each 110 amp 25 volt pack will be approx 3.1 x 18 x 10.7 inches. If there is a problem a pack can be removed (or even bypassed if out and about).

The BMS PCB dealing with more than one string of batteries saves cost and space. Dealing with 7 batteries strings on one PCB is a good compromise.

Being able to monitor and regulate charging with the BMS makes no odds whether it is a single cell or 50 in parallel. Having a water cooled plate means that I can bypass quite high current during charging to equalise fully.

By choosing a 18650 battery with an internal PTC and a pressure interrupter a faulty cell will disconnect itself. On the final version I may add an individual fuse to each cell as further protection.

Madmac


----------



## sailfish11 (Sep 15, 2007)

Whisperer, you may have already seen it, but Electro Automotive as Voltsporsche kit (for the 914) that fits your desired specs. It's not cheap (~$12.5) but it might give you an idea of what you can do.

It's a DC-based kit, which leads me to the other reason for this post. I'd like to hear any comments people have on Whisperer's original question about the merits of AC vs. DC. I too am very interested in AC. The only downsides I see are cost and the need for at least 144V.

I'm willing to pay a little more if the gain in efficiency warrants it. AC seems like the better choice, but the overwhelming majority of conversions are DC.


----------



## skullbearer (Jul 9, 2008)

sailfish11 said:


> Whisperer, you may have already seen it, but Electro Automotive as Voltsporsche kit (for the 914) that fits your desired specs. It's not cheap (~$12.5) but it might give you an idea of what you can do.
> 
> It's a DC-based kit, which leads me to the other reason for this post. I'd like to hear any comments people have on Whisperer's original question about the merits of AC vs. DC. I too am very interested in AC. The only downsides I see are cost and the need for at least 144V.
> 
> I'm willing to pay a little more if the gain in efficiency warrants it. AC seems like the better choice, but the overwhelming majority of conversions are DC.



The issue I see with saying AC gains efficiency is that most DC conversion are with brush motors, and if you go to a brushless DC they seem to be just as compact and efficient as the AC motors are, and you can eliminate the fewer voltage options and the AC-DC converters that you have with an AC system.


The only real reason I've ever gotten out of an engineer so far about the difference was being told that DC systems can build capacitance, however I found out from another (more qualified) EE that it only takes a little safety system he wouldn't take the time to explain to me that apparently uses about $10 worth of diodes and things to ensure the system will NEVER have that so called 'capacitance' problem.


So I really can't see that AC actually is _advantageous_, except in that you can grab a lot of industry motors used for good prices.


----------



## Madmac (Mar 14, 2008)

> and you can eliminate the fewer voltage options and the AC-DC converters that you have with an AC system.


The same circuitry is used to drive both types. Just the software is different. 


The failure mode of concern for DC motors is the controller going short circuit and giving the motor full power. Not what you want in slow traffic. It is very difficult to detect the fault from deliberately driving the motor at full. With an AC (or BLDC) system any fault means that the correct sequence of drive stops and the motor stops.

As more DC system end up on the road there will be a case of pedestrian being injured or killed by a motor drive failing. At that point EV detractors will push to have DC motors banned. All DC systems should have an emergency cutoff with easy reach of the driver. 

Madmac


----------



## smd (Jul 7, 2008)

most DC controllers are protected against that. converted golfcart hardware may be not as safe tho... and it's a good idea to have a kill switch in the cockpit.


----------



## aeroscott (Jan 5, 2008)

skullbearer said:


> The issue I see with saying AC gains efficiency is that most DC conversion are with brush motors, and if you go to a brush less DC they seem to be just as compact and efficient as the AC motors are, and you can eliminate the fewer voltage options and the AC-DC converters that you have with an AC system.
> 
> 
> The only real reason I've ever gotten out of an engineer so far about the difference was being told that DC systems can build capacitance, however I found out from another (more qualified) EE that it only takes a little safety system he wouldn't take the time to explain to me that apparently uses about $10 worth of diodes and things to ensure the system will NEVER have that so called 'capacitance' problem.
> ...


A brush less dc motor is a ac motor commutation is either is done with brushes or with electronics . but brushless perm magnet motors are easy to control compared to induction motors but it's still a ac system


----------



## aeroscott (Jan 5, 2008)

Madmac ,on the cut off switch . I was thinking about a crow bar safety . this is a dead short that is activated by some system fault or hand switch that trips fuse or breaker that might not trip on a fault .


----------



## Madmac (Mar 14, 2008)

> most DC controllers are protected against that.


It is something worth checking before buying a controller. Most controllers protect against over voltage, over temp, and over current, not output failure.



> I was thinking about a crow bar safety . this is a dead short that is activated by some system fault or hand switch that trips fuse or breaker that might not trip on a fault .


Under fault the current is limited by the motor. Providing the contactor is rated correctly just breaking the holding voltage will do the job.

Madmac


----------



## sailfish11 (Sep 15, 2007)

In terms of efficiency, doesn't AC get more efficient at higher voltages? If I have a 144v system, for a given vehicle (and battery pack), won't I get better range with an AC setup than a DC one?


----------



## Khors (Jul 31, 2008)

Madmac said:


> The Li-ion batteries are sourced through a Asian industrial supplier that my company uses for a lot of other items. That price does not include shipping and also import duty if any is applicable.
> 
> Madmac



but still, what kind of capacity are they holding ? at 1.20 a pop you could put 60 or so in pretty easily instead of a lead acid battery . if the capacity was anywhere near comparable i would be down for that =D


----------



## skullbearer (Jul 9, 2008)

aeroscott said:


> A brush less dc motor is a ac motor commutation is either is done with brushes or with electronics . but brushless perm magnet motors are easy to control compared to induction motors but it's still a ac system


An AC motor operates by having a rotating magnetic field and a separate free-wheeling magnetic field within the rotating one. The free-wheeling field draws more energy from the rotating field as the difference in speed increases (in that the rotating field is faster), or provides power if you shut off the rotating field... or so this is my vague understanding.

A brushless DC motor operates on the same principles as a brush DC motor, except that instead of using brushes and basic electrical circuit design, it uses computer activated coils. It is NOT an AC system, nor does it operate as an AC system. What you might be referring to is that the coils alternate rapidly (especially at high rpm) the direction of electrical flow, in the same way AC power alternates direction rapidly according to its frequency.

That is _similar_ to AC but is NOT AC. The brush DC motors do EXACTLY the same thing, except that the alternating is controlled by the brushes and not by a chip.


Am I completely off on this?

NOTE: Some brushless DC motors do not have permanent magnets, and use electromagnets instead, which must be powered to generate the magnetic field.


----------



## sailfish11 (Sep 15, 2007)

First, let me say this has probably all been covered somewhere before, but I've searched and searched and can't find the answers.
I've started my EV project -- a 1965 Mustang convertible. I chose this donor because style is important to me, and the ICE curb weight isn't too outrageous (~2500lbs). But now I'm stuck with the range problem....
My goal is to find the cheapest/best way to get 50+ miles. The two choices that jump out me are: 1) AC vs. DC and 2) AGM vs. LiFePo4. Since this thread isn't about batteries, I'll ask about that choice elsewhere.
But as for AC vs. DC, this seems like the best place. So which way should I go?
I have to admit, I'm starting off wanting to convince myself that AC is the best choice, but I'm having trouble finding the necessary info to make an informed decision.
Like, what's the minimum voltage required for AC? I've heard 144, but I've also heard higher. Perhaps it depends on the motor, which then leads me to: which motor? My Mustang is on the heavy side, so I'd think I'd need at least a 9", maybe even an 11".
If I go AC, then I probably wouldn't be able to afford LiFePo, so I'd have to go with 12+ AGMs. AC motors are big -- not sure how many AGMs I could fit in my Stang. I'd like to avoid completely filling my trunk, both for the storage space issue and because I don't like the idea of 700+ pounds in my rear end!
Any thoughts/advice?
Thanks!


----------



## mjcrow (Jan 5, 2008)

Sailfish11,

You might find this page interesting http://www.evwa.com.au/ see the bottom of the page . That's right us Aussies like Mustangs too (I have a 65 coupe myself)

I'm sure Robert (you'll find his details on the "contact us" part of his site) will be happy to help you with battery questions as he imports TS to Oz and has some in his stang.

Another aussie link is the vehicle of the AEVA President, nice AC conversion with industrial equipment - http://www.evalbum.com/1149

Good luck with your EV project

mjcrow


----------



## sailfish11 (Sep 15, 2007)

Thanks! I'm looking forward to the project. I'm slowly closing in one my power train choice. I'm leaning toward an AC system (probably the Solectria AC55 from Electro Automotive) and then just load up on the batteries. I'm thinking of using the UB121100 because it seems to have the best Ah for the price. I'll get as many as the car will hold -- hopefully 20 (although I have no idea where they'll go). The car has a beefed up suspension, so I'm not too worried about the weight.

Anyone like to give feedback on my motor and battery choices?


----------



## whisperer (Sep 1, 2008)

sailfish11, wow, very cool. I own VAP and sell vintage Mustang parts. If you need something stock Mustang let me know or go to www.vintage-automotive.com. (Not trying to advertise here, just pointing....  )

Here's a few links to other Mustangs that are being converted. Chris Jones's is very well done and he has loads of pics and info on his site.


http://www.geocities.com/[email protected]/EV/1.htm

http://www.evalbum.com/733

http://www.evalbum.com/1835

http://www.evalbum.com/98


----------



## sailfish11 (Sep 15, 2007)

Thanks Whisperer -- I'll definitely check out your site (once my wallet's recovered from the conversion)!
I've gotten some PMs warning me against putting so many lead-acid batteries in the car -- even with the beefed up suspension.
I may have to choose to either go back to DC, or try some 'creative accounting' to convince my wife that splurging for the LiFePo's is a good idea. 
I've actually been email Chris Jones about his 'Stang and gotten some great advice from him. I'm going to try to do this conversion right -- I just hope it doesn't bankrupt me!


----------

