# Newbie question: AC or DC?



## TheSGC (Nov 15, 2007)

I guess you would have to define better. 

AC is usually more efficient because of regenerative breaking, but its really expensive. DC is low cost, easy to work with and requires less battery power than AC. (AC is around 200-300 volts and DC is 48-144 volts depending on the power wanted.)

DC is the most common used in homemade EV conversions since its price is the least expensive. (=my version of "better")

Choosing between the two depends on what you want to spend. Set a price and then start looking at the two systems. AC systems can go for $10,000 just for the motor and controller, where as one can get a full DC kit, with all the parts one needs for $5,000 - $6,000.


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## david85 (Nov 12, 2007)

Also pay attention to the efficiency rating of the motors, solectria sells some low cost AC motors (electro automotive carries them), but the smaller ones are barely over 80% efficient, where the ADC motors are close to 90%.

Siemens makes a nice AC drive that Metric Mind carries. It can go past 200Hp, but you need a high voltage battery for that, it will run at 144V, but at a reduced output. The liquid cooling also means a higher constant output than a DC motor, and the higher max RPM means you can get away without shifting. The inverter is programmable, and also liquid cooled. The setup is pretty old by now (though not as old as the ADC motors!) but I still think its one of the best out there.

But your budget will have a lot to do with which systems are most suitable.


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## celsomenaia (Jul 26, 2007)

i´m building my second electric car , and it will be AC

i´m going to use a ZAPI AC4 , it will work from 72V - 96V and it´s 550A
it´s not cheap, but to buy DC controller I whould have to import from USA, and to add up the shiping cost, I also have to pay 21% sales tax,and other import taxes so a good ADC motor and controler (DC) would be almost the same price of the best motor a the ZAPI controller (AC) .

best regards


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## little ghost (Feb 17, 2008)

Thanks for the replies people... Although i'm still mistified i must admit. Well i suppose i always wanted regen braking so that limits my options to an AC system. I've been a petrol head for years and years and know engines very but this is... another language to me. I've been looking at alot of home build kits lately and i didn't realise the choice is so massive and confusing.

Another question... Some companies offer kits based on the size of the car... if i was to buy a kit for a heavier car and put it in a lighter car, would i get more performance? Or is there much more to it than that.

I'm looking for 80mph and 100miles range. I'm sure it's possible as there's a kit for a Porsche with those specs. Donor would be Civic or possibly Mazda 323. Any help would be much appreceiated

Alex


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## Bugzuki (Jan 15, 2008)

The is also the option of Brushless DC (which is an AC motor with permanent magnets on the rotor instead of just a squirrel cage). These motors are quite effecient, but expensive. www.uqm.com is one company. I would probably go that route if I had 25 thousand to spend on the motor.

Personally I would want my car to go about 100 miles so the AC motors look better to me for that purpose. With higher voltage you draw less amps from the batteries, but at the sametime you pack will be bigger. If you had 12v batteries to get to 360 volts would take 30 batteries. If you had space for some 90 AH batteries that would give you around 32Kwh batteries. This would give you enough power to make the 100 miles. The AC motors are usually more powerful so you would be able to handle the Freeway speeds much better also. 

Trying to fit that many batteries in a civic or 323 might be pushing it a lot. Unless you could get some Lithium Ion batteries, but then for that amount of power it would cost around 20 thousand more in Lithium Ion.

If cost is an issue right now DC is probably the best option.


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## celsomenaia (Jul 26, 2007)

hello

There is also Perm Mag motor , there have AC and DC motors with permanent magnets 

DC motors from 24V to 72V , 0,72 kW to 7,2 kW
AC motors from 24V to 96V , 0,5 kW to 30 kW 

the price are from 309 € to 810€ for the DC
and for the AC from 626€ to 2120€

if any one like send an e-mail and I will send the pdf with price list and specs, the are made in Germany I think.

I almost bought the 20kW AC motor , but i found the Best motor cheaper, but the Perm-mag is about 26 kg (57lbs) and the Best Motor 68 kg (150lbs)

best regards


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## John (Sep 11, 2007)

I would say that AC is technically superior to DC where as DC is economically superior to AC. AC requires a much more complex control system. The DC controller generally uses pulse width modulation (PWM) to chop the average voltage presented to the motor so basically it turns the power on and off rapidly with a big power transistor of some sort and varies the ratio of on and off times to control the voltage. The AC controller has six big power transistors two for each phase. One of those transistors switches the phase positive and the other one switches the phase negative and these switches are modulated on and off to approximate a sine wave output (note that both switches cannot be on at the same time as this would result in a dead short so there is usually a lull between phase positive and phase negative). Within that is interlaced a higher frequency pulse width modulation to control the voltage. Also the basic three phase frequency has to be synchronised to the speed of the motor as induction motors run asynchronously and DC brushless run synchronously. This requires some system to feed back the rotor position to the controller. The net result is that AC controllers cost many times what DC controllers cost because they cost more to design and produce. It also results in a flatter torque curve for AC with easy reversing and regenerative braking. The absence of brush drag and the electrical losses associated with the brushes can provide a marginal improvement. The safety aspect should also be mentioned. If the control stage of a DC controller failed it could potentially go full throttle (though I've never heard of this occuring) whereas this cannot happen with an AC controller. Another benefit of the AC motor is its potential power density. The absence of a commutator and brushes allows the AC motor to safely spin at higher RPM's than its DC counter part and in a shorter package. The higher RPM coupled with the flatter torque curve will translate into increased power as power is a product of torque and RPM. People like Tesla Motors (using a licensed AC Propulsion design) have succeeded in producing 180 kW out of a 35 kg motor.
I should add that while a series would DC motor is not reasonably able to provide regenerative braking a sepex DC motor can. A number of the benefits of AC can be had by separately controlling the motor field such as easy electrical reversing and reliable regenerative braking and a flatter torque curve. The draw back of sepex is reduced starting torque over series motors. Zapi have a 96v sepex controller that combats this by boosting the field strength during start up. I'm not sure why sepex motors aren't more popular although the lack of a powerful controller might have something to do with it.


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## celsomenaia (Jul 26, 2007)

hello

just a correction Jonh, the Tesla roadster motor has 50kg  , 

And the AC controllers, indeed have six power stages, insted of just one , but for the same Power they have less Amp's rating making them more cheap

I have been cheking prices and a DC controler can cost about $1500 (USD) as for the same power AC controller é can get for 1500 € ($2200 USD), but in the end for me it's about the same because of shiping and taxes .
I can say that if you live in Europe one can get a decent price for an AC motor/controler , but in the USA it will be prefered to buy DC .

If we look at a map , we can seen that most of the AC motors, (bestmotor, Perm-mag, Mes , Brusa, etc ) are europe made , and DC motors (ADC, Netgain, AD&D,etc) are USA made


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## Thalass (Dec 28, 2007)

One thing with AC motors that is difficult to do with DC is reversing. An AC motor is simple to reverse, and so is easier to do direct-drive with AC motors. This saves you weight and reduces the number of complex mechanical parts you have to maintain. They also lack brushes, so you don't have to replace them or get the commutator re-milled as the motor ages, either.


Also: Unless I'm mistaken, an AC induction motor (squirrel cage) is asynchronous and therefore by definition does not need to track the position of the rotor. The rotation speed, perhaps, but it's position would be irrelevant since the rotor is, electrically, one piece of metal with no polarity - all shorted together. 

The rpm of the rotor would be close to that of the stator field, though never exactly the same as it needs _some_ difference to induce the rotor field. This would also vary according to the mechanical load. A higher load would bog the rotor down, which would induce a stronger rotor field which would try to accelerate the rotor until a new balance was reached. 


Anyway I personally lean more towards AC motors. I'm planning to build my EV using at least one (possibly two) AC Induction motors, driving a diff to a pair of wheels each. (ie: one motor at the front, one at the back)

I think if I can get two smaller motors I can use standard size diffs, rather than having to buy chunky stronger ones, but that is assuming I can get that type of motor in a smaller size. Then fill the former engine bay up with LiFeP04 batteries! (assuming I can afford them!)


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## mattW (Sep 14, 2007)

Does someone wanna summarize this and chuck it in the wiki? Just make a new topic called AC vs DC.... that would be great.


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## little ghost (Feb 17, 2008)

Thanks for the replies chaps. Looks as though i'll be going with an AC kit with regen. $12,000 US, but with the pound so strong against the dollar right now i'm looking at £6,000 GBP. Car i imagine will cost £500-1000. Can't wait to start!

Cheers,
Alex


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## celsomenaia (Jul 26, 2007)

hello

May I ask you what was you choise ? (Band, model)

Best regards


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## Bugzuki (Jan 15, 2008)

Yea, you gotta tell us what you decided to get. And where you are getting it from.


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## little ghost (Feb 17, 2008)

I haven't made a one hundred percent choice yet, but the favourite choice so far is probably the AC Light Vehicle Direct Drive Kit from Electro Automotive: ($11,090.00 plus shipping) I need to find out if this kit provides regen braking first though as i'm not sure if it does. Although i know the VoltsPorsche one does.

http://www.electroauto.com/catalog/ackits.shtml#ldirect


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## John (Sep 11, 2007)

If you look at the list of features for the controller right at the top of the list is regenerative braking.


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## celsomenaia (Jul 26, 2007)

Hello

Do you know this site ? 
http://www.solarvan.co.uk/index.htm

It´s a AC system

Best regards


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## little ghost (Feb 17, 2008)

cheers John, obviously being blind with the old regen there. Thanks for the link cels, it looks like a great site, but I can't view it at the mo as my Internet is down (writing this on iPhone!) anyhoo, think I'm better of with a light or medium car system? Donor is probably going to be civic or mazda 323. Cheers


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