# AC vs DC



## green caveman (Oct 2, 2009)

bigdawg7299 said:


> What are the pros and cons of each assuming the application is a conversion from ICE to EV?


Hopefully someone with greater expertise that me will chime in, but my understanding is that the AC systems are technically superior in every regard but are much more expensive.

The reason to go with DC is that the motors/controllers/etc. are all lower cost.


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## JRP3 (Mar 7, 2008)

Bang for the buck series DC is hard to beat, especially for low end torque.
AC gives you built in regen and easy electronic reversing, plus a built in safety factor where failure means a shutdown instead of possible full power condition. Pretty limited selection for AC at the moment.


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## Qer (May 7, 2008)

green caveman said:


> but my understanding is that the AC systems are technically superior in every regard but are much more expensive.


Now, now. Superior is quite a claim. Depending on what parameters you prior it's also a false claim. For example, if you want high power and low weight, DC is better. Also, if your budget is limited:



JRP3 said:


> Bang for the buck series DC is hard to beat, especially for low end torque.
> AC gives you built in regen and easy electronic reversing, plus a built in safety factor where failure means a shutdown instead of possible full power condition. Pretty limited selection for AC at the moment.


DC can also do regen since all motors can also be generators. It's a pain to get it to work with series wound motors (and almost noone does) but for example with a SepEx it's pretty simple to achieve. SepEx can also be reversed rather easy (no contactors needed).

So I think the best thing is to start with the question: What do you want/need in performance, weight, size, features etc? And what is your budget?


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## etischer (Jun 16, 2008)

AC motors provide torque over a larger range of RPM allowing a single gear ratio to be used. 

I would say AC motors (water-cooled) are capable of more continuous power per pound.

AC motors are brush-less which makes them more reliable and more efficient. 

Only downsides I see are cost, availability, and generally they require higher pack voltage.


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## JRP3 (Mar 7, 2008)

Qer said:


> For example, if you want high power and low weight, DC is better.


I guess Tesla didn't get that message 



> DC can also do regen since all motors can also be generators. It's a pain to get it to work with series wound motors (and almost noone does) but for example with a SepEx it's pretty simple to achieve. SepEx can also be reversed rather easy (no contactors needed).


So where's that SepEx Soliton then if it's so easy  And what motors would you use it with, especially above 120 volts?


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## Tesseract (Sep 27, 2008)

JRP3 said:


> I guess Tesla didn't get that message


They had other requirements to consider... DC motors do, on average, deliver more power per pound than an equivalent size ACIM. It's not a huge difference, though, and ACIMs built with laminations appropriate for 400Hz nominal operation can be significantly lighter for a given power output.



JRP3 said:


> So where's that SepEx Soliton then if it's so easy  And what motors would you use it with, especially above 120 volts?


Your second question sorta answers the first one. It's simply a case of there not being much demand. And then you inevitably get someone saying, "but the Kelly is so much cheaper..." So, we just don't want to bother with it.

That said, Kostov makes some EV-sized SepEx motors. They keep bugging us about a controller for them, too, but unless I see some compelling sales volume figures from them we've got enough projects to work on at the moment.

To the OP - there have been NUMEROUS debates about AC vs. DC. On a weekly basis it seems like... Here's the real standout one: Pure horse power DC vs AC


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## corbin (Apr 6, 2010)

bigdawg7299 said:


> What are the pros and cons of each assuming the application is a conversion from ICE to EV?


Hi BigDawg,
I recently did a bunch of research on this. In the end, I went with DC.

Here's my short analysis and opinion, which has a few links to sources:

http://www.corbinstreehouse.com/blog/2010/04/plug-bug-ac-vs-dc/

Then, if you decide on AC, you'll have to find a motor. Assuming you want to use a regular EV motor (and not try to make some industrial AC motor work), then here's your choices:

http://www.corbinstreehouse.com/blog/2010/04/plug-bug-ev-ac-drive-motor-controller-selection/

corbin


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## Richard Wood (Jun 27, 2008)

Thanks Corbin, this is a good piece of research. I am currently planning to use a 9 inch DC motor, but am taking a side-track to explore AC before I finally commit. 
The HPEVS AC50 had caught my eye in terms of its excellent price and target car size but you've got me wondering about necessary power for the 2km 8 degree climb i'm planning at the end of every commute. I am thinking Lithium batteries might help.
If it has a top speed limited by the controller voltage then can you just use the gears a bit earlier? (In my case I'm thinking auto gear box and have posed a question about the pros/cons of of an AC/Auto combination in another thread.)


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## JRP3 (Mar 7, 2008)

I'm using the HPEV AC31 system in my Fiero with 36 SE 100Ah cells. I can hit 65mph in second gear, can go up a mile long hill in third at 55mph with no problem. I have some steering issues I have to work out so I've never gone faster than 65 and don't think I've ever used 4th, definitely not 5th.


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## corbin (Apr 6, 2010)

Richard Wood said:


> Thanks Corbin, this is a good piece of research. I am currently planning to use a 9 inch DC motor, but am taking a side-track to explore AC before I finally commit.
> The HPEVS AC50 had caught my eye in terms of its excellent price and target car size but you've got me wondering about necessary power for the 2km 8 degree climb i'm planning at the end of every commute. I am thinking Lithium batteries might help.
> If it has a top speed limited by the controller voltage then can you just use the gears a bit earlier? (In my case I'm thinking auto gear box and have posed a question about the pros/cons of of an AC/Auto combination in another thread.)


Richard - I think the HPEV AC50 would basically make a VW bug feel just like a bug; it puts out about the same max horsepower (~50hp). I was looking for something a little more powerful so I could easily cruise around my mountain hills in the Santa Cruz mountains. That is my main reason for not using it. 

Note that I also have zero real work experience with these motors! After I get my bug built and running I'll be able to provide better first hand input on my thoughts.

corbin


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## Plamenator (Mar 6, 2009)

Tesseract said:


> That said, Kostov makes some EV-sized SepEx motors. They keep bugging us about a controller for them, too, but unless I see some compelling sales volume figures from them we've got enough projects to work on at the moment.


I think that DC indeed beats AC as long as voltage is under 250-300V.
Above that DC looses the battle as commutation worsens progressively.
Tesla was at 400V I think...and they did not have Soliton/Kostov back then 

The reason I was "bugging" you for a SepEx controller is because I see a potential in it (part of the reason being our competitors make sepex motors with advanced brushes which cannot do regen so kind of kill the idea in its infancy).
Series is only better than sepex in terms of peak overload associated with serious racing. For an OEM sepex at 120-192V is perfect - no spin off to destruction, does have regen and a complete 30kW motor+controller package can come for about 3000-3500$.
The only HV controller there is the pitifull Kelly at 120V but that is really a golf cart model.
I think a sepex controller will quickly steal market share from existing series controllers - even for the hobby market. I suppose most people will trade peak overload for regen.
If you wait to get certain high volumes for sure, than you are doomed to bring your product to the market late.

It is an interesting topic so all opinions are welcome.


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## JRP3 (Mar 7, 2008)

corbin said:


> Richard - I think the HPEV AC50 would basically make a VW bug feel just like a bug; it puts out about the same max horsepower (~50hp).


I can promise you that my Fiero with the AC31 doesn't feel like any bug I've ever been in. Did you see the part where I can hit 65 in second gear? The AC50 has a little less peak torque than the AC31 but holds that torque almost 1000RPM longer.


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