# AC motor DC Combo



## Inframan (Jan 30, 2011)

Ok I just got finished reading a thread entitled "Hybrids aren't electric cars" the OP explains that electric motors have more torque than gas so when using the electric at the start in a hybrid then allowing the gas motor to take over at higher speeds results in greater efficiency and better mileage. This got me to thinking has anyone at all used a combination of AC and DC motors in a full electric conversion? Since AC I have heard uses much less amperage at the very start then you could switch over to the DC motor for top speed. Could an already converted car using a DC motor be retrofitted with a small AC assist motor like the prius to achieve greater efficiencies?


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## aeroscott (Jan 5, 2008)

ac is higher eff. at all rpms just not as easy to build controllers for it.


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## DawidvC (Feb 14, 2010)

Inframan said:


> This got me to thinking has anyone at all used a combination of AC and DC motors in a full electric conversion? Since AC I have heard uses much less amperage at the very start then you could switch over to the DC motor for top speed. Could an already converted car using a DC motor be retrofitted with a small AC assist motor like the prius to achieve greater efficiencies?


If you are serious about using both ac and dc, use a 4-wheel drive setup, with the dc motor driving the back wheels. That gives you the possibility of better acceleration from a standing start, as well as regen during braking.

Dawid


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## Inframan (Jan 30, 2011)

Do you know of a good ac motor to chose for the front wheels? How could I integrate the ac controller with my warpdrive controller? Or would I have to have two throttles?


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## JRP3 (Mar 7, 2008)

Inframan said:


> Since AC I have heard uses much less amperage at the very start then you could switch over to the DC motor for top speed.


Except that DC motors have higher torque at start up so they would accelerate you faster. AC motors have higher RPMS so would allow you to go faster in a given gear. Your setup is sort of the opposite of optimal.


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## DawidvC (Feb 14, 2010)

Integrating two such different motors is going to be quite an interesting exercise, and it would also depend a lot on the individual motor characteristics on what would be needed to do something like this.

You would want your front (AC) motor to be big enough to power the car at your normal cruising speed. The rear (DC) motor must be big enough to deliver your expected acceleration figures. I would suggest looking at something like an Arduino or PicAxe board to control the individual motors, as a direct linkage, even with two potboxes, is not going to give optimal control over all aspects of the combined drivetrain.  Some experimentation required 

Regards

Dawid


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## JRP3 (Mar 7, 2008)

Why not use a single pot controlling both motors?


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## helluvaengineer96 (Oct 8, 2010)

Inframan said:


> Ok I just got finished reading a thread entitled "Hybrids aren't electric cars" the OP explains that electric motors have more torque than gas so when using the electric at the start in a hybrid then allowing the gas motor to take over at higher speeds results in greater efficiency and better mileage. This got me to thinking has anyone at all used a combination of AC and DC motors in a full electric conversion? Since AC I have heard uses much less amperage at the very start then you could switch over to the DC motor for top speed. Could an already converted car using a DC motor be retrofitted with a small AC assist motor like the prius to achieve greater efficiencies?


I think it is important to keep in mind the real advantages and disadvantages of AC vs DC.

1) Power is power. When looking at power there is not a significant efficiency advantage between AC and DC.
2) AC makes it easier to regen power
3) AC can maintain a constant top speed while voltage decreases. It is the phase difference that controls speed not voltage.. A DC motor's top speed is directly proportional to the voltage, so as voltage drops so does the DC motors top speed.

If you have done the heavy lifting (paid the $) of installing a AC controller, then I see no benefit of adding a DC motor with the AC motor.

Then add in the fact you have twice the controller weight, twice the leakage current, twice the motor weight, and worse yet half the mean time between failures.. Yikes.

One of my pet peeves with hybrids is the increased complexity and reduced mean time between failures. 

But to build on your idea....
What if you wanted 4 wheel direct drive EV? In that case maybe it would make a little sense to have AC on the front two wheels and DC on the rear to wheels to make the vehicle a little less expensive while getting most of the benefits of AC. (Kind of like putting rotors on the front and drums on the rear).

Wow this just made me realize something..... I am doing and AC EV and the car is rear wheel drive.. dang. I would get a lot better regen out of the AC motor if it were a front wheel drive vehicle. Oh well too late to change know.


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## helluvaengineer96 (Oct 8, 2010)

DawidvC said:


> If you are serious about using both ac and dc, use a 4-wheel drive setup, with the dc motor driving the back wheels. That gives you the possibility of better acceleration from a standing start, as well as regen during braking.
> 
> Dawid


Ha.. I reached your same conclusion but it took 20x the words. You sir are much more efficient than I.


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## JRP3 (Mar 7, 2008)

helluvaengineer96 said:


> If you have done the heavy lifting (paid the $) of installing a AC controller, then I see no benefit of adding a DC motor with the AC motor.


Low end torque. I actually have a future dream project using a DC and AC motor. DC gives me the killer low end torque and AC gives regen and a flatter torque curve. It would not be great for efficiency.


> Wow this just made me realize something..... I am doing and AC EV and the car is rear wheel drive.. dang. I would get a lot better regen out of the AC motor if it were a front wheel drive vehicle. Oh well too late to change know.


Unless you need more regen than your rear tires have traction it's not an issue. I'm using AC with a rear wheel drive Fiero and I can put over 200 amps of regen into the pack and I've never locked up the rear. Of course I also don't drive in the snow so that could be an issue, but it should be something like anti lock braking since as soon as the motor stops turning the regen stops and the wheels should turn again.


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## helluvaengineer96 (Oct 8, 2010)

JRP3 said:


> Low end torque. I actually have a future dream project using a DC and AC motor. DC gives me the killer low end torque and AC gives regen and a flatter torque curve. It would not be great for efficiency.
> 
> Unless you need more regen than your rear tires have traction it's not an issue. I'm using AC with a rear wheel drive Fiero and I can put over 200 amps of regen into the pack and I've never locked up the rear. Of course I also don't drive in the snow so that could be an issue, but it should be something like anti lock braking since as soon as the motor stops turning the regen stops and the wheels should turn again.


I'd suggest a transmission or getting a AC motor with the torque you desire.

Transmission needs no explanation.

With a wound rotor motor both AC and DC motors have a constant torque region. After you leave that region AC torque curve follows 1/RPM^2 for DC it is 1/RPM. So the low end torque for equivalent motor is the same.

Unfortunately, I can add no detail on the motor design side, it's been a number of years since any emag/motors classes.. 

I think there may also be a way to generate more torque by playing with the waveforms.... Guess that is one of the reasons to do these projects, experiment and learn/relearn.

Bottom line I would not consider low end torque an inherent disadvantage for AC, just a design parameter with trade offs.


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## DawidvC (Feb 14, 2010)

helluvaengineer96 said:


> I think there may also be a way to generate more torque by playing with the waveforms....


Field Orientated Control (FOC) give the same torque characteristics as a sepex motor to an AC motor. It will not be the same as a series motor. At high speeds the AC motor is definitely the better choice.

Regards
Dawid


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## efan (Aug 27, 2009)

hey guys,
I am not really sure how to start a new thread , so I decided to post this here...
[h3]The Bad: Not your best hill-climbing device.[/h3]
The Electrical Engineering training series says, quote: "Series motors cannot be used where a relatively constant speed is required under conditions of varying load." Unfortunately, that pretty much sums up what an electric car would be doing in hilly terrain, like my neighborhood. The DC series motor in your NEV might not climb those hills too briskly. Oh, you noticed this already? So did Zenn and Miles, actually...and this year(2008), their new cars have AC drive systems installed.


many of you probably already know this, so I wanted to ask just how much of a challenge is it for a dc conversion to go up a hill at 60-65 mph...for lets say 2 miles at the time...and I am not really sure about the grade but lets say it is 25%....
doesn't higher voltage system fix this problem?


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