# Questions about AC vs DC conversions



## electric! (Sep 3, 2011)

Firstly, i notice that most AC motors i'm finding are rated for lower voltage than their DC cousins. It's not hard to find a DC motor that takes up to 144 volts, while i'm having difficulty finding AC motors that accept over 100 volts.

The AC motors i'm finding are rated for lower horsepower than the DC motors, but that's to be expected since AC motors don't provide as much torque.

I can find links to actual products if anybody needs specific examples.

My second question is about controllers. I know that i'm going to need a DC to AC inverter in order to run an AC motor, but is this its own separate part, or is it included in an AC motor controller?

I'm also seeing 1000+ amp DC controllers, and the AC controllers i'm looking for are closer to 500.

Are these differences (lower voltage and amp allowances) side effects of the efficiency of AC motors, or will i be riding in a significantly underpowered car if i opt for an AC motor?


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## major (Apr 4, 2008)

electric! said:


> My second question is about controllers. I know that i'm going to need a DC to AC inverter in order to run an AC motor, but is this its own separate part, or is it included in an AC motor controller?


Hi electric,

The AC motor controller is the device which converts DC to AC with a variable voltage variable frequency output to the motor to control torque and speed.

Didn't actually see question #1  Used to be AC motors were only available in higher voltage like 230 VAC requiring 330 VDC battery. Now a few are offered for lower voltage systems. Pound for pound, the AC system should provide higher power and efficiency. But you have to deal with available packages, so you may not find that to be the case.

Look around. You can find examples of guys happy with the AC package in their conversions here on this forum. The DC path is most commonly used and performs well.

Regards,

major


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## tomofreno (Mar 3, 2009)

You might do some searches. No shortage of discussions here on AC versus DC. Also check the evalbum and the garage here for performance info on specific vehicles/motors. What vehicle do you intend to convert and what do you want in performance? If zero to sixty in less than about 12 seconds in even a fairly light vehicle you will need to go with DC or spend around $15 - $20k for a high performance AC motor/controller combo. The lower cost AC options are the combos from HPEVS (formerly HPGC - see thread in the Motors forum), and some BLDC combos from Current Tech EV - see advertisement at right on your screen (don't know of anyone who actually has one of these running though). These are mostly below 100 HP peak.


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## efan (Aug 27, 2009)

Hey major and tomofreno, 
I have a question on AV vs. DC and want to ask for your input. 
I am not really interested in regen braking and efficiency, but am curious about torque vs. rpm

so if you were to run an AC and a DC motor at identical voltage and current. assuming that they can operate at the same high rpms, would the torque vs. rpm curves be the same, meaning peak torque will be approximately the same and torque will start tapering off at the approximately same rpm for both motors?

here is a link to Remy's HVH250 motor:
http://www.remyinc.com/docs/Remy_HVH_250_Sep09.pdf

so looking at the 300v torque curve, would I be able to get simmilar torque profile if I had a warp11 (HV) or kostov 11 and run it with say soliton jr at 300v (300amps peak, I think that's what Remy's max amps are as well)?


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## tomofreno (Mar 3, 2009)

> so if you were to run an AC and a DC motor at identical voltage and current. assuming that they can operate at the same high rpms, would the torque vs. rpm curves be the same, meaning peak torque will be approximately the same and torque will start tapering off at the approximately same rpm for both motors?


 Short answer is yes, mostly. For example see the attached graphs of estimated torque (bottom two) comparing 8" and 9" series DC motors with an AC50 in my car with similar peak current controllers and same pack voltage FB1-4001 is the 9" motor). But the actual relative performance of any two will depend on the details of the diameter, windings, poles, and other things that major knows more about than I.

View attachment AC50, ADC 8 inch, ADC 9 inch, wheel torque.pdf


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## major (Apr 4, 2008)

efan said:


> so if you were to run an AC and a DC motor at identical voltage and current.


That's an interesting question efan. And I think tomo gave a good reply. I doubt that Remy and a Warp11 are going to overlay exactly, but likely have similar shapes. Obviously the current limit torque curve is a horizontal line up to base speed for both and then torque dives down as speed continues to increase. The Remy (IPM) rotor has associated torque from the magnets as well as reluctance, so it is the winner in my opinion in this region over a series wound DC. If you can relate that portion of the torque curve back from the power curve, the series DC power curve will have a parabolic shape. Meaning the Remy has more torque at higher RPM or maybe better said a wider power band. This could be somewhat equalized by running the series DC at half the gear ratio and doubling the current limit. But then you lose the apples to apples.

Enough to confuse you 

major

ps. I've been impressed with the Remy on our Lightning MC


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## aeroscott (Jan 5, 2008)

The brush motor will have a upper limit on max voltage that brushes can handle . The Remy does not , why put a voltage limit on the Remy . If we look at the Remy KW output at each voltage, we see max torque will stay with it to 5000 rpm @ 700 volts , it looks like full torque @ 10,000 rpm would take about 1200 volts . yielding blistering acceleration or a smaller motor/controller .


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## Tesseract (Sep 27, 2008)

aeroscott said:


> The brush motor will have a upper limit on max voltage that brushes can handle . The Remy does not , why put a voltage limit on the Remy .


Not quite... iron losses go up at approximately the 1.6 power of frequency so there is definitely a point of diminishing returns.

And there are practical limits on voltage, too, since higher voltage not only requires thicker insulation which decreases the coupling efficiency of magnetic flux from the windings to the stator, it also results in degradation of the insulation and bearings from capacitively-coupled current (so-called dV/dt effects).

While the Remy motor does deliver impressive performance for its size, I'm not a big fan of using rare earth magnets in motors, mainly because of the cost: historical pricing of rare earth metals. It doesn't take a genius to see that ALL of those graphs are trending steeply upward in price!


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## Jan (Oct 5, 2009)

Tesseract said:


> I'm not a big fan of using rare earth magnets in motors, mainly because of the cost..


Tesseract, I asume you compare it now with pure AC. Or not?

If so, have you any idea why PM motors are more popular in hybrids? I sometimes read that PM is more efficient and is better at regen. But I can not find any proof for that in real existing motor data I can find.


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## Tesseract (Sep 27, 2008)

Jan said:


> Tesseract, I asume you compare it now with pure AC. Or not?


Yep. More specifically, against a plain old induction motor.



Jan said:


> If so, have you any idea why PM motors are more popular in hybrids? I sometimes read that PM is more efficient and is better at regen. But I can not find any proof for that in real existing motor data I can find.


Keeping in mind that I'm not a motor expert, my understanding is that the flux from PMs in AC motors is used to replace some or all of the magnetizing current and also make it easier for the inverter to develop high torque at low RPM.


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## Jan (Oct 5, 2009)

Tesseract said:


> ...the flux from PMs in AC motors is used to replace some or all of the magnetizing current...


Yet, induction motors often claim -just the motor, without the inverter- 99% efficiency at a certain RPM and torque. I can understand that what you say is true, but in the end, it's not noticable in efficiency.

[quote
...and also make it easier for the inverter to develop high torque at low RPM.[/QUOTE]

Yep, I found a few PM with a higher torque. But not much, it doesn't impresses me. It doesn't make me see why car manufaterers choose PM over simpler and cheaper induction.


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