# DC vs. AC, insanity edition



## Hollie Maea (Dec 9, 2009)

I've noticed that there is a big gap (about an order of magnitude) in how much people push their motor above rated power for DC compared to AC.
What I mean is, for example a Warp9 and an AC50 are similarly sized motors, both in terms of physical size and in terms of continuous rating. But people will strap a zilla2k to a Warp9 and pull around 600kW from it for a few seconds. That's like 10 to 20 times higher than rated power. Whereas no one even talks about peaking an AC50 at more than 2 or 3 times higher than its rated power.

Is there a theoretical reason for this, or is it more due to the fact that you can get monster DC controllers, and DC motors are cheap enough that people were willing to risk ruining them in an effort to see what was possible?

I know there aren't any AC controllers available that come close to something like a Zilla2k, but if there were, would it work to try to go nuts (for a few seconds) with a motor like an AC50? Or is there some physical reason why DC motors can handle it and AC ones cannot?


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## onegreenev (May 18, 2012)

If I could push more amps in the AC-50 I would. I believe it could handle more. I also believe that the DC Series motor is able to handle high amps due to the size of the copper internally but the commutator is also a limiting factor as is the voltage. 

The gap you see is really a matter of available controllers. The Curtis is limited in voltage and amperage. A Soliton1 allows for very decent voltages and gobbs of amperage.


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## Hollie Maea (Dec 9, 2009)

That's sort of what I was thinking. I should note that I'm specifically talking about induction motors. Permanent magnets are a whole different story and WILL die if you push them too far for even a moment.


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## major (Apr 4, 2008)

Hollie Maea said:


> I've noticed that there is a big gap (about an order of magnitude) in how much people push their motor above rated power for DC compared to AC.
> What I mean is, for example a Warp9 and an AC50 are similarly sized motors, both in terms of physical size and in terms of continuous rating. But people will strap a zilla2k to a Warp9 and pull around 600kW from it for a few seconds. That's like 10 to 20 times higher than rated power. Whereas no one even talks about peaking an AC50 at more than 2 or 3 times higher than its rated power.
> 
> Is there a theoretical reason for this, or is it more due to the fact that you can get monster DC controllers, and DC motors are cheap enough that people were willing to risk ruining them in an effort to see what was possible?
> ...


I think your question is basically flawed. Show me 600 kW out of a Warp9. We were able to witness 170 hp (wheels) last summer. So I might believe 200 kW with a 50/50 chance of zorch in 3 seconds. We also saw a Tesla do 270 hp with an AC motor which was smaller. The difference between AC and DC motors means you get the peak power in different ways. Breakdown torque in the ACIM and demag in the PMAC motors mean you can't just pump in more armature current and the commutator on the DC machine means you can't just increase the frequency.


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## Tesseract (Sep 27, 2008)

Expanding a bit more on what major alluded to, with the induction motor there is a limit to how much torque you can get from it _regardless of how much current you force through the phase windings!_ This is called "breakdown torque", and depending on the design of the rotor, it is generally in the range of 3-4x rated full load torque. Note that in the induction motor, locked-rotor torque is always less than breakdown torque, so once you lock the rotor you have effectively lost control of the motor unless the drive utilizes some form of sensored-feedback vector control scheme. Dare I say that this type of control scheme has the singular goal of attempting to _emulate_ a DC motor? 

For a PM field AC motor ("BLDC", PMSM, etc.), the maximum torque is limited by the coercive force of the field magnets. If you attempt to overflux the armature (ie, which is usually the stator, but this is not always the case!) you will not only not get more torque out of the motor, you will also demagnetize the field magnets (whether gradually or catastrophically depends on the magnitude of the overcurrent).

In contrast, a series DC motor has a field strength which is always proportional to that of its armature (if not always precisely aligned to it, hence zorching) so there is no such thing as "breakdown torque" or "catastrophic demagnetization". This is why a series DC motor will keep putting out torque in proportion to current right up until its windings vaporize.

That's pretty much the only advantage that a series DC motor has over it's AC counterparts, but it is a pretty compelling one for most folks.

(Disclaimer - yes, I design the hardware for Evnetics, which currently only makes DC controllers... so, if you want to assume that means I am talking my book here, well, I won't try to argue otherwise.)


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## Brute Force (Aug 28, 2010)

I've been pondering this question myself. I haven't seen any reason why an ACIM controlled to emulate a series DC motor, as Tesseract put, wouldn't perform in a similar manor. I think the Tesla and AC Propulsion units corroborate that.


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## Hollie Maea (Dec 9, 2009)

Thanks for all the insights. Good stuff. Concerning tesseract's statement "unless you have sensored feedback vector control" of course I am assuming that. Open loop vector control, or, God forbid scalar control probably shouldn't be mentioned when talking about performance controllers. Yes, I know they are hard to make, but any AC controller worth it's salt will have it.


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## mizlplix (May 1, 2011)

> Yes, I know they are hard to make, but any AC controller worth it's salt will have it.


Can anyone name an AC controller "Worth it's salt" that I can buy without moving to Germany?

Miz


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## major (Apr 4, 2008)

mizlplix said:


> Can anyone name an AC controller "Worth it's salt" that I can buy without moving to Germany?


Worth its salt? Sure. Rinehart. We proved it on the salt  And on the track. And up Pikes Peak. But you got to work with them to tune it to your motor. These things aren't just plug and play.

http://www.rinehartmotion.com/

And there are others. Mission comes to mind. But these places design and build to your application. 

Or you can pick up a surplus Azure drive and hope somebody hacks it and then hope you can figure out how to tune it to your motor.


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## subcooledheatpump (Mar 5, 2012)

Tesseract is right. You can push an induction motor beyond 4 times nominal current, but it basically just overheats. I did actually manage to burn out a 10 HP industrial induction motor by over current. That's why I have a 30 HP motor now. I think it's pretty well known among most EVers that high speed is what you want from an induction or PM motor and not high torque, with the use of a reduction gearbox. Just as major said, Tesla manages to get pretty good results from relatively small motors. They just use high speed and not high torque. So in a sense then, yes they do manage to push their AC motors, at least compared to a standard AC motor anyway


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## Bowser330 (Jun 15, 2008)

mizlplix said:


> Can anyone name an AC controller "Worth it's salt" that I can buy without moving to Germany?
> 
> Miz


Keep in mind the price for the Rinehart Motion entry level PM100DX inverter is ~$7000USD

...and what motor would you use with the Rinehart inverter?
Remy HVH250?
EVO AFM140?
Last time i checked they were both selling for ~$11,000

So 100-150kw for ~$18,000, not to mention any extra amount for the custom setup, since the inverter needs to be setup for that specific motor.

There is another option, rewinding an industrial framed AC motor for EV use, this company does it, and has decent prices...
http://www.customevperformance.com/p/8535813/100kw-ac-induction-ev-motor-.html
$3000 for 100kw, so with the PM100DX, total ~$10,000??

OR....

Warp11HV $3,500 & Soliton-1 $3,000, total $6,500, ~3X less costly than the 100-150kw Rinehart and Remy/EVO combo...for equivalent torque and power output...


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## mizlplix (May 1, 2011)

> There is another option, rewinding an industrial framed AC motor for EV use,


Yes, I know...








Like this one, I just took out of my car. It performs nicely with a 1238R controller, but I am afraid I will get tired of it after a while. (I am bad that way) 
It was wound with 42' of 18ga wire per phase. It is going to the shop and being rewound for 56' of wire per phase and skipping 4 slots. (a long story)

ivanbennett dot com/forum


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## Bowser330 (Jun 15, 2008)

mizlplix said:


> Yes, I know...
> 
> 
> 
> ...


Would you say $3000 for a rewound 165m frame AC motor capable of 100kw is a fair price?


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## Hollie Maea (Dec 9, 2009)

Yes, we know that DC is cheap. This thread isn't really about that.


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## mizlplix (May 1, 2011)

________________________________________________________


> Would you say $3000 for a rewound 165m frame AC motor capable of 100kw is a fair price?


If they started out with a new motor (new price)
It could come to that.

Mine was a salvage pile motor = $100
Parts to rewind (wire, thermister, terminals,Paint)= $400
Labor to rewind ( 16 Hrs @ $25/per hr) = $400
________________________________________________________
Tax on parts= $ 32
Total = $932
Add for new motor price = $1500
________________________________________________________
Possibly-----------------------------------------------------------$2,500
Then any other charge for a large operation I may have missed.

Then if they use a new frame, add any machine work to adapt to a coupler, then...yes.

Mine cost $500 because I did the labor, machine work and shopped around for a perfect 
core. It weighs 220 Lb (82Kg), has the longer stator and a skewed rotor.

They are not that hard to rewind by hand, but if you can not or do not feel comfy doing it, then Yea, $3,000 might be fair.

Miz


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## Bowser330 (Jun 15, 2008)

mizlplix said:


> ________________________________________________________
> 
> If they started out with a new motor (new price)
> It could come to that.
> ...


Thanks Miz, what is the maximum rpm for your rewound motor?

Hollie, I figured your original questions were answered by Major, Tess and others so I was replying to Miz about his interest in the rinehart inverter. 

However if you have additional on-topic questions to ask then by all means fire away...


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## ndplume (May 31, 2010)

Thanks for the interesting thread. its like Mac vs PC without all the emotion and embellishment.

So here is something I read about, but can't find links to the article -

About 2 months ago, in the News folder, there was an article about a company that was using harmonics to drive A/C motors. I read the article and I found it interesting, but can't find it now.

From memory - Their controller used Harmonics to utilize ...Back EMF? 
With their *Patented* A/C controller, they were able to use smaller drive transistors (IGBTs, ect) which generated less heat and less cost. The project featured was an electric tug for moving airplanes.

In order to accomplish this feat, they had to re-wind the motor from the "standard" config to.... ?

Anyone else read that article? How would that change the A/C vs DC discussion?


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## major (Apr 4, 2008)

ndplume said:


> Thanks for the interesting thread. its like Mac vs PC without all the emotion and embellishment.
> 
> So here is something I read about, but can't find links to the article -
> 
> ...


http://www.chorusmotors.gi/ Has been around for about 10 years or more and hasn't caught on. Maybe they have something; maybe not. But it is a lot of complexity


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## ndplume (May 31, 2010)

major said:


> http://www.chorusmotors.gi/ Has been around for about 10 years or more and hasn't caught on. Maybe they have something; maybe not. But it is a lot of complexity


Thats it. Thanks for the link. 
I took some time to read more on it, the wheel tug is actually an electric wheel on the jet, not a separate vehicle. They make a case to use it and are testing it on some jets now. Aviation changes come slowly, so time will tell on that.

Also found a (thin) link for cars (http://choruscars.com/) but it doesn't seem to have much information. 

What I like about this thread is the opportunity to look at the AC vs DC motor question and try to understand the cost vs capability vs suitability.

So this is how I understad the Chorus solution :

The "Meshcon controller" IS in the A/C motor camp, requires a 17 phase wound motor (rewind of 3 phase). I assume this means 17 leads to the motor from the controller (vs 3 for a 3 phase).
The controller uses harmonics to create extra low speed torque (compared to 3 phase a/c solutions). 
Lastly, the controller can use lower cost power electronics (SCRs vs IGBTs) as well as lower power rated ones. Although, it sounds like it will need 17 power stages (vs 3 for 3 phase).
They claim that lower power parts will generate less heat and cost less. (Of course, lower system costs, as we know, doesn't necessarily = lower customer cost.)

Maybe we should add an over-riding category : Availability.
Unobtanium seems to be very common, so I wonder if a meshcon motor/controller is even available in a size that would suit a conversion?


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## ndplume (May 31, 2010)

(located another thread on the mescon)

http://www.diyelectriccar.com/forum...-harmonically-controlled-ac-motors-23775.html


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