# DC vs AC motors



## hydromagnet (Jan 7, 2015)

Hi I wanted to ask about the pros and cons of using each type of motor.

Not sure if there is a guideline somewhere here on the forum already explaining...(?)

Specifically, i'm wondering if the Curtis controllers for AC motors (or others) provide Regenerative battery charging?

Thanks and happy tinkering into the new year!
Marty


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## Sunking (Aug 10, 2009)

It boils down to cost. DC is cheep compared to AC.

If money is no object AC wins as it has many advantages.


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## major (Apr 4, 2008)

hydromagnet said:


> Hi I wanted to ask about the pros and cons of using each type of motor.
> 
> Not sure if there is a guideline somewhere here on the forum already explaining...(?)
> 
> Specifically, i'm wondering if the Curtis controllers for AC motors (or others) provide Regenerative battery charging?


Specifically yes. Curtis AC controllers regenerate. Any AC controller worth considering will regenerate.

And the AC/DC question is age old. Many times here and on any other EV forum there is. Unfortunately it is difficult to search for that subject. Check the forum wiki. And maybe scroll thru the motor sub-forum directory to find some of those threads.

Welcome aboard,

major


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## jonescg (Nov 3, 2010)

Sunking said:


> It boils down to cost. DC is cheep compared to AC.
> 
> If money is no object AC wins as it has many advantages.


Further to this - those who have gone AC will never regret it, and always recommend an AC motor. I'm one of them 

Those who have gone with DC motors will generally be ambivalent - if they had the money they probably would have gone AC, but the old brushed motors and controllers are considerably cheaper.


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## Duncan (Dec 8, 2008)

My tuppence worth

DC can be cheap 
My motor cost $100 and my controller $600
That gave me a system that cannot be matched by an AC system less than $12,000 

AC is the way of the future but if you want a bit of grunt you have to go DC unless you have very very deep pockets


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## lithiumlogic (Aug 24, 2011)

Are any of you guys using your DC motor on the highway? The biggest objection to DC seems to be that the commutator can overheat if you spend over 30 minutes > 60 MPH, even though these motors can put huge amounts of power down over short periods on the drag strip.

That said, probably not too many EV are used that way. While technically possible, longer range highway capable builds are costly, most people just want a weekend car or commuter vehicle.


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## Moltenmetal (Mar 20, 2014)

Most of your investment is going to be the pack, unless you are going with a cheaper pack from a scrapyard Volt or Leaf. Put the motor/control choice and cost in perspective to the cost of the pack and the choice might become obvious- unless you're going for dragstrip torque as your primary objective, then DC wins hands-down.


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## Sunking (Aug 10, 2009)

In the days of past DC motors had an advantage of high torque at low RPM's with precise speed control. Those days are gone with the advances in solid state VFD's.

AC motors are more efficient, more reliable with much fewer repairs if any, can be ran in either constant torque or hp. Just abou tall the advantages DC motors had is gone with the exception of high torque at very low RPM. 

Today comparing DC and AC motors is like comparing lead acid batteries to lithium. Once you have made the switch you will never go back.


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## rwaudio (May 22, 2008)

Duncan said:


> My tuppence worth
> 
> DC can be cheap
> My motor cost $100 and my controller $600
> ...


unless you use the same type of thriftiness on the AC side as well.
I have a Siemens/DMOC combo
400 for the controller (no shipping)
1850 for the motor (+shipping)

While it doesn't match your $700 combo (that's a steal!) it gets into AC for far less than retail.

If I used retail prices for a DC comparison it would make my AC system look cheep.

In general though, DC is cheaper than AC regardless of source but for fairness people should compare apples to apples, new vs new, used vs used etc.


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## dcb (Dec 5, 2009)

I was kinda hoping someone would snatch these 6 1000amp 1200v igbts for maybe $75 apiece and hook them up to a $350 homebrew controller (+ caps)  Though don't know how to find suitable AC motor(s) on the cheap (8 x ac24ls? 12x? how do you capitalize on [email protected]?). 


http://www.ebay.com/itm/281491687939?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT


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## rwaudio (May 22, 2008)

dcb said:


> I was kinda hoping someone would snatch these 6 1000amp 1200v igbts for maybe $75 apiece and hook them up to a $350 homebrew controller (+ caps)  Though don't know how to find suitable AC motor(s) on the cheap (8 x ac24ls? 12x? how do you capitalize on [email protected]?).
> 
> 
> http://www.ebay.com/itm/281491687939?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT


Well you need one controller per motor and you need 6 igbt's per controller so that would make one.... and you shouldn't use it all the way to rated current, however the right person could make one very powerful AC Inverter if there was a matching motor.


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## hydromagnet (Jan 7, 2015)

Thanks alot guys for all your help!


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## SHARKBITEATTACK (Nov 12, 2012)

I come from the world of electric bikes but is it implied in this discussion that the DC motors are series wound and the AC motors are all 3 phase Induction?


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## major (Apr 4, 2008)

SHARKBITEATTACK said:


> I come from the world of electric bikes but is it implied in this discussion that the DC motors are series wound and the AC motors are all 3 phase Induction?


I did not see that implied. However, for the DIYer and highway capable EV projects, the series wound DC or AC induction motor are the favored types. Occasionally you see a SepEx DC. Rarely, if ever, will you see a commutator PMDC machine robust enough for an EVcar. Non-ACIM like BLDC or PMAC are starting to become available to the hobbyist with deep pockets or as salvage from OEM EV/HEVs.


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## dcb (Dec 5, 2009)

rwaudio said:


> Well you need one controller per motor and you need 6 igbt's per controller so that would make one.


Well since induction motors slip, it is *possible* to run several off the same 3 phase buss, might not be ideal for a car, but seperate inverters isn't technically a "need".


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## SHARKBITEATTACK (Nov 12, 2012)

major said:


> I did not see that implied. However, for the DIYer and highway capable EV projects, the series wound DC or AC induction motor are the favored types. Occasionally you see a SepEx DC. Rarely, if ever, will you see a commutator PMDC machine robust enough for an EVcar. Non-ACIM like BLDC or PMAC are starting to become available to the hobbyist with deep pockets or as salvage from OEM EV/HEVs.


Gotcha. Why is there such a limited selection of PM motors available for the DIY community? All the electric bike conversions are using BLDC motors. Most of them hub motors. It was my understanding that just about all the EV's and hybrids on the market use these types of motors too (except Tesla). 


major said:


> I did not see that implied. However, for the DIYer and highway capable EV projects, the series wound DC or AC induction motor are the favored types. Occasionally you see a SepEx DC. Rarely, if ever, will you see a commutator PMDC machine robust enough for an EVcar. Non-ACIM like BLDC or PMAC are starting to become available to the hobbyist with deep pockets or as salvage from OEM EV/HEVs.


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## dcb (Dec 5, 2009)

Induction motors are simpler, rugged, plenty efficient, and powerful. You don't need a rare earth magnet supplier to make them, they're just aluminum and steel and copper. I imagine the supply line and costs (and supply risks) had something to do with tesla's decision, and as they have shown induction motors work remarkably well in a car, no need to over-complicate it that I can see.

http://www.coppermotor.com/2011/10/...plug-in-and-hybrid-electric-vehicle-industry/


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## major (Apr 4, 2008)

SHARKBITEATTACK said:


> Why is there such a limited selection of PM motors available for the DIY community?


The volume of sales for DIY EVcar motors was never enough to motivate anyone (or any company) to develop and tool a PM motor. The series wound motor could draw from the lift truck market and the ACIM from the industrial market.


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## paulsEV (Aug 12, 2014)

jonescg said:


> those who have gone AC will never regret it, and always recommend an AC motor. I'm one of them
> 
> Those who have gone with DC motors will generally be ambivalent - if they had the money they probably would have gone AC, but the old brushed motors and controllers are considerably cheaper.


(NOTE: that quote was in reponse to Sunking saying that DC is cheap compared to AC, but when money is no object, AC wins and has many advantages)

Aside from potential issues relating to the brushes/ commutator, what else could there be that contributes to the passion for AC over DC?
I am open to hearing the logical reasons, but am more interested with the experiential basis that makes up this attitude/ belief.


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## dcb (Dec 5, 2009)

ac motor starts at peak torque and stays there till rated speed (~peak hp) then goes into a "constant power" mode of sorts. More efficient, there is no fixed commutation angle, field weakening, regen, enclosable, durability, simple rotor, very high rpm with enough volts, complicated controller (but becoming a thing in DIY land), motors in the wild often needing higher pack voltage or rewinding. Higher pack voltage reduces charging options.

DC (e.g series wound) is great off the line, peak horsepower is at low rpm, but for drivability you need power on the high end too (i.e. wind resistance/passing). Complicated lossy rotor, commutator, limited dynamic adaptability, brushes often the weak link in making power via more volts and/or amps. Simpler controller though and usually lower voltage requirements. Lots of "standard" voltages can work, 72/96/144. 

Either can be put into service if you understand the tradeoffs. I've got an ac motor and controller kit for a motorcycle, I understand all of it pretty well, but am *considering* going DC anyway because of the lower voltage requirements and the ac motor and controller is a smidge too big. I am mostly interested in getting it rolling, regardless of AC or DC  And a DC motor anywhere near as large (12") would probably be way too much off the line for a bike (only one way to find out).

followup: re-evaluation done, sticking with AC for the bike.


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## jonescg (Nov 3, 2010)

paulsEV said:


> (NOTE: that quote was in reponse to Sunking saying that DC is cheap compared to AC, but when money is no object, AC wins and has many advantages)
> 
> Aside from potential issues relating to the brushes/ commutator, what else could there be that contributes to the passion for AC over DC?
> I am open to hearing the logical reasons, but am more interested with the experiential basis that makes up this attitude/ belief.


Efficiency is the big one. My PMAC motor is 92% efficient across the vast majority of the speed/torque range, and only dropping as low as 75% in the worst cases. Under ideal circumstances it peaks at about 96%. Brushes will damage commutators in a big way at sustained high speeds, and burn out rather quickly. Likewise, climb a very steep driveway with a DC motor and you will overheat the armature.

IP66 is the next big one. A liquid cooled AC motor can have sealed bearings and be driven under water if you so choose. A brushed DC motor will never like it. This means not only are there no brushes to replace, there's no laminations to rust, no bearings to gunk up and no moving parts to get so hot they need cooling (that said, Tesla's induction motors have a liquid cooled rotor).

Finally power. Let me tell you about power to weight! My AFM140 motor puts out 75 kW all day long, and nearly 170 kW for up to 60 seconds, or 200 kW for 10 seconds. That's 1.9 kW/kg continuous, and approaching 5.0 kW/kg peak. DC motors cannot achieve this sort of power density. For a racing motorcycle application, it was right on the money. 

The inverter I use to deliver 140 kW at the rear wheel is a Rinehart PM150DZ and it weighs in at about 11 kg. It's also pretty compact, but the power to weight of the whole package is still very impressive.

Regen is beautiful. With regen, the only major maintenance headache of your EV; brakes, is reduced even further. It's so seamless and smooth, and infinitely adjustable, you can have single pedal/wrist riding through the streets.

It's a shame that the inverters are expensive, but the Rinehart (about AUD$9k
delivered) is a very solid piece of equipment and will never let you down.


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## Duncan (Dec 8, 2008)

OK - the future is AC

However there are a few misconceptions
(1) - AC is NOT more efficient
As far as I can see any efficiency gains in the motor are offset by losses in the controller

(2) - The brushes/commutator are NOT an inherent limitation
These are used in high power applications in industry with lifetimes in the equivalent of millions of miles

The likes of the Tesla uses a purpose designed AC motor 
There is no surprise that it is better than a DC motor designed for a forklift and slightly modified for use in an EV

That does NOT mean an AC motor designed for some other industrial use will be any good in an EV


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## dcb (Dec 5, 2009)

Duncan said:


> (2) - The brushes/commutator are NOT an inherent limitation


Lol  https://www.youtube.com/watch?v=oxuUEQereK8

Still a nice machine!


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## Duncan (Dec 8, 2008)

So two 10Kw (design rating) brush/com sets get a bit sparky with 800Hp (600Kw) going through them
That's only a 3000% overload!

And yes there are brush/com sets that are designed to take that sort of load - only they are a tadge heavy!


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## SHARKBITEATTACK (Nov 12, 2012)

Not certain about this so someone correct me if I'm wrong but I think the controller on an AC powered EV is more "failsafe". If the inverter or VFD in the controller fails there is no way to turn your batteries DC voltage into AC so the motor stops producing torque and freewheels. With a DC motor if you burn out the PWM controller and don't have a properly sized fuse in place the motor may go WOT.


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## jonescg (Nov 3, 2010)

Duncan said:


> OK - the future is AC
> 
> However there are a few misconceptions
> (1) - AC is NOT more efficient
> ...


I don't think it's a misconception that AC motors are more efficient. The most efficient DC motor used in EV applications I've ever seen was the Agni, at about 90%. Most series wound DC motors are around 80-85%.

And while brushes are used in high power applications, the motors are generally the size of a 44 gallon drum, so not exactly applicable to an EV.

Considering the ideal product of efficiency, size, weight and maintenance, AC wins on every front except cost.

And yes, DC motor speed controllers can fail in the WOT mode, even one recently in Tasmania. A big red button to kill the battery is a good idea.


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## dougingraham (Jul 26, 2011)

SHARKBITEATTACK said:


> Not certain about this so someone correct me if I'm wrong but I think the controller on an AC powered EV is more "failsafe". If the inverter or VFD in the controller fails there is no way to turn your batteries DC voltage into AC so the motor stops producing torque and freewheels. With a DC motor if you burn out the PWM controller and don't have a properly sized fuse in place the motor may go WOT.


While somewhat true the DC controller can detect this situation and open its contactor so it isn't something that should ever cause a WOT condition. I know the Evnetics Soliton controllers do this and I am almost certain that the Zilla controllers do as well.

There are at least six IGBT's in an AC controller compared to the one in a DC control. If one IGBT fails it is almost certain that its half H paired IGBT will also fail and this looks like a direct short across the battery. It should be able to detect this situation and open the contactor or considerable heat will be generated inside the controller and wiring. Both a fire and a WOT are potential problems but neither are very likely with a well designed and implemented system.

What I am saying is this is not a valid reason to choose AC over DC.


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## DavidDymaxion (Dec 1, 2008)

This could be a worry with a direct drive car, particularly one with a high gear ratio (like the drag racers that go 120 mph on one gear). It takes lots of current with gearing that high, and the motor is turning very slowly so individual brushes get lots of current and get hot. Adding insult to injury, low motor speeds means the motor fan is turning slowly. Even so, I haven't heard of a direct drive EV over heating due to climbing a steep hill, but I could see how that could happen.

If the DC EV has a transmission, it is a different story. In first gear, a DC EV can climb very steep hills with no problem. For instance, in my car, the rear parking brakes went over-center and jammed stuck. I climbed a hill in this mode, no problem. Likewise, my DC electric tractor climbs very steep hills whilst mowing tall grass no problem, and has pulled out stumps no problem. If you use first gear generally I don't think there will be a problem with a properly sized DC motor, if you keep it spinning, and it has good air flow. BTW these things are true for AC, too, if you undersized it, over geared it, undercooled it, and climbed a steep hill it it also would overheat.



> ... Likewise, climb a very steep driveway with a DC motor and you will overheat the armature. ...


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## Duncan (Dec 8, 2008)

The efficiency question has been answered by the real experts like Tesseract

My understanding is any increase in efficiency in the actual motor is offset by a reduction in efficiency in the controller

From my point of view it's like a hot rodder looking at which engine to fit
The Ferrari V12 is a lovely unit
But he is more likely to fit an American V8
Its not as nice but it's a lot cheaper!

If you want a car with a bit of snot you can pay
(DC) Second hand motor and DIY controller - less than $1,000
(1400 amps - 350v) - 490Kw
(DC) New motor and nice controller - $7,000
(1000 amps - 300v) - 300Kw

To almost match those with an AC system you will need $18,000 (200Kw)
(and the DC system will still be more powerful)

This is changing
In another few years you will be able to get more powerful AC units cheaper
But not now


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## dcb (Dec 5, 2009)

If you aren't reaching for 200kw then the difference comes down a bit (and is less of a factor compared to battery costs), but even so you *can* diy a suitable AC inverter for far less than a grand, if you are scrappy. My project so far: $250 motor (50kw peak), $350 homebrew controller kit, $110 in caps, $180 for 6 600A/1200v igbt, and about $2k for 16 leaf batteries. Throw in another $100 for misc (copper bus bars/snubbers/etc) Sans batteries, I'm up to: 
$740 for 50kw of AC motor and controller (1/4 of 200kw). So the diy AC route is here, today, and is very affordable, but a much steeper learning curve than DC.

But with no regen (again series DC assumed) there really is no efficiency comparison with AC. AC wins on efficiency by a wide margin under normal driving conditions. Unless you are a hypermiler, then more power to ya.

But accelerating fast for cheap, it seems impossible to beat DIY DC, though even forklifts and etc are switching to AC, so the supply of DC motors is probably going to shrink and already suitable motors are getting harder to find on the cheap, but there seems to be a surplus of silicon switches, and moreso as the latest switching advances make replacing them cost effective in industry. Plus induction motors themselves are cheaper to make and getting better quality controls all the time.

I'm not entirely sure of the limitations of rewinding induction motors (more magnetic losses at higher speeds), but the v/hz slope on my meager ac24ls would put it at 100kw at 10krpm, theres a fair bit of untapped AC potential out there, though it is already at 3x the rated 60hzspeed, don't know if it will like 6x. *If* folks start seeing reliable and repeatable 130+ hp bursts from an 88lb, $200 motor though...


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## Duncan (Dec 8, 2008)

_So the diy AC route is here, today, and is very affordable, but a much steeper learning curve than DC._

More power to your elbow! 
But the expertise to design your own controller is rare!

DIY DC is available to everybody (even electronic numpties like me) almost entirely due to the Paul & Sabrina OpenRevolt which can be built on a "monkey see monkey do" basis

When we have the equivalent on the AC side THEN DIY AC will be viable!

Maybe it will be a "dcb" AlternatingRevolt


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## MPaulHolmes (Feb 23, 2008)

I made 2 beta 350v 400ampRMS controllers. The first one is about ship to Canada to "thingstodo". The other is going to "astro" in Australia. Both of those guys are on ecomodder.com. Right now, I'm making a few additions to the board like voltage monitoring, but am keeping it largely through-hole. The control section and 6 isolated drivers (each capable of maybe 8-10 watts) are all on one board. You just solder it, and then plug the board directly into 3 IGBT half bridges (that have been bolted down with the right spacing! haha). No wiring necessary. No software libraries. Just a couple .C files for a lowly dsPIC 5v microcontroller running at around 14.5MHz running field oriented control code for ACIM or PMSM (I tested both!).

At the moment it costs around $220 for the board and parts for populating the board and current sensors if I bought enough for 3 boards. All you would need is 3 IGBTs and a film capacitor, and a couple copper sheets, a nomex insulator for the sheets, and a base plate and enclosure. There are new IGBT half bridge modules on Mouser that are 600v 400amp that are around $150 each. The best 600v 1000uF dc bus link capacitor in the world is now $150 right from their website. Easy peasy for up to 350v 400ampRMS. Or use 1200v 600amp IGBTs and a higher voltage bus cap, and run the controller up to 700v 400ampRMS. The drivers can drive the IGBTs hard too, to reduce switching losses. I'm using 3.3Ohm gate resistors with a 24v supply.

Also, the driver section has no electrolytics, and the MTBF for each of the 8 22uF MLCC film caps is something like 6000 years at 105degC. haha. 

The reason I keep picking 600amp as the peak is, the board plugs into any igbt half bridge from 75amp up to 600amp. 600v and 1200v packages are the same footprint. If you use a 600amp component, there will be about 0.25" of space between the IGBTs. 

And the neat thing is, you can just stick different software on it (which is done, by the way!), and connect all 3 phase cables to a series wound DC motor's M+ terminal and drive away! haha. I mean, can't we all be brothers?! What better way to bring unity, than to make DC and AC the same thing (except for the motors. Those two will always be at enmity.)?


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## dcb (Dec 5, 2009)

Think about it Duncan 











Duncan said:


> But the expertise to design your own controller is rare!


Already designed... I bought a kit from johannes remember?



Duncan said:


> ...which can be built on a "monkey see monkey do" basis


What do you think I'm doing? 

http://www.diyelectriccar.com/forums/showthread.php/another-homebrew-ac-controller-45909.html

Honorable mention:
http://ecomodder.com/forum/showthre...eap-3-phase-inverter-ac-controller-10839.html


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## hmincr (Jan 20, 2012)

Wouldn't building monkey see-monkey do, require a signal generator and O-scope, for testing along the way, and knowing how to use them ?


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## MPaulHolmes (Feb 23, 2008)

Nah. I've had probably 100 people build motor controller kits, and most of them had almost no electronics experience. If the directions are good, and they are careful, it can work fine without any special equipment for testing.


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## SHARKBITEATTACK (Nov 12, 2012)

dcb said:


> If you aren't reaching for 200kw then the difference comes down a bit (and is less of a factor compared to battery costs), but even so you *can* diy a suitable AC inverter for far less than a grand, if you are scrappy. My project so far: $250 motor (50kw peak), $350 homebrew controller kit, $110 in caps, $180 for 6 600A/1200v igbt, and about $2k for 16 leaf batteries. Throw in another $100 for misc (copper bus bars/snubbers/etc) Sans batteries, I'm up to:
> $740 for 50kw of AC motor and controller (1/4 of 200kw). So the diy AC route is here, today, and is very affordable, but a much steeper learning curve than DC.


Wow, so I would think for less than $1000 (and some drivetrain modification) one could take their existing DC powered EV and add an induction motor to be used as a generator specifically for regen. Interesting


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## dcb (Dec 5, 2009)

@hmincr, an oscope (while equally useful for dc or ac) is a glorified voltmeter (volts vs time), nothing fancy really. And nothing expensive needed for starters, have you seen what johannes used for his kit? It is mostly a concern if you are modding for more power or whatnot.









@sharkey, wat?


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## hmincr (Jan 20, 2012)

Years ago, I bought a business project that was being phased out. It required knowledge of electronics, so, I hired the same tech that did the repairs. I bought a signal generator and an O-scope. The business wound down and the Tech left. I still have 2 working machines AND the signal generator and O-Scope. 

I had a working machine that was obsolete, so, I went to an EE and tried to have him remodel the machine to a newer smaller motherboard, so I could use this device underwater, hand held. He did his best to change this machine from a Magnetometer to a Metal Detector. 

I asked him if he would like to talk with the tech that did the repairs on these, and, the EE got ALL bent out of shape. Lastly, he declared the schematics to be ALL wrong. I quickly developed a mistrust of EE's, and, took a few years to get confidence back for EE's. 

After all this months of frustration, word got out what we were trying to do, and, someone else designed the same type machine, and I lost out, BIG TIME. 


If I knew how to use the 2 machines for electronic troubleshooting, I might get into doing some electronic experiments. I have a few devices that did not work upon receiving them, but, not expensive enough to return to China, so, I just put them in a box.


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## dcb (Dec 5, 2009)

Wow, life story huh? Or you could watch some monkeys on youtube test their oscilloscopes with their signal generators...


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