# In a nutshell: what are the relative strengths and weaknesses of AC vs DC motors?



## Thaddeus_The_Bold (Apr 11, 2016)

Hi,

What are the relative strength and weaknesses of AC vs DC motors?

I am attracted to AC from what I've read because regenerative braking seems to come "standard". But DC sounds like it's higher performance.

Just what is the rundown? Thumbnail it for a newbie, please!


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## miscrms (Sep 25, 2013)

Broadly speaking, here's what I've gleaned:

DC: Great low end torque (if your battery/controller can keep up), more kW per $, lower rpm limit so tends to need multiple gears to take full advantage of low end torque while keeping reasonable top speed.

AC: Less low end torque, regen braking, higher efficiency, higher rpm limit (can translate to higher peak hp), control is more complicated contributing to higher price.

Here's some data I had collected while planning my conversion. DC motors are generally hard to compare to AC, but I found at least one example of a dyno run done by EVTV that was helpful. In this case I believe the Warp 9 is limited by current availability particularly at low rpms. 










Rob


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## dcb (Dec 5, 2009)

something seems horribly wrong with that warp 9 graph, I would believe this more:


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## miscrms (Sep 25, 2013)

Thanks, can you provide a link to this data? I think the data I had was from an earlier run on the same car, and as I recall they were unhappy with how the Soliton was performing. Perhaps they got that resolved resulting in the new data?

Rob


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## Duncan (Dec 8, 2008)

I agree with dcb the graphs for the warp9 are wrong - the power graph looks weird nd the torque graph is the same as the power graph!

My tuppence worth
DC is old - the modern way is AC

There are several options

Forklift motor and DIY controller - 
$1000, and 150+hp but 19th century

Second hand bits from Leaf/Volt/Tesla - 
$3000 and 150+ hp + 21st century but you need to be smart

New DC
$6000 - a good bit nicer than second hand but effectively the same

New AC
$9000 - Nearly as nice as the second hand AC stuff but a LOT easier - not as much power


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## Thaddeus_The_Bold (Apr 11, 2016)

So which motor/voltage combination would reliably shove a 3000 lb car 0-60 in 5 to 6 seconds?


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## Sunking (Aug 10, 2009)

I am going to chip in. First if you can afford it, AC is the hands down choice. 

DC Motors Advantages. 

1. High starting torque
2. Very simple speed control. 
3. Can use lower voltages.
4. Wide selection of motors in a mature market.
5. Lower initial cost.

DC Motors Disadvantages

1. Higher maintenance (Brushes, Wear, Carbon Dust)
2. More prone to failures.
3. Low usable RPM range requiring a transmission.
4. Torque is inversely proportional to RPM. Max Torques at 0 RPM, No torque at maximum RPM. Which means good acceleration from a standing stop, but fades as sped increases. It negates the high starting torque.
5. Less efficient requiring more battery capacity for a given range. 

AC Motors Advantages.

1. Virtually maintenance free. 
2. Sealed which means it can be used in explosive or wet environments.
3. Can be configured for Constant Flat Torque from 0 RPM to maximum RPM. That means it accelerates strong from a standing stop to full speed. My HPEV AC15 develops 70/ft lbs from 0 to 5800 RPM.
4. Much higher RPM operating range which means you do not need a transmission.
5. More efficient. 
6. Motor braking with regeneration. Which means when couple with #5 longer range and less brake wear. 

AC motor disadvantages. 

1. Generally requires higher voltages.
2. Controller more complicated and expensive. 
3. Higher initial installation cost. 

So as you can see, if the budget is not an issue, AC motors win hands down.


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## Duncan (Dec 8, 2008)

I would agree with Sunking except for a couple of tiny niggles

Efficiency - there simply is nothing in it, the DC motor is a tadge less efficient than the AC motor
But the AC controller is a tadge less efficient than the DC controller
So it cancels out!

D_C motor Torque is inversely proportional to RPM. _- 
No! torque is proportional to current - 
Current only goes down after you have reached 100% at the controller
If you have a higher battery voltage then you keep your torque for longer (higher rpms)

DC motors do have a lower rpm limit - that is from the brushes and because almost all of the DC motors we use are re-purposed forklift motors and they were initially designed for much lower revs

But he is right - if you can afford it go AC


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## Thaddeus_The_Bold (Apr 11, 2016)

This is all really valuable. Thanks so much!


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## dcb (Dec 5, 2009)

miscrms said:


> Thanks, can you provide a link to this data? I think the data I had was from an earlier run on the same car, and as I recall they were unhappy with how the Soliton was performing. Perhaps they got that resolved resulting in the new data?
> 
> Rob


I've no clues on the complete history, but that picture is from here:
http://evtv.me/2011/04/the-graphs-is-always-greener/

edit: lol, I could totally see someone in Anna IL naming their shop "slingblade"


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## electron bom (Dec 4, 2014)

Arguably, one of the largest benefits of an AC motor is the ability to accept liquid cooling. DC motors are, in part, limited by the heat they generate due to relatively inefficient air cooling.


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## Thaddeus_The_Bold (Apr 11, 2016)

None of the motors I am seeing offered online for sale have liquid cooling. I guess I'm looking for glycol in all the wrong places...


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## dcb (Dec 5, 2009)

pretty much all production ev or hybrid motors (i.e. from the boneyard) are liquid cooled.

http://www.goldenmotor.ca/products/72-Volt-10KW-BLDC-Motor-Liquid-Cooled.html

http://www.hpevs.com/HPEVS-AC-Electric-Motors-Oil-Cooled-Marine.htm

plus numerous diy conversions/attempts.
i.e. https://www.electricbike.com/hub-motor-pikes-peak/


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## Sunking (Aug 10, 2009)

Thaddeus_The_Bold said:


> None of the motors I am seeing offered online for sale have liquid cooling. I guess I'm looking for glycol in all the wrong places...


Maybe no water, but there is oil cooled using ATF from HPEV Motors is one I know of.


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## RIPPERTON (Jan 26, 2010)

Thaddeus_The_Bold said:


> None of the motors I am seeing offered online for sale have liquid cooling. I guess I'm looking for glycol in all the wrong places...


If its not water cooled, water cool it.
Just about everything you buy for your EV will have to be custom taylored or modified in some way. Heres a few fittings I put on my Kelly Motor for splash Glycol cooling ie like a dry sump, not completely flooded. Of course you cant do this with a standard forklift motor.









3 water injectors in between the 3 phase terminals.


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## Thaddeus_The_Bold (Apr 11, 2016)

I've built a lot of crazy sh8t in my day but I'm actually hoping to keep the fabrication of parts to a minimum on this project -- off the shelf goodies to the extent possible, even though that's the most expensive way of doing things.


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## matt4x4 (Dec 10, 2015)

I keep reading about forklift motors being used, what is the practicle limit then for the curb weight of a vehicle, and could you use 2 forklift motors chained together. Basically whats the cheapest method to power a 1/2 ton truck, to drive 30 miles a day, no load. Is it better to go with a 1/4 ton truck like a ranger.


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## piotrsko (Dec 9, 2007)

Ranger is 1/2 ton rated. Mine is about 4400 lbs driving around. Not too much different from a f150 except for possibly less aero drag at highway speeds but not by much.

Cheapest way is volt or leaf salvage battery pack, salvage forklift motor and home built controller like the revolt. Used is good too.


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## electron bom (Dec 4, 2014)

matt4x4 said:


> Basically whats the cheapest method to power a 1/2 ton truck, to drive 30 miles a day, no load.


My Warp 11 that is for sale in the classified section.


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## miscrms (Sep 25, 2013)

Thx, DCB! Here's the updated graph using the 3rd gear data. Seems a lot more reasonable.


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## electron bom (Dec 4, 2014)

great graphs, thanks for sharing


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## miscrms (Sep 25, 2013)

Duncan said:


> Efficiency - there simply is nothing in it, the DC motor is a tadge less efficient than the AC motor
> But the AC controller is a tadge less efficient than the DC controller
> So it cancels out!


I wonder what the fundamental limits of each technology are? I thought ANL's measured data showing the combined efficiency of the Leaf Inverter and Motor to be >90% over a wide area, and up to 96% peak was pretty impressive. 










http://energy.gov/sites/prod/files/2014/03/f13/ape006_burress_2013_o.pdf

It was also pretty tantalizing that they found the Leaf Motor / Inverter were thermally stable at 80kW (7000rpm, 65C coolant) indicating that this is probably a continuous rating. I really hope we'll see what this motor can do when pushed some day soon!

Rob


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## miscrms (Sep 25, 2013)

Part of the trick of course is taking these curves and extrapolating anything useful from them. Its rarely as simple as one motor is just clearly better.

If anyone wants to play around with it, here is a customized version of the "Excel Dyno" that I've been using. As represented by the graphs above, I've brought in data for the Leaf Motor (published and dyno), the published data for the AC51 and dual AC35, and the Warp9/Soliton1 Dyno data. The "Upd" version is the new more reasonable looking data.

This spreadsheet was not designed with EVs in mind, and there's lots of things I'd like to see added into it for EVs, but its the most comprehensive attempt I've seen at modeling acceleration with various motors, gear ratios, and vehicle attributes. Very fun to play with, though accuracy and relation to real world is unknown to me. What I can say is that when I put in the Leaf vehicle characteristics, gearing and motor data the results were quite close to real world reports on 0-60 and 1/4 mile times.

Note that I have tire friction set unrealistically high for now to eliminate the wheel spin limitations. 

https://drive.google.com/file/d/0B1-9FM7ibmbyQ2NIYU94bV9oZ28/view?usp=sharing

Also note that I had nothing to do with the original development, all credit goes to the original developers here:
http://www.offroadvw.net/exceldyno/

My Mac version of Excel 2011 seems to butcher the formatting a bit, you might be better off just copying the data you are interested in from my version into a fresh copy of the original. The biggest issue I have had with it is single speed transmissions seem to break the calculations when changing motors. I get around this by using a two speed with nearly the same gear ratio and the shift point set at redline.

Rob


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## Batterypoweredtoad (Feb 5, 2008)

There's either a log file or a dyno run on here of a K11 Alpha with a stiff pack at the voltage max of a Soliton One. If I recall correctly it carried its 275'ish ft lbs of torque all the way to near 6K rpm for north of 275 hp.


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## Batterypoweredtoad (Feb 5, 2008)

Found it. (Near 6K rpm, not quite all the way) 

http://www.diyelectriccar.com/forums/attachment.php?attachmentid=16376&d=1370379903

From this thread: http://www.diyelectriccar.com/forums/showthread.php/planning-another-miata-conversion-73842p10.html?highlight=alpha+dyno


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## Sunking (Aug 10, 2009)

Duncan said:


> I would agree with Sunking except for a couple of tiny niggles
> 
> Efficiency - there simply is nothing in it, the DC motor is a tadge less efficient than the AC motor
> But the AC controller is a tadge less efficient than the DC controller
> So it cancels out!


I see your point, but do not share that POV. Series Wound DC motor efficiency is on the order of 70 to 85%, where BLDC and Induction is above 95%. PWM speed controllers are not known for efficiency either and are only efficient at 100% modulation. 

Secondly you discounted Regenerative Braking. 

It really becomes appearant to me in manufactured Golf Cart technology. Old school is DC motors with 48 volt 190 AH batteries made today. However also available today are AC carts with 48 volt 160 AH batteries have longer range, are quicker, and faster than the DC counterparts. Some like the EZGO RXV do not even have mechanical brakes. They use an motor clutch brake when parked. All breaking is Regen Motor braking.


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## miscrms (Sep 25, 2013)

Pretty impressive. Looks like its a log file, would be very interesting to see it on a dyno. 

Here's what I get trying to pull the data off the curves. I'm assuming controller/motor efficiency of 85% to convert power into the controller to power out of the motor. I guess it makes sense that it would be controller limited so it falls pretty much right on the Warp9 / Soliton 1 curve at lower rpms, but then thanks to the higher Voltage can keep it up at higher rpms?

Would love to see that Kostov with 2k Amp controller


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## major (Apr 4, 2008)

Sunking said:


> I see your point, but do not share that POV. Series Wound DC motor efficiency is on the order of 70 to 85%, where BLDC and Induction is above 95%. PWM speed controllers are not known for efficiency either and are only efficient at 100% modulation.
> 
> Secondly you discounted Regenerative Braking.
> 
> It really becomes appearant to me in manufactured Golf Cart technology. Old school is DC motors with 48 volt 190 AH batteries made today. However also available today are AC carts with 48 volt 160 AH batteries have longer range, are quicker, and faster than the DC counterparts. Some like the EZGO RXV do not even have mechanical brakes. They use an motor clutch brake when parked. All breaking is Regen Motor braking.


It is a shame how the series wound DC golf cart motor evolved. It started out as a decent machine but over many years became a commodity which was cost reduced to a piece of crap by using cheap (or less) materials and processes. It was during a time when the market really didn't care about energy and it was cheaper to use more battery than a pound of copper in the motor. Cost was the driving force over everything including quality.

Recently, last decade or two, the market changed. And technology also. So it is not just the change from DC to AC, but a whole different design philosophy along with energy efficiency, performance and quality. The golf cart DC series motor is a poor representative of EV DC motor drives.

The typical EV DC series motor will see peak efficiencies in the high 80's or low 90's. Normal running efficiency is typically about 85%. And the DC PWM motor controller is actually a bit more efficient than its AC counterpart because it has a single series switch and the AC drive always has two.

But the AC drive packages for modern EVs, including golf carts, are superior to the DC drives IMHO. 

major


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## Sunking (Aug 10, 2009)

major said:


> It is a shame how the series wound DC golf cart motor evolved. It started out as a decent machine but over many years became a commodity which was cost reduced to a piece of crap by using cheap (or less) materials and processes. It was during a time when the market really didn't care about energy and it was cheaper to use more battery than a pound of copper in the motor. Cost was the driving force over everything including quality.
> 
> Recently, last decade or two, the market changed. And technology also. So it is not just the change from DC to AC, but a whole different design philosophy along with energy efficiency, performance and quality. The golf cart DC series motor is a poor representative of EV DC motor drives.
> 
> ...


No argument from me Major. I have built 3 custom carts. First two were DC Series Wound custom motors from D&D. My third I considered using a DC motor from Plum Quick. Both DD and Plum Quick make decent motors. Both have the same limitations, RPM and Torque DC motors for carts top out at 5000 RPM for speed motors, and torque motors at 3600 RPM). 25 to 30 MPH is about the limit without serious modifications to the cart in the way of lift kits, high speed gears, and larger wheels/tires. Even still doing all that the RPM and Torque limits restrict speeds to 40 to 50 with poor acceleration and horrible hill climbing. 

So I went the AC route using an HPEV AC15 motor (same as AC9 except at 96 volts). I get 70 mph and I limited it to 70 mph via RPM limit in the controller. Could easily go 77 yo 80 mph. Still had to do a lift, not as much, speed gear, and larger wheels and tires, just not as much as would be required for DC. 

Of course as you know I do not have to give up Torque and RPM's. I get great acceleration up to 50 mph before the torque starts to fade at 5000 RPM and still strong acceleration up to 70 mph at roughly 7000 RPM. Motor is rated at 16 HP @ 8000 RPM continuous. Peak HP is 60 Hp at roughly 5000 RPM with flat torque of about 70 lbs from 0 to 5000 RPM. With a 6:1 speed gear is quite a bit of torque on a 800 pound vehicle. 0-60 is quicker than most cars. Hills are no problem.

But once you have tried AC motors there is no going back to DC. Only thing that baffles me is why the AC motors cost more than DC. On the commercial and industrial side where I work, AC motors are less expensive than DC. Controller I understand why AC cost more, but not motors.


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