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Discussion Starter #1
Hi,

I know, it is discussed before, but how do you think, how much difference is between AC 3 phase induction motor and DC series motor efficiency?

Example DC series motors: Kostov K9'' 144V / Netgain Impulse9 144V
Example AC motors: Siemens 1PV5135-4WS28 / ABB 3GAA 131 316
These motors have different power ratings, but main question is about efficiency (enery loss).

I've heard that generally DC motors have ~60-70% efficiency, while AC motors have ~85-95%.

But I have doubt about that because:
* DC motor brushes have no such big friction, to give 20% difference?!
* both motors have strator fields, that use electricity.
* assuming that I would use same voltage for AC or DC (~144V).
 

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Various parameters come to play.

First is resistance. I^2R is very dominant.
Second is field angle vs rpm. Efficient torque production through a wide RPM range is difficult.

And somewhere the last issue is friction. The air resistance of a vehicle is far great than brush or bearing friction.

Hope this helps.

If you are technically and financially ready for it, go AC.
(Not being negative about DC but its not as 'green' as AC :rolleyes:)
 

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I know, it is discussed before, but how do you think, how much difference is between AC 3 phase induction motor and DC series motor efficiency?
Hi Yohn,

Generally speaking for the size and power range of EV motors, I'd give AC about 2 or 3% advantage over DC. And then maybe a percent or so is given back in the AC controller versus the DC counterpart.

Towards the higher power end of the EV range, AC drives will run higher voltage systems like 300V batteries. There are some AC drives available now in the lower to medium power range rated for about 100V batteries.

That's the way I see it :)

major
 

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Seach for motor performance curve. You can see than the K9 motor seem have 84% peak efficiency and Impulse seem have 86% peak efficiency. This peak efficiency is only reachable in a tight range of the motor rpm.

Another really good point for DC motor is the high efficiency of the controller at 98-99% (sometime near 99.5%) compare to 90-95% efficency for AC controller.

Exemple of motor / controller efficiency:

DC: 86% x 0.99 = 85.1%
AC: 90% x 0.95 = 85.5%
 

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Various parameters come to play.

First is resistance. I^2R is very dominant.
Yes, and that is true for either AC or DC motors. The problem with a lot of AC vs. DC comparisons is that DC motor efficiency is often given for intermittent duty loads while AC motor efficiency is often given for continuous duty loads. Unless you can get efficiency ratings for both motors at the so-called "S2-60" duty then I wouldn't read too much into them; you aren't comparing apples to apples.

Second is field angle vs rpm. Efficient torque production through a wide RPM range is difficult.
Not really. If you amend that statement to say "Efficient torque production at low RPMs is difficult" then I'll agree with you. This applies to either type of motor, btw.

And somewhere the last issue is friction. The air resistance of a vehicle is far great than brush or bearing friction.
For larger motors, sure, windage and friction are trivial causes of inefficiency. Of course, the rotor of the ACIM can be made much "smoother" than the rotor (armature) in a DC motor, so that gives it an advantage at higher RPMs (e.g. - above 6000).

If you are technically and financially ready for it, go AC.
(Not being negative about DC but its not as 'green' as AC :rolleyes:)
Statements like this, however, infuriate me. How do you figure that AC is "more green" than DC? That's not even a scientifically quantifiable statement! It costs a lot more to make an AC inverter and they use silicon much less efficiently (at best: ~70%) than a DC motor controller (~100%) so they are less "green" right from the start. And even if the AC controller + motor combo is more efficient, how long will it take to pay back the difference in price between the two systems? None of the AC crowd ever thinks about this, they just hide behind the "known fact" that AC motors (only) are more efficient and they can do regen so they must be superior.

Ok, maybe, but please run some numbers first. In fact I'll do it for you. Let's assume that you have two motor + controller systems, DC and AC, that deliver the same average power of 20kW with the same average efficiency (AC motors *can* be more efficient than DC motors. but DC controllers are always more efficient than AC inverters for the same power rating). Let's be extra generous and say that the AC system can recapture an average 25% of the energy with regenerative braking (5-10% is more typical).

In other words, the AC system uses 15kW while the DC system uses 20kW to do equivalent amounts of work. Wow... 5kW sounds like a lot. Well, at an average price of $0.10/kWh here in the US that means we save $0.50 in electricity per hour of operation.

How much does the AC system cost vs. the DC system? How long will it take to pay that back? Here's a representative of each:

Curtis 1238 AC system

There are numerous combinations of DC controller + motor, but ones of equivalent size to the above would cost around $2000 (e.g. - Alltrax 7245 controller + D&D ES-31B motor).

A price difference of $2500 divided by $0.50 means you have to drive for more than 5k hours (an estimated 250k miles) to recover the difference in cost between the two systems.

That regenerative braking is a real benefit when driving in hilly areas is a good reason to go with AC, but claiming it is so much more efficient it will save you money or that it is more "green" is total nonsense.
 

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Statements like this, however, infuriate me. How do you figure that AC is "more green" than DC?
I knew I was pissing into the wind with posting something like that :D
Green is a very vague statement, I wouldn't take that too serious.


Maybe I was in an unscientific way trying to promote the use of AC.

Where did you get the figure (at best: ~70%) from? Wild statements like that.... ;)
 

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In other words, the AC system uses 15kW while the DC system uses 20kW to do equivalent amounts of work. Wow... 5kW sounds like a lot. Well, at an average price of $0.10/kWh here in the US that means we save $0.50 in electricity per hour of operation.

A price difference of $2500 divided by $0.50 means you have to drive for more than 5k hours (an estimated 250k miles) to recover the difference in cost between the two systems.
Adding an additional 25% battery capacity to my car would cost me $3000 bucks and add 175 pounds to my car. Not to mention the space needed to make them fit!
 

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Discussion Starter #8
Where did you get the figure (at best: ~70%) from? Wild statements like that....
This told one expirienced EV converter, but he couldn't say exactly where are so big losses are generating, so I wanted to know some thoughts / facts from you guys, in this forum.

That regenerative braking is a real benefit when driving in hilly areas is a good reason to go with AC, but claiming it is so much more efficient it will save you money or that it is more "green" is total nonsense.
Yeah, regen is one good extra for AC systems, but as you regularry fill little amount in batteries, does'nt this shorten their lives (LiFePO4)? (use more cycles doing so?) Also regen seems to be difficult to set for everyday use, because of need for coasting. So you must adjust regen somehow with brake pedal, etc.?


So maybe we can do conslusion: AC vs DC systems have almoust the same efficiency, if not counting regen, and higher voltage usage possibility?
Interesting, that DC system in motor actually looks like AC system (because of brushes alternating field)? :) So only real difference is where this AC is generated - in controller (inverter) or in commutator! :)


Ehh, I want to build controller myself, and it seems that go for DC and OpenRevolt will be best to do. This AC thing is very inviting, but I am afraid that I could stuck there with controller / inverter build.
 

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Where did you get the figure (at best: ~70%) from? Wild statements like that.... ;)
Because the switches in the inverter must be modulated at all times - they can't ever go to 100% on. The highest average modulation depth is 70.7%. There are some exceptions to that (shifting the neutral point around, etc.) but, in general, you have to turn all of the switches off in an inverter for some meaningful percentage of time in order to create sine wave currents in the motor. No way around that, and not so wild after all, eh?


Adding an additional 25% battery capacity to my car would cost me $3000 bucks and add 175 pounds to my car. Not to mention the space needed to make them fit!
This is assuming that having equivalent range for the two technologies is the main criterion for comparison - a different, and altogether much more difficult argument to make as it depends on terrain, driving style and the ratio of time spent accelerating, braking and at a constant speed.

And, anyway, 25% return from regen is exceptionally generous in the first place and depends on a most favorable confluence of the above factors. Anything less than 10% variation in range is probably unnoticeable by the average driver unless they drive a very precise route every day and are running right at the edge of capacity.

Like I said in my original response, you can make a rational case for AC over DC, but not by invoking economics or "greeness".
 

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and higher voltage usage possibility?
On which side?

Motor? Depends on the motor. Yes, most DC motors run below 200 Volt, but so does some AC-motors. The voltage is also pretty irrelevant since what will determine the performance of the car is how much torque and power you get. Torque is proportional to the current, but that is PER MOTOR! If you take two different types of motors and run them at the same current you might very well get totally different torque. Power is power, power in equals power out + losses. So a higher possible motor voltage is just that; higher possible motor voltage.

Battery? All controllers smart enough to have a setting for it can limit motor voltage to a suitable max motor voltage no matter what battery voltage your pack has (ok, it can't increase the voltage, just decrease). You can run for example a Soliton at 300+ Volt pack and only allow up to 170 Volt over the motor. The controller also convert power to power so by having a high pack voltage means your pack doesn't have to handle full motor current since Umotor*Imotor=Ubattery*Ibattery. If Umotor<Ubattery it means that Ibattery<Imotor.

I've said it before and I say it again. You must compare complete systems, you can't just compare AC versus DC and ignore everything else. Before you put your system in a vehicle with a suitable pack it's just a very theoretical discussion without much bearing in real life.
 

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Many good points brougth up . I remember seening a voltae /eff.chart for the venerable CM 77 ( jet starter/ generator ) used on many early ev's . motor rating 400 A , 24 V , less that 50% at the lower volts and over

90% in the 100 volt range . I was looking over Remes pmac eff. chart peaks at 95% but can go into the 70's % at some rpms . I think a bigger saving can be had by using a transverse motor / transmission rather then the hypoid differential .
 

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Just got off the phone With Jim Rowe of Metric Mechanic , he builds racing BMW engines transmissions , differentials . On the super flow dynamometer they get a 27% loss on a stock 318is in the drive train . He thinks that 3-4% in the transmission ,3% in the drive line , 3-4% for the half shafts,the high loses in the shafts is do to the angles he said . He said his nephew built a Ev Geo Metro and he liked the weight and drive train (transverse motor) efficiency of it. sorry for the hijack

WWW.metricmechanic.com
 

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Discussion Starter #14
Hello, guys, thank you for answers!

I tried to search for reasonably priced AC motor!
And I got such question - if AC motors are simpler than DC, why then they are more expensive? :)

Maybe someone knows good high voltage (>200V) / >15kW continuous AC 3 phase induction motor for ~1500$?
I have an idea, maybe if I could find cheap AC motor, I could try to build DIY inverter-controller? (I know it's not close to word "easy", but seems like worth trying).
 

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Discussion Starter #16
Thank you, JRoque!
Wow that's really interesting! OK this will sound very newbie - but can such industrial motors be used for EVs? :) they seems to use 460V by default!
 

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Hi. My take on this is that if you can find a motor with the right weight (they're usually heavy) and have sufficient voltage to drive it, you're set. Note that the 300+ volt battery pack to run a 220VAC motor is slightly heavier and more expensive (to connect) than a lower volt setup.

From my searches I've noted that you'd do better with an inverter rated motor due to the high start/stop and current requirements of an EV application. Inverter class motors are also better insulated and can usually spin faster. Look for motors with both C-face and foot mounts. If you have a 460V pack, get a 220V motor and limit the current if necessary.

Don't forget that there will be some tinkering necessary with the inverter to adapt the accelerator, regen and braking. Not terribly difficult but not plug and play either. An inverter with vector drive is essential and one with shaft position input is better.

JR
 

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Here's a few 40 HP (~15Kw) motors:
Um, 40 HP is about 30 kW; 40 HP (continuous) motors are way bigger than you would want. The shipping weight for the first one I saw was 500 pounds... even if a quarter of that is packaging, it's way too heavy and likely way too big (I can't read American style frame sizes... I think it said 324T).

You want 20 to maybe 30 HP motors (and probably compact 30 HP models) for most sedan sized conversions.

Edit: you also don't want to "match" the power level of the inverter and motor. A 20 HP motor should probably have a controller that peaks at about 60 or 80 HP. You'd need to see what the 1 minute rating of the controller is; I'm guessing that the first 40 HP inverter I saw would be called 25/30 kW in Australia, and would have a 50% overload for 1 minute (over the lower figure), in other words about 25 * 1.5 = 37.5 kW peak (about 50 HP). That might be enough for a very small conversion, if high performance was not a goal.
 
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