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AC vs Series DC efficiency

52K views 82 replies 20 participants last post by  gunnarhs 
#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).
 
#2 ·
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:)
 
#5 ·
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.
 
#3 ·
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
 
#4 ·
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%
 
#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.
 
#10 ·
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.
 
#11 ·
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 .
 
#12 ·
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
 
#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).
 
#17 ·
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
 
#25 ·
Hi. I too thought you were saying AC inverter switches can only be turned on at a linear 70.7%.

Aren't DC motors more efficient at starting but AC do better at full load running? In terms of cost, AC systems like those from HPEV are a bit more expensive than similar DC systems but not much more.

JR
 
#32 ·
...
Aren't DC motors more efficient at starting but AC do better at full load running? In terms of cost, AC systems like those from HPEV are a bit more expensive than similar DC systems but not much more.
I wouldn't make any broad statements about efficiency differences between AC and DC motors. While it is technically possible to make an AC motor cmore efficient than a DC motor for a given frame size, it is not absolutely true.

But this gets hashed out over and over again. I really am agnostic on motor technology. We will eventually make an AC inverter, so why would I trash talk them? However, there is precious little justification for making such a product for the DIY market now - there just aren't enough of you people out there that want to convert a car, much less pay 50% more for an AC system to do so.
 
#27 ·
This thread is not very good anymore at all......... I feel sorry that because of a hint / joke I made some start putting nonsense claims to facts and the other way arround.

Hope this helps to put some clarification back to this thread:

1. Silicon utilisation , modulation depth (ratio on/off time of the igbt), switching patterns has effects on efficiency not describable in absolute more or lesser efficient.

2. Stall torque != to do Efficiency. Stall torque describe steady state (0 rpm) losses.

3. More torque vs less torque at low rpms != to do with efficiency nor the required amount of energy to go to a certain speed .


edit: != means NOT
 
#28 ·
I think that choosing the AC motor is the good choice because of the regenerative capabilities (brake, range extender... ), motor control capabilities, maintained torque below 2000 - 3000 RPM, constant torque reduction on constant RPM increase, high efficiency in all the range between 3000 RPM to max RPM and better heat dissipation.

Graphics of DC motor:
http://www.onosokki.co.jp/English/hp_e/products/keisoku/torque/ts7700.htm
http://homepages.which.net/~paul.hills/Motors/MotorsBody.html
http://www.go-ev.com/motors-warp.html

Graphics of Ac motor:
http://www.metricmind.com/images/mes_200-250_efficiency.jpg
http://www.metricmind.com/line_art/5135ws28.gif
http://www.metricmind.com/motor.htm
http://evcrv.blogspot.com/2011/02/motor-hpgc-ac-50.html

For me the question is Induction motor or permanent magnet motor, here there is a nice discursion:
http://www.doria.fi/bitstream/handle/10024/31238/TMP.objres.448.pdf?sequence=1
 
#30 ·
I think that choosing the AC motor is the good choice because of the regenerative capabilities (brake, range extender... ), motor control capabilities, maintained torque below 2000 - 3000 RPM, constant torque reduction on constant RPM increase, high efficiency in all the range between 3000 RPM to max RPM and better heat dissipation.

Graphics of DC motor:
http://www.onosokki.co.jp/English/hp_e/products/keisoku/torque/ts7700.htm
http://homepages.which.net/~paul.hills/Motors/MotorsBody.html
http://www.go-ev.com/motors-warp.html

Graphics of Ac motor:
http://www.metricmind.com/images/mes_200-250_efficiency.jpg
http://www.metricmind.com/line_art/5135ws28.gif
http://www.metricmind.com/motor.htm
http://evcrv.blogspot.com/2011/02/motor-hpgc-ac-50.html

For me the question is Induction motor or permanent magnet motor, here there is a nice discursion:
http://www.doria.fi/bitstream/handle/10024/31238/TMP.objres.448.pdf?sequence=1
this last link is intense .
 
#34 ·
While it's true AC systems require more silicon transistors, AC systems eliminate the brushes and commutator. I'd rather have microprocessor controlled solid state switches than mechanical commutation and reversing contactors.

Dollar for dollar, AC systems are cheaper to build despite having more transistors. This is why AC motors are used almost exclusively by electric car companies and in factory automation. I think the DIY electric car market is the only application where AC systems are more expensive than DC systems.
 
#41 · (Edited)
I have a system designed from scratch using an AC motor. My experience, while AC motos can indeed be more efficient that the DC counterparts this is only true at a given RPM range.

AC motors have many drwabacks and they relly heavily in a good control algorithm which is expensive

Regenerative braking does not make a big difference if you are not in constant stop/go traffic and even so the difference is not very significant. The constant need for syncronisation from the motor to controller also adds extra losses. It is however a great upgrade to your braking system, speacially if you dont have ABS as the motor will not allow the wheels to lock, if properly set up.
DC motors dont have regeneration but their low torque characteristics also make them efficient to drive in traffic at low speeds. On the contrary for motorway operation an AC version may be more efficient.

I had a DC motor on an e-Bike, after that bought one (that I currently have) with a brushless DC as well and have no reason not to recomend any of both. Same story for Induction motors, both having advantages and drawbacks

There is no better option. You'll need to do some reseach on what you want/have and see which options better suit your needs. Thats the best option I would recomend when thinking about doing any conversion.
 
#42 ·
You mentioned in the other thread that you were using V/Hz mode. That's not really a good basis for comparison. If you can't get vector control, then AC isn't worth having. The difficulty of getting an inverter with a well implemented vector control system is a legitimate issue, but considering the performance of AC in scalar mode isn't particularly germane.
 
#43 · (Edited)
You mentioned in the other thread that you were using V/Hz mode. That's not really a good basis for comparison. If you can't get vector control, then AC isn't worth having. The difficulty of getting an inverter with a well implemented vector control system is a legitimate issue, but considering the performance of AC in scalar mode isn't particularly germane.
I was not exactly comparing with my conversion. The advice I posted was generic, not necessarily related to the issues on my build.

In fact, I can work on closed loop using V/Hz with slip control. Vector mode is just a fancy way to be in closed loop using no external encoder, by using two current transducers to check current and rotor speed. This is generally cheaper to implement and requires no extra wiring, but requires a unit with a powerfull processor to make changes quickly enought for the unit to be responsive.

By controlling the slip of the motor I can control the torque and acceleration, hence have a torque mode, the point is how much harder or more expensive the controller ends up being. On my case I will require external circuitry, for those that buy an already made controller it simply means more expensive.

AC motors have many drawbacks and they relly heavily in a good control algorithm which is expensive
On a DC motor there is not much tweeking to do with the controller as the motor on its own is a torque source and this is probably what you are comparing to when refering to V/Hz. On an AC motor working in V/Hz this is mainly a small energy efficiency and confort as the changes are not so sudden with torque vector. On the other hand I quite like to drive in V/Hz as I can control the car using only one pedal. It gives me a very accurate control, which I will keep when implementing the variable torque mode {-100; 0; +100}.
 
#44 · (Edited)
The OP asked which was more efficient, AC induction or DC brushed.

This is a bit like asking which is more efficient, a Pontiac Bonneville, or a Lincoln Continental. Neither design is very efficient.

The problem with each motor is given in the name.

Brushed DC. The problem is the brushes. Go into a dark room and spin a regular plug-in brushed drill. You will be amazed at the fireball of sparks in there. Obviously an efficiency, heat, and longevity problem.

Induction AC. The problem is you have to use a bunch of extra energy to create the inductive field. These motors are hugely inefficient at low rpms and low loads. So driving mellow won't help with an induction AC



If you look at the 10 kw line it is 80% efficient at 1500 rpm and only goes up to about 84% at 6000 rpm. That is pretty bad.

If you are looking for the most efficient motor type that is commonly available it is the permanent magnet motor (call it AC or DC if you prefer). The magnet creates the field for "free" so no energy is wasted. They are also good at low loads down to much lower rpm's than induction. Obviously, they have no brush losses.

With the magnet motor below it is 90-95% efficient at 10 kw from 1500-3000 rpm. Exactly the rpm's you typically use while driving.




Dewalt and all the other cordless drill manufacturer's already figured out which motor type provides the highest efficiency and longevity and the lowest weight.

They all use magnet motors.

If you want to know the weakness, again, look at the name. Magnets. They are more costly and become heat damaged at a slightly lower temperature. They areusually rated at temperature class F (311 F) instead of H (356 F)). Some make a big deal out of this, but it is only a few degrees. If you are getting any motor over 300 degrees it will not last long.

Cheers. :)
 
#45 · (Edited)
The OP asked which was more efficient, AC induction or DC brushed.

This is a bit like asking which is more efficient, a Pontiac Bonneville, or a Lincoln Continental. Neither design is very efficient.

The problem with each motor is given in the name.

Brushed DC. The problem is the brushes. Go into a dark room and spin a regular plug-in brushed drill. You will be amazed at the fireball of sparks in there. Obviously an efficiency, heat, and longevity problem.

Induction AC. The problem is you have to use a bunch of extra energy to create the inductive field. These motors are hugely inefficient at low rpms and low loads. So driving mellow won't help with an induction AC



If you look at the 10 kw line it is 80% efficient at 1500 rpm and only goes up to about 84% at 6000 rpm. That is pretty bad.

If you are looking for the most efficient motor type that is commonly available it is the permanent magnet motor (call it AC or DC if you prefer). The magnet creates the field for "free" so no energy is wasted. They are also good at low loads down to much lower rpm's than induction. Obviously, they have no brush losses.

With the magnet motor below it is 90-95% efficient at 10 kw from 1500-3000 rpm. Exactly the rpm's you typically use while driving.




Dewalt and all the other cordless drill manufacturer's already figured out which motor type provides the highest efficiency and longevity and the lowest weight.

They all use magnet motors.

If you want to know the weakness, again, look at the name. Magnets. They are more costly and become heat damaged at a slightly lower temperature. They areusually rated at temperature class F (311 F) instead of H (356 F)). Some make a big deal out of this, but it is only a few degrees. If you are getting any motor over 300 degrees it will not last long.

Cheers. :)
That is not true.

An induction motor has the capability to reduce the magnetic field and "overdrive" using very little energy, or work in saturation at low revs with high torque required for direct drive applications. It does, however, require complex control algoritm to operate in an efficient way. They can be driven hard or gentle with similar efficiency curves. Similar because as with any motor as you increase current you'll also increase winding, magnetic or bush losses as heat, everything else being equal.

Permanent magnets... You get nothing for free. The initial cost is higher.
Try to spin a permanent magnet or a stepper motor and you will feel the lag the magnets create just like if you try to push a magnet away from a piece of iron. They are suited for low revs at high torque, an example are e-bikes, at 350RPM typical. There are other types with weaker magnets optimized for higher revs but they will provide little torque (HP = RPM* Torque).

You can not overdrive them. if you want more speed you need more voltage. If you have an high speed PM you wont have torque at low revs, same as a shunt wired DC motor - you have a constant torque at a constant speed, reason for which they are not used as traction motors, but rather applications where a constant speed is required as the load changes.
As with temperatures, yes it is a big deal, because magnets loose their magnetic properties if heated above a certain point and this limits how hard a motor can be pushed, which is much less that what a comparable series DC motor can, so continuous ratings are considerably smaller to avoid this and they are usually more suited for motorbikes


Best application for cars for both efficiency and high operating range is either induction, series brushed DC or syncronous with field powered rotor (Same as a car alternator) where the field can be used to either provide high torque at low revs or high revs at low torque for the same net power output as you would expect on a car (1st gear as much greter torque than 5th gear, but at lower speed and vice-versa).

My opinion: There are no winners, each type has advantages and disavantages. AC is prefered over series DC on the industry because of low maintenance and lower manufacturing costs, where they have to run 24/7 during many years. On a car you would change brushes once or twice unless the motor is too small, however AC is more expensive in terms of control equipment. Foir hybrids is usually cheaper to use a syncronous or even PM because they operate at an optimun RPM range before the ICE takes over and the controll is much simpler than that of an induction motor. Again linking the best of each motor to its own application.
 
#46 ·
Yet I hear that induction motors win the efficiency contest with PMs at light loads because they have control of the field. And you trade the mechanical commutation for electronic commutation in the AC-DC argument or in other words; replace carbon switches with silicon switches. So it is a tradeoff of efficiency, durability, cost and which type you are offering for sale, isn't it ;)
 
#48 ·
Guys, I just posted the efficiency map put out by Siemens using their own inverter. How can you possibly claim that with a proper algorithm they are much better. Here is one of the best companies matching their own motor and controller. The efficiency results are not good. I provided data, not wishful thinking.

As I said, the name of the motor gives the weakness. Magnet motors have magnets which are more expensive to produce and have their own limitations such as lower heat resistance and lower rpm potential.

Magnet motors do not all "cog" unless the leads are connected (regen). This is a huge misconception. My 180 kw magnet motor can smoothly be spun by hand much easier than an 8" brushed dc.

Major, if I thought induction motors were better I would use those. They aren't, so I don't. You were just touting the efficiency of the Remy magnet motor on another thread. Called it a 'real' motor if I remember correctly.

Cheers. :)
 
#49 ·
Shocking - ruckus thinks the BLDC motor is the second coming incarnate!

What non-engineers seem to always fail to grasp is that engineering is a study in compromise. There is no "ultimate" motor. Just like there is no ultimate switch, transformer, resistor, capacitor, etc...

Yes, some portion of the phase current rating of an inverter is squandered creating the field in an induction motor, but that current sloshes back and forth between the input capacitor and winding inductance, with just a few percent lost to the switches. In other words, creating the field in an induction motor does not result in a huge drain on the battery pack.

In contrast, AC motors with PM fields have *terrible* efficiency at light loads because you are stuck with maximum field strength all the time, whether you need it or not.

As for demagnetization from overtemp, I agree that that's an unlikely scenario, but what you seem to be overlooking is that it is quite possible to demagnetize the PMs from too much phase current, or if the phase current is energized at the wrong time (from encoder error or the like).

In the end, BLDC/PMAC/PMSM motors are somewhat delicate and therefore perhaps not the best choice for use in a demanding environment like an on-road vehicle.
 
#50 · (Edited)
Shocking - ruckus thinks the BLDC motor is the second coming incarnate!
har har, that must be why I listed several downsides of each motor type, including magnet motors, right? The question is which is most efficient. I provided the correct answer.

In contrast, AC motors with PM fields have *terrible* efficiency at light loads because you are stuck with maximum field strength all the time, whether you need it or not...
The data I provided proves just the opposite. You can't 'wish' something true. You actually have to show data if you want anybody to believe you.

..but what you seem to be overlooking is that it is quite possible to demagnetize the PMs from too much phase current...
Really? What gave you the idea I was overlooking that? Last I checked, every motor can be destroyed by too much current. At least, that's what the non-engineer types tell me (the ones that actually build and use EV's).


In the end, BLDC/PMAC/PMSM motors are somewhat delicate and therefore perhaps not the best choice for use in a demanding environment like an on-road vehicle.
I guess Toyota and GM didn't get the memo.. :rolleyes: ..or all those people working in mining, or the folks who build battleship turrets for the Navy..
 
#53 ·
One thing that people sometimes forget is that the magnitizing current in an induction motor ( or any inductive load for that matter) does not consume energy. I does increase current, but that current is out of phase with the voltage so it does not consume real power. It does decrease the power factor, and the resulting higher current will lead to some real power losses as it passes through resistive elements, but the magnetization itself does not require power.
 
#55 ·
I'm not sure this is a fair comparison. The AC motor seems to be designed for 8000 RPM while the BLDC is 4000 RPM. It's just a simple change in number of poles or a 2x change in drive ratio to make it more comparable.

The 80% efficiency line of the BLDC applies at 678 Nm at 1000 RPM (71 kW) and 325 Nm at 500 RPM (17 kW). The 85% line applies at 474 Nm at 1000 RPM (50 kW). The 90% line applies at 271 Nm at 1000 RPM (28 kW) and 881 Nm at 3000 RPM (277 kW). But I have difficulty believing that last figure, and the Scott Drive 200 system motor (which I assume this to be) is rated at 200 kW.

Now for the ACIM. The 80% efficiency line applies at 200 Nm at 1500 RPM (31.5 kW) and also at 50 Nm (8 kW), and 10 Nm at 8000 RPM (6.8 kW). The 85% line applies at 190 Nm at 2500 RPM (50 kW) and at 20 Nm at 8000 RPM (17 kW). The 90% line applies at 150 Nm at 4600 RPM (72 kW) and 30 Nm at 8000 RPM (25 kW).

If you use 1/2 the RPM values for the induction machine, you get 80-90% efficiency at "normal driving speeds", which translates to typical ICE values of 1000-4000 RPM. And the induction motor is still 80% efficient at the equivalent of 750 RPM and 400 Nm, where the BLDC is also 80%. :p

The BLDC may be doomed for large scale use in EVs, mostly because of the rare earth magnet materials whose cost will skyrocket if the demand goes up by 2x or 3x. BLDCs probably account for only 10% of EVs and a much lower percentage of full size DIY conversions. And the magnets can be ruined by just a short application of very high current, as may happen with a shorted IGBT or MOSFET that blows a fuse at 100 mSec but 20x rated current. An ACIM would easily "shrug it off" with no damage whatsoever. ;)

You may want to check my calculations and I may not be reading these graphs correctly, but that notwithstanding, motor efficiencies of 80% or better are quite good, and if that occurs at low power levels, the actual losses are not that bad.
 
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