Motor Junkie article spreads FUD: "20 Drawback Of Electric Vehicles Drivers Commonly Overlook"

azbill

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An electric motor has an efficiency sweet spot. Usually the drive train is geared to put this point at about 45 mph as that gives the best overall EPA rating. But if you change the gearing slightly so that this sweet spot falls at 70 mph that is where the car will be most efficient at the expense of efficiency at lower speeds.
For electric motors the "sweet spot" is at the halfway point on the speed versus torque curve (which is actually a straight line). Maximum torque can be applied at 0 RPM, and 0 torque can be applied at maximum RPM. For all the electric cars without shifting gearboxes, it is easy to know the "sweet spot", just divide the maximum speed by 2. My Bolt has a max speed listed as 85, so it has maximum efficiency at 42.5 mph. The Rivian is listed as max speed of 125, so maximum efficiency is at 62.5 mph. I have not looked up any Tesla models top speeds.

Thus, if the Lucid and Taycan can shift gears, they can be efficient at two different speeds, plus they can be capable of faster top speeds.

Even gas and diesel engines have the same type of speed versus torque curve, but their usable RPM range is much more limited, thus shifting gears is an absolute necessity.
 

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Could it be that the Rivian is electronically limited to 125 mph, in which case that does not reflect the maximum speed of the motor?

Electronically-limited Lucid Airs have attained 217 mph on test tracks and 235 mph with the limiters removed. (These tests were done with induction motors before Lucid switched to permanent magnet motors.) That would imply the efficiency "sweet spot" of those cars was 117.5 mph.

Can that be the case?
 

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The "sweet spot" of the motor/gearing has to be factored by the aero (and other) penalties imposed by speed (which will vary by vehicle). Maximum range will almost always be attained at <30 mph.
 

ajdelange

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...speed versus torque curve (which is actually a straight line). Maximum torque can be applied at 0 RPM, and 0 torque can be applied at maximum RPM.
IMtorque.png


You may have just not worded things the way you intended to but the speed vs. torque curve for an induction motor is hardly a straight line though there is a portion of it which is nearly linear between the "breakdown point" and the speed where the rotor and electric field are going at the same speed. This is the "stable" part of the characteristic where the motor can be controlled and thus where it is operated in the BEV application.

Maximum torque can be applied at 0 RPM, and 0 torque can be applied at maximum RPM.
s=1 on the curves represents 0 rpm. As the plot clearly shows the torque available at s = 1 is considerably less than the maximum available torque at the breakdown point. One can get higher locked rotor torque by increasing rotor resistance but this is at the expense of efficiency. There is also chicanery whereby high rotor resistance for maximum starting torque can be switched out for more efficient operation in the linear region.

As there is no relative motion at s = 0 there can be no torque produced at that speed but for s < 0 the torque becomes negative i.e. the "motor" starts to act like a generator (regen).


For electric motors the "sweet spot" is at the halfway point on the speed versus torque curve
For all the electric cars without shifting gearboxes, it is easy to know the "sweet spot", just divide the maximum speed by 2.
Think about this. What is the maximum speed? On the torque curve above I suppose you could call the maximum speed Ω corresponding to s = 0 but that's just the electrical speed and the electrical speed in a BEV is all over the map depending on how fast you are going and what slip you want.

In fact the most efficient speed is a function of rotor and stator resistances and the leakage inductances.

Now that's the story for induction motors. My Tesla has one of those but it also has a PMSRM with a somewhat different theory so to illustrate the point that things aren't as simple as you try to make them I'll just put up this efficiency contour diagram for a real experimental motor (more about it than that I do not know). I don't recall what Rivian is using for motors. The SW concept is very appealing because of no rotor copper loss (or expense) and 0 no rotor hysteresis loss.
Efficiency 2.png

The sweet spot is about 1/3 of the way between the origin and the maximum plotted torque and maximum plotted speed. The real message here is that the "spot" is huge.


My Bolt has a max speed listed as 85, so it has maximum efficiency at 42.5 mph. The Rivian is listed as max speed of 125, so maximum efficiency is at 62.5 mph. I have not looked up any Tesla models top speeds.
I hope I have adequately explained why it doesn't work that way.

Even gas and diesel engines have the same type of speed versus torque curve, but their usable RPM range is much more limited, thus shifting gears is an absolute necessity.
Ice engine torque curves are more inverted U shape than electric motor curves.
 
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ajdelange

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The "sweet spot" of the motor/gearing has to be factored by the aero (and other) penalties imposed by speed (which will vary by vehicle). Maximum range will almost always be attained at <30 mph.
Indeed the "spot" isn't a spot. It is a range of speeds and that range does, of course, depend on a whole host of things besides the motor but the motor is clearly a major contributor. The data I presented earlier show that the sweet range is, for my Model X as, where, and when I have been driving it, between 30 and 47 mph or so but the absolute best performance comes at 37.714 mph.
 

ajdelange

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Could it be that the Rivian is electronically limited to 125 mph, in which case that does not reflect the maximum speed of the motor?
No because there is, broadly speaking, no maxiumum speed for an AC motor. You want it to go 10% faster you just increase the drive frequency by 10%. Now of course there are practical limits such as vibration, hysteresis loss, skin effect, inverter efficiency etc. all of which relate to frequency. But when it comes to a BEV what limits the maximum speed is, ultimately, the drag and the rate at which power can be safely drawn from the battery. When you accelerate out of the blocks you need lots of torque which translates to thrust and accelerates the car. a = F/m in which a is the acceleration, m the mass and F the thrust. F = r*T in which T is torque and r wheel radius and the power is T*Ω = Ω*m*a/r. When Ω, the wheel speed is small, the power required to accelerate is small but when speed builds up it is large and, as the battery/inverter can only supply so much a must be reduced. Furthermore, as the speed goes up drag begins to become the dominant sink for energy. Eventually drag plus wheel resistance etc. consume all the power that the battery can deliver and none id available for further acceleration. At that point one can go no faster. This is what sets the maximum speed.

Now the total available power can be set by the manufacturer. His supply may be capable of delivering 300 kW but he may wish to limit what he allows it to deliver to 250 kW for various reasons (the obvious one of which is that 300 kW is lots more stressful on system components than 250 kW).

Electronically-limited Lucid Airs have attained 217 mph on test tracks and 235 mph with the limiters removed.
Knowing, as we do, that drag is the dominant power sink at such speeds we can easily calculate that those extra 18 mph will require 17 % more power from the batteries. That could be up to 50 kW which is a lot of power.


That would imply the efficiency "sweet spot" of those cars was 117.5 mph.
No, it wouldn't. The car's sweet spot has little to do with it's top speed.



Can that be the case?
No.
 

Hmp10

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The car's sweet spot has little to do with it's top speed.
I was responding to azbill, who posted:

"For all the electric cars without shifting gearboxes, it is easy to know the 'sweet spot', just divide the maximum speed by 2. My Bolt has a max speed listed as 85, so it has maximum efficiency at 42.5 mph. The Rivian is listed as max speed of 125, so maximum efficiency is at 62.5 mph."

Both you and azbill are engineers. Yet, if I'm understanding each of you, you're giving conflicting views here.

I have found there are very few technical questions I try to research about EVs for which I don't find almost diametrically opposed answers from different sources. While it can help the time pass while waiting months or years for one's new EV to arrive, It's very confusing for a non-engineer to sort out. I guess most buyers just have to rely on the assumption that the manufacturer has made the optimal trade-offs for producing whatever mix of performance characteristics are being advertised.
 

ajdelange

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Both you and azbill are engineers. Yet, if I'm understanding each of you, you're giving conflicting views here.
Yes, definitely.

I have found there are very few technical questions I try to research about EVs for which I don't find almost diametrically opposed answers from different sources.
Yes, there is definitely a LOT of misinformation out there. The idea that the most efficient speed for a vehicle is half it's top speed is an example. My X's top speed is 155 mph. If the 1/2 rule applied its most efficient speed would be 77.5 mph and, as the data I have posted shows it is actually about half that (37.7 mph) or 1/4 its top speed.


While it can help the time pass while waiting months or years for one's new EV to arrive, It's very confusing for a non-engineer to sort out.
Yes, I understand that and I spend a lot of time trying to make people understand where there are errors and why the presented information is not correct. An engineer should perceive, based on understranding of his art, that the notion of greatest efficiency based on half top rated speed is absurd. The Taycan has been driven at 167 mph. Does anyone think it is most efficient at 84 mph? Not anyone who understands anything about the aerodynamics of cars.

I guess most buyers just have to rely on the assumption that the manufacturer has made the optimal trade-offs for producing whatever mix of performance characteristics are being advertised.
As for the internet in general I don't know what to say. I guess you can go to sites like this one and try to figure out who really knows what he is talking about and who doesn't. The real problem is that often those most assertive in their proclamations are the ones that know the least (Dunning-Kruger effect). It's easy for me to spot those guys in part because of my engineering background but also because they exhibit certain patterns. A non technical person can look for those I suppose. At the worst the discussion turns from the subject at hand to ad hominem attacks. More usually, fortunately, the main sign is that if challenged with a reasonable counter position they will ignore what the challenger says or change the subject. But they will almost never admit they are wrong. It's hard to do (I've had to do it).
 

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An undated Motor Junkie article does it's best to spread FUD on EVs.

"20 Drawback Of Electric Vehicles Drivers Commonly Overlook"

The article mentions the Mustang Mach-E so I'm assuming the article can't be too old. Regardless, the lack of references and incorrect or exaggerated information peppered throughout the article makes this IMO a hit piece on EVs. Unfortunately the article does not have a Comment section, but FWIW there is a Contact Us section. ;) I'm working on my email.
Wow. An uninformed compilation of hearsay and selected but not broadly applicable facts. Not sure we will ever get back the time put into reading this. So you may want to reconsider putting more time in on an email to "Contact Us". Curious though to see if it evokes any response. They may only be counting clicks.
 

azbill

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Yes, definitely.

Yes, there is definitely a LOT of misinformation out there. The idea that the most efficient speed for a vehicle is half it's top speed is an example. My X's top speed is 155 mph. If the 1/2 rule applied its most efficient speed would be 77.5 mph and, as the data I have posted shows it is actually about half that (37.7 mph) or 1/4 its top speed.


Yes, I understand that and I spend a lot of time trying to make people understand where there are errors and why the presented information is not correct. An engineer should perceive, based on understranding of his art, that the notion of greatest efficiency based on half top rated speed is absurd. The Taycan has been driven at 167 mph. Does anyone think it is most efficient at 84 mph? Not anyone who understands anything about the aerodynamics of cars.

As for the internet in general I don't know what to say. I guess you can go to sites like this one and try to figure out who really knows what he is talking about and who doesn't. The real problem is that often those most assertive in their proclamations are the ones that know the least (Dunning-Kruger effect). It's easy for me to spot those guys in part because of my engineering background but also because they exhibit certain patterns. A non technical person can look for those I suppose. At the worst the discussion turns from the subject at hand to ad hominem attacks. More usually, fortunately, the main sign is that if challenged with a reasonable counter position they will ignore what the challenger says or change the subject. But they will almost never admit they are wrong. It's hard to do (I've had to do it).
I was talking about DC motors, which I have designed control systems for. I was not thinking about induction motors. The sweet spot I was talking about was for horsepower, not for range. Some electric vehicles are using induction motors, some are using DC. I think I heard one of the Tesla's has some combination of the two. Not sure what Rivian is using.
 

azbill

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I investigated some existing EVs and this is what I found:

DC Brushless motors - Bolt, Leaf
Synchronous AC motors - I-Pace, Niro, Kona, Tesla M3
Induction Motors - Tesla S & X, Etron
 

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ajdelange

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A lot of the misinformation you post could be avoided if you would only check on what you write before you hit the Post button. For example:
I was talking about DC motors, which I have designed control systems for. I was not thinking about induction motors. .... Some electric vehicles are using induction motors, some are using DC. I think I heard one of the Tesla's has some combination of the two. Not sure what Rivian is using.
Followed by:
I investigated some existing EVs and this is what I found:

DC Brushless motors - Bolt, Leaf
Synchronous AC motors - I-Pace, Niro, Kona, Tesla M3
Induction Motors - Tesla S & X, Etron
So no car is using a DC motor and a couple of Teslas have a mix of induction and permanent magnet motors (the X has one of each and I assume the S does too but I wouldn't state that as a fact unless I checked it). It would be so easy to do the verification of the facts first and then not make the first post or to at least go back and edit it.

Even if DC motors were in use your original statements about them were incorrect as you could easily verify with a simple online search. Apparently it has been a long time since you have worked on motor controllers. Please check what you post.
 

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@ajdelange i think we all need to understand it’s a forum and not a fact site. Don’t want to make people worried of backlash from others on what they post.
 

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