Electric 600V Design Questions

[coleasterling]

Thanks everyone!

I don't mean to get too far off the topic, but ~300V has been pretty normal for hybrids. An interesting note is that many of them use boost converters to up battery voltage to the inverter. The older Prius boost converter takes voltage from ~200V to ~500V. I believe it was rated for 20kW? The Lexus RX450h ups that to boost to 650V for their rear diff motor.

I don't have much of an opinion as to whether higher voltages should be allowed or not, although I do see some extra safety concerns. What I would really like to know is why European teams feel the need to run such high voltages, anyway. I would understand if teams were using salvage EV motors that were designed for 500V+, but teams are getting custom motors, anyway. Torque density, assuming the same stator and rotor configuration, is directly related to copper fill and that's (probably?) not significantly changing between winds. Heavier cables?



Higher voltages do a lot of things that benefit electric motors... which are not at all my specialty but I'll give the summary a go. The TLDR is power equals Torque * rotations and higher voltages let you stick in more power in a system to get out more rotations. Gear that to give you more torque and you're in business. Additionally, race cars need to be light. P = V^2/R, so if you want to shove the same amount of power into a motor winding, a 12V motor would need wires with 100 times less resistance than a 144V system. That's additional weight to handle the same power output. There are certainly diminishing returns when you go higher and higher in rpms because of emf etc, but I think the crossover point for efficiency with current tech still points to higher voltages than 300V being ideal, which must be why ever car manufacturer has packs at a higher voltage.



@Jay you're on the right track. The high voltages let us use less current which translates to lighter components and more efficient designs. Comp limits the power used and the voltage used, with that you can easily figure out the current needed to apply max power at competition. Also remember your battery voltage will drop as power is drawn and the pack empties so that increases currents if you want the same power (this is why some EVs slow down as endurance finishes). It all ends in high currents which require special, heavy and hard to find parts. There's a reason Formula E uses ~1000V system: it's light and powerful.


I'm not sure I agree with that line of thought, at least without seeing the math. Some things are simple like cables. Yes, they have to be bigger, that's obvious. How much weight does that actually add?

Look at the accumulator to start. For a given pack capacity using the same cell, assuming similar cooling efficiencies, heat wasted in the cells is going to be approximately equal in either low or high voltage configurations. How good are your cell to cell connections? Take a very low voltage system, let's say 100V. That's around ~24 series cells vs. ~140 for 600V. You have around the same number of total connections, but far fewer series cells to contribute to the IR, with better current sharing for more parallel cells. Total pack resistance is easy to drastically increase from cell connections that are sub-par.

On the motor side: For a given stator and rotor configuration, torque is going to be equal, neglecting difference in copper fill. You probably would gain some torque with a higher voltage wind since copper fill would be slightly better with more turns of smaller diameter wire, but is it significant?

I don't know enough about the inverter design to make any comments on it. I'd love to hear about why higher voltages helps/hurts inverter efficiencies, though!

[Adam Farabaugh]

Want to start off saying that I don't think the voltage limit is TOO debilitating. I don't think it ruins the EV competition in the US. That being said there are some limitations.

1) Most off-the-shelf equipment that would work well for FSAE (talking specifically powertrain components) has a performance sweet-spot around 400-450VDC. Lower voltages effectively put a speed limit on off-the-shelf equipment (I'm looking at you, EMRAX). There are some creative ways to skirt this but the easiest would just be higher voltages.

2) I spent a lot of my FSAE-E time with the accumulator. Raising voltage means you can lower currents. I would love to have less cells in parallel and more in series. That would mean you don't have to worry about fusing between parallel strings, currents through your interconnects are lower so you can worry a little less about interconnect resistance, as well as the oft-quoted weight savings from longer conductors. I believe it's easier and safer to design around dielectric criteria to deal with higher voltages than to design around thermal criteria to manage large currents.

3) the 300V ceiling is not high enough to open up the design space. It's a no-brainer to run as close as possible to a 300V bus if you want to be competitive, especially with off-the-shelf equipment. If the limit is 600V, I don't think you'd actually see 600V buses... I think you would see a lot of teams actually making a choice rather than taking the de facto best option.

[JulianH]

Want to start off saying that I don't think the voltage limit is TOO debilitating. I don't think it ruins the EV competition in the US. That being said there are some limitations.



I have no experience in motor design. I just know that a higher voltage has some advantages (and that's in the end why the most are using it in Europe).
It does not explicitly ruins the competition. It is though implicitly ruining the competition as it simply outlaws the European cars. As you design a car for your "home-turf" and maybe make and oversea event as add-on (like a lot of the US teams do, and the European combustion cars), you will not limit yourself to 300V in order to compete in the US. Simple as that.

[sekl]

I would not call myself an expert on this either, my experience is more with batteries than with inverters and motors, but here's some thoughts...

Battery:
From a cell perspective a higher voltage has not much of an impact. P=U*I and E=U*C... You need a certain amount of energy to move your car far enough and a certain amount of power to move it quick enough, so you will increase the capacity of the pack if the voltage is lower, which in turn means the cells will allow larger current draw, so the available output power remains unchanged. There is a weight penalty on the contactors, the fuse becomes more difficult to handle, conectors and connections in general must be sized bigger... However, the amount of cells needed does not change as this is driven by the energy needed. Main limitation on the battery, I guess, is that cells that the lower voltage limit reduces the amount of cells that match the requirements well - as in, enable one to physically build a pack with the target energy and power.

Adam, what do you mean with "fusing between parallel strings"? I can not imagine a reason why you would go with a series first, instead of parallel first, configuration, help me out please.

Inverter:
Most power modules (typically IGBTs) are available in either 600V or 1200V technology, effectively limiting the maximum voltage possible. Higher voltage technology typically means higher DC losses. At the same time, gate capacity and inductance of IGBT modules limit the maximum switching frequency. Now, power losses and switching frequency are limiting factors in inverter design. Switching losses go up, DC losses go down at higher voltages. It depends on the application, but to my understanding often times the higher voltage benefits more than it hurts as there is ways to lower switching losses but not much can be done to lower DC losses on application level. Specially at low RPM and high torque output the DC losses are an issue to my understanding.

Motor:
Now, here I think is the key for FSAE application. A higher voltage will allow a relatively higher peak power output for a given machine. That means, a smaller machine can give a higher peak power at a higher voltage. The continuous power output of an electrical machine is mainly determined by how good you can cool it while the peak output for short duration is mainly defined by P=U^2/R. The maximum voltage is limited by the isolation properties of the machine. In FSAE, the continuous power needed is rather low, let's say 25kW. To get decent torque out of the machine, specially at high RPM, however, it takes some more power. So, what you want is a machine that is just big enough to cool away the losses at 25kW but still can deliver 80kW peak power for short time. To achieve this ratio of cont. to peak power is easier at higher voltages, is my understanding.


On the safety concerns - I recommend to stay in safe distance from 300V just as from 600V. The risk of an electric shock causing serious damage to your body is given at 300V already. Risks due to high currents are also given in any case, even for far below 300V systems, any metal that shorts a high power lithium battery will become liquid and that can cause series injuries. Actually, I consider the risk due to short circuits and resulting excessive currents higher than the risk due to electric shock, so protect your eyes and skin when working with batteries.


I agree that FSAE could run at 300V without problem from a technical perspective. The issues I see with the limitation is that it reduces available off the shelf parts that fit FSAE well, teams that consider custom parts as impossible take what they can get and not even think about what they actually need - as in, start with system design before you look at specific components... If the rules change in Europe, teams change their designs and those at the top now will still perform well - because they do the system design part well and then do what it takes to realize what they think is best.

[coleasterling]

Thank you all for the comments!

I believe the reason Adam wants to go with series strings that are then paralleled is mainly due to the fusing requirements in the rules. In 2011, we ended up running series strings in parallel because it drastically reduced the number of fuses needed for the pack per the rules. Pretty much no other reason than that. It certainly isn't the most robust way to build a pack.

I think you're right on the money as far as the accumulator goes. For a given cell and capacity target, I^2 losses will be the same for any configuration, neglecting differences in interconnects and the like. Weight-wise, it isn't like we're talking thousands of amps here. A lowly EV200 is rated at 500A continuous(with large conductors of course, but no one will be running 500A cont.) and weighs just under a pound. What about higher voltage rated fuses vs. higher current rated? Weight is a lot closer than you'd think for your average semiconductor fuse. I'm sure there are some outliers of some configuration that are much lighter, but I haven't exhaustively searched for it.

I'm not sure I agree with you motor-wise. P = V^2/R doesn't really apply as we're always current limited. Yes, increasing voltage increases power assuming no change in current, in general. The problem with that reasoning is that the current limit for a given motor goes down with an increase in voltage. The winding resistance is necessarily higher for a high-voltage wind so your power handling doesn't increase.

Look at the Emrax spec sheet here: http://www.enstroj.si/images/stories/emrax_228_tech_data_table_dec_2014.pdf

Notice that the efficiency, peak torque, and peak and continuous power are the same across all configurations. Also note that the rated speed doesn't change. The only thing that will give you more power for a given motor is to drastically increase the amount of copper in the gap, which increases torque. You can over drive the motor, but that can be done in any winding configuration.

Edit: I should also say that it is possible to drive a motor to rpm's high enough that non-copper losses (Iron, mechanical mainly) start dominating and you drastically lose efficiency. I would think (although not 100% sure!) that the peak power for a given configuration would occur at an RPM just before those losses take over. You'd want to stay on the controller's current limit and to get there your voltage would have to be high enough to keep the BEMF from killing your motor current. That would give you the absolute max rpm the motor could sustain and keep the current pegged, giving you the highest power possible for that configuration. In that case, higher voltage is better, but only to a point. If you've already chosen an extremely high voltage wind, then that doesn't leave you with any head room for this scenario or to account for sag for that matter.

[sekl]

I don't understand which rule you refer to, can you post some more detail here which rule you mean and how you place fuses, please? Maybe I just don't understand the words...

On component weights in the battery some arguing is possible but I think that's mostly fine tuning already, not so much the topic for this discussion.

For the motor, I was more thinking of characteristics as shown in this datasheet.
http://www.yasamotors.com/wp-content/uploads/2014/07/Datasheet-YASA-750_en-ID-15637.pdf
Yes, current is not really short circuit current in the end, I guess the time would become too short at some point, but I do not see a technical reason to limit to constant power. All you need to ensure is that the isolation materials used do not overheat, so higher peak power for shorter time is possible and being less conservative is an option, too. For this, enough detailed knowledge of the component is needed, but in general it is possible to my knowledge. To give you an example, I have seen an FSAE car run a machine designed for 150V at 400V. Peak power output was significantly increased while lifetime was significantly decreased, but as it was designed for 10 years lifetime, that was no problem. It was possible because the supplier did support the team with simulation and application support.

[Thijs]

Want to start off saying that I don't think the voltage limit is TOO debilitating

Agreed. As a matter of fact, plenty of European top 10 teams run closer to 400V than 600V. I'm convinced it's possible to build a 300V E-car that beats 95% of all combustion cars.
It's just that if European teams think going to 470 or 588 will make them 5% faster, why wouldn't they do so, given how competitive the races are?
That absolutely doesn't mean that you need that in order to be able to for example run sub 4 second acceleration runs.

You made a point on the other thread that probably explains a lot more about the current state of the US E-competition:


It seems to me that the euro teams who dropped ICE and switched to EV already had good ICE programs, and passed down their knowledge of how to build a good car (nevermind what powertrain)

For example, DUT11 was our first electric car. It won both FSUK 1A and FSE with just 56kW peak power from off the shelf motors, 400V and no aero, against teams with a year more experience in FSE, and more power on board.
It ran a 3.84 on acceleration in FSG and in autocross at FSUK it was faster than all combustion cars except for GFR (who came fresh off an FSAE Mi win, and were running the first of their amazingly successful aero designs).
(obviously, DUT11 would get crushed by the top 5 cars today, but just to show that there's no excuse for running 5 second acceleration runs).

Of course, the reason it did well wasn't because it had the best theoretical specs attainable within the rules, which it clearly didn't, but the same reason all successful FSAE cars do well:
It was a simple, lightweight car which got the basics right. It had great build quality, which largely explains why it was reliable enough to have some proper testing and driver training before the events.

In other words: FSAE-E teams do themselves a huge disservice by losing any sleep over things they're missing out on for lack of volts (or money for that matter). Those will only get you the last 10 or 5% once you've got the first 90 or 95% down.
In order to build a (more than) acceptable E-car, teams first have to focus on what makes a good FSAE car good, no matter what powertrain.

The first team to have the guts not to accept 250+kg (or rather 200 actually) and crappy build quality and the lack of reliability that results, will be the guys to set their team on a path to consistent top placings for years to come, and since they will have a car that actually runs, testing their car throughout the season and proper driver training will only result in extending that lead over the years.


European teams only stand to gain from having a healthy Electric competition in the States, so like Julian said, most teams will be happy to help out. In my experience, teams are usually simply proud of what they've built in a year, and are eager to discuss their designs with anyone, except perhaps for tiny details they think competitors haven't really figured out yet that give them an edge, but there aren't too many of those...
However, the best place to do this would definitely be at the competitions. Everybody is always busy throughout the year, and exchanging emails will never come close to the opportunity to look at dozens of finished cars and talk face to face to team members. So book your tickets to Germany now! :) (But first, have a closer look at fast ICE cars in Lincoln)

[coleasterling]

Thijs,

THANK YOU! This is exactly what I have been getting at. The voltage limit should not be an issue for most teams, and I believe shouldn't be an issue for ANY team. Whining about not being able to use higher voltage is useless when there's still so much you can do. It "slightly" limits the design space as Adam said, but there are plenty of options, especially if teams look outside the normal Yasa or Emrax.

That said, I intended this to be a technical discussion of why high voltage is actually better, but haven't really seen anything to convince me of that beyond that we know interconnects are heavier.

Sekl,

EV6.1.4 If multiple parallel batteries, capacitors, strings of batteries or strings of capacitors are used then
each string must be individually fused to protect all the components on that string. Any conductors,
for example wires, busbars, cells etc. conducting the entire pack current must be appropriately sized
for the total current that the individual fuses could transmit or an additional fuse must be used to
protect the conductors.

The YASA has different power levels shown because it is the same winding configuration between the two plots. The time gets too short in less time (ha) than you would think. What good is 1000hp if you can only do it for a millisecond? How far do you have to stretch that out before it becomes useful?

Yes, you can significantly over run most motors, especially industrial. As I said before, RPM is limited by how fast it can spin before non-copper losses dominate or it mechanically fails. You can do this with ANY winding configuration of that motor, so you could get that RPM with a lower voltage wind and more current just as easily as with a high voltage wind.

The continuous power limitation is pretty much just thermal.

[Thijs]

Cole,

I'm not sure you'll get a much more satisfying answer than what has already been offered. In the end it comes down to meeting your performance goals:
- tire limited rather than torque limited
- power limited by rules rather than powertrain
- not (significantly) limited by top speed
(in that order)
All while carrying enough energy to drive 22km at speed.

Using higher voltages will let you do that at lower mass. Of course any efficiency gains translate directly into lower mass as well, because your battery capacity can be smaller.

You may either underestimate how much there is to be gained (lost?) or not be impressed by 'only' losing a couple of kg's, but I can't really help you there.
Looking at it differently though: If you have nice off the shelf 500V motors available (or you're going to design custom motors anyway), you might as well go for the option that saves you some extra mass.

Although it didn't really address your question, my previous post was written primarily because I hope that no US teams would get the impression from this thread that the 300V/600V thing is what's holding them back, but I'm glad to see that we already agree on that.

BTW, that doesn't mean that I don't still find it a very lame move of the US competition to effectively exclude European teams from coming to the US. I can't help but think they knew exactly what they were doing when they set that limit. I hope they see that not only did that strategy not help, it has completely backfired.
When FSUK was first organized in 2001, of course 6 teams out of the top 7 were from the US. So what. I'm sure that exposure only benefited European teams in the long run.
Perhaps the SAE was hoping to give US E-teams some time to mature, but instead they now lag the European competitions (where the level has increased significantly between 2012 and 2015) by an extra 3 years.

Thijs

[Swiftus]

My complaint about the 300V limit in the US is exactly what Thijs has said. Why create a situation where 3/4ths of the competition is capable of designing a car which is ineligible for one continent's competition? It just seems silly even before you get in to any details about which way is better. The idea behind the international rules committee is to create a set of rules which are universal throughout all of the competitions, encouraging students to travel and see each other's engineering solutions.

A top ten Euro-spec electric car would easily beat any US electric car under the current situation. The problem is the isolation create for the US competitions has failed to spark enough innovation in thinking for the US students to get their cars up to a world stage competitiveness (2 puns in 1 sentence - woo!). I'm not trying to insult the thinking of US students by any means - just trying to demonstrate that when you see one of the top electric cars on track you begin to start second guessing your understanding of physics. They just look out of this world.

But in a more general sense, more open rules require students to think through their design decisions more and figure out how their overall vehicle concept affects the subsystem design requirements. Do you sacrifice everything for the sake of lightness? Put around the track in endurance because your battery doesn't have the capacity for an all-out attack the entire time? Or do you pack as much motor and battery as you can in to the car to guarantee full speed throughout endurance? Or is the choice in the middle.

The difference between 300V and 600V in the matters of safety regulations seems a little odd to me. 300V+ systems have been running in American buildings for many years. Our machine shop at OSU has machines which run on 480V and are 25 years old. As I said in the EPA thread, it seems that most of the electric cars being sold to the consumer market are in the 450V range. The equipment is out there for more than 300V and it seems unfounded to me as to why it is prevented from being used.

Maxing out a rules set doesn't always work. In combustion, GFR had run less than 610cc for 7 years and been quite successful while doing so. But at the same time, teams like Stuttgart and Florida have managed to make cars equally as quick on 610cc platforms. Different overall concepts with largely the same overall result. Narrowing the rules narrows the possibilities which forces design differences in minutia rather than the overall systems - since everyone moves to an identical vehicle concept.

[Thijs]

Just for fun, I made a graph that shows battery voltage distribution at the last German competition.

http://www.fsae.com/forums/attachment.php?attachmentid=1003&d=1455817303

And as a bonus:

http://www.fsae.com/forums/attachment.php?attachmentid=1001&d=1455816599

A grand total of one team would have been eligible to run in the US
(namely the Indian team with the 134V battery. The guys from Beijing actually cited 302V, so I guess they wouldn't have been allowed either)

Update: just found out that the 300V team runs 2 300V batteries in series, so actually 600V. I've updated all graphs accordingly

Thijs

[nat45928]

There is a team from the Czech Republic registered this year. I am not sure if they have plans for any European comps or what voltage they run.


http://students.sae.org/cds/formulaseries/electric/teams/ (http://students.sae.org/cds/formulaseries/electric/teams/)

[Thijs]

According to their website, CTU were running 260V last year (their fourth car).
They've previously competed in Italy, Hungary and the Czech Republic.

I hope UTA and Washington will make it to Germany this year.

[nat45928]

Interesting. I'm hoping the bump in teams in the US helps this year across the board. The competition in the US should be well ahead of where it is with or without the voltage limit.

One thing I noticed last year is most teams had one or two people who handled electronics in the car (ourselves included). I think that is having a big impact right now in the US as teams get a car together that works or kinda works, the electrical people graduate, the knowledge disappears and the team slips. We've been having lots of issues finding people interested in electronics since our school is geared toward power systems (substation and power plant type work). We are still trying to figure out how to get that "A team" Claude has talked about.

I would be interested in seeing what types of continuous powers and currents teams are seeing with their cars at the lower and higher voltages. Our team had a very limited car last year: realistic 50kW peak, 20kW cont and 100A cont. for reference which makes me wonder how much of a weight penalty there is with the lower voltage limit.

[coleasterling]

Thijs,

That's some great information! I, too, think it would be interesting to see what kind of power teams are quoting for their stated voltage, as well as their actual competition weight.

I guess one of the main points I haven't articulated well is that I think there are much greater contributors to car weight than the voltage spec'd for a system, at a given power. Still assuming you'd gain <5lbs total at 300V vs. 600V, bare chassis design weight alone varies much more. Tire choice, too. What I see from the graphs is that the average voltage of the top 10 cars is around 450V. I haven't kept up with competition placings, but how much more dominant are the top few cars over the rest of the top 10?

[Kevin Hayward]

Thijs,

Thanks for posting the graph, it is very interesting. For clarity I have put a rough line for the Pareto set which shows the boundary of the data.

http://i.imgur.com/Ho4ROS0.jpg

Given the number of teams around 450 and 600V a t-test on the data might be interesting.

Kev

[Thijs]

more graphs!


I haven't kept up with competition placings, but how much more dominant are the top few cars over the rest of the top 10?

pretty dominant:
http://www.fsae.com/forums/attachment.php?attachmentid=995&d=1455752367

Although it's worth mentioning that (based on their autocross times) three teams would have probably been able to finish around top 5 or 10 but broke or DQ'd in endurance, so it may not be quite as bad as the above graph suggests.


I, too, think it would be interesting to see what kind of power teams are quoting for their stated voltage, as well as their actual competition weight.

http://www.fsae.com/forums/attachment.php?attachmentid=999&d=1455816589

This uses stated power and official competition weights.
Zürich is the only team that's above a boundary that appears to limit all other data points.
Do keep in mind that the highest specific power values are achieved through enormous amounts of power, that you're not actually allowed to use at the competition
(1/3 of all teams have 100+kW which is well over the power limit, and 5 teams have over 130kW)

disclaimer: I'll happily leave it up to everyone to decide for themselves if they want to conclude anything from these graphs, but yay, data! :)

[Kevin Hayward]

I performed a two-tailed t-test on the data looking at final placing. The two populations were teams above 500V and teams between 300 and 500V. The average placings were as follows:

> 500V : 16.2
300-500V : 21.2

Given the difference in mean it is worth considering a t-test. Making the assumption of unequal variance the p-value was 0.17. On that basis from the data supplied we cannot make the claim that having a voltage of above 500V is a significant indicator of finishing place when compared to 300-500V.

Obviously more data could be used to refine this value, and may show that there is a significant difference.

It is probably better to do this against dynamic points rather than position, but you work with what you have.

Actually considering just outright performance it might be worth just looking at the autocross event as the final points will be very influenced by reliability of the endurance event.

Kev

[Thijs]

Actually considering just outright performance it might be worth just looking at the autocross event as the final points will be very influenced by reliability of the endurance event.

The data is there. We can always make more graphs
http://www.fsae.com/forums/attachment.php?attachmentid=1000&d=1455816595

Although obviously this too is still very noisy data from very small data sets (6 teams from the top 10 only drove two laps) and we have no idea of the influence of driver skill, but I guess it's the closest thing we have to putting a number on outright speed for each car.

[Kevin Hayward]

Thijs,

Glad to see another graph fanatic :)

Similar look to the Pareto set, and I would expect almost no change to the t-test results. It is a shame not too have the data of a lot of 300V cars competing as well to see whether you could make a call between US and Europe rules.

Kev

[BillCobb]

I like graphs. Is there a single source for engineering parameters (weights, voltage, wheelbase, track, tires, suspension types, N-motors, motor type) vs. position and scores ? Just curious. We love Mahalanobis distances.

[Thijs]

Bill, Cole,

Unfortunately there isn't a publicly available source that has all that data in one convenient overview.
Weights, position and scores I took from the formulastudent.de results section, that stuff is freely available of course.
I would expect the Racecar Engineering data to be reasonably accurate when it comes to things like tire types, electric motor specs and basic measurements (except for weights, but there we have more reliable data available).

Cole, I think Kevin meant to say that for the sake of looking at these relations it's a shame that there aren't more 300V cars running in Europe, not that it would help to include the US competition results in the same overview.

As a side note on the specific power/voltage graph that I posted earlier: I would be very careful in trying to extract any data from that on how much choosing higher voltages contributes towards a lighter car. There may well be a correlation, but I would expect at least some of the teams that actively pursue relatively minor weight savings by switching to a higher voltage to be the same teams that were more focused on building lighter cars anyway.

Thijs

[Thijs]

You're right Kevin, points paint a fairer picture than overall positions so here's an additional comparison:

same as before for comparison:
http://www.fsae.com/forums/attachment.php?attachmentid=1001&d=1455816599

same as before but points in stead of placing:
http://www.fsae.com/forums/attachment.php?attachmentid=1004&d=1455818213

same as above but only dynamic points:
http://www.fsae.com/forums/attachment.php?attachmentid=1005&d=1455818217