11.1v, good or bad?

[c0sie]

This is a 'debate' that the micro section drivers are having at the moment and id be interested to hear everyones opinions on this.

Basically it has been suggested that running 11.1v LiPo on a brushless system will make the electrics (ESC and motor) run cooler, eliminate any form of cogging or initial throttle hesitation and will not cause issue with the increased power and revs available.

The argument against is that due to the increased input power the electrics will actually run hotter, and that the increase in speed/revs/available power will generally increase the change of less experienced drivers having accidents.

There are two main parties arguing this one out:
One has experience of 11.1V in 2WD and 4WD (micro) and although seemingly has no issue in 2WD, has admitted to having to turn the throttle EPA's down to reduce the top end of their car.
The other party has limited experience with 4WD 11.1v, mainly due to blowing up a ESC AND motor whilst using his 11.1v cell pack for the first time under my supervision.

Bear in mind that currently 99% of micro scale brushless systems are safely rated past 11.1v by their manufacturers.

Again, purely to satisfy my curiosity im keen to hear the opinions of others :)

[Skiddins]

Might be worth asking on an RC helicopter forum etc as they use packs up to at least 22V :eek:

Perhaps someone will there be able to compare uprating the voltage?

Skiddins

[c0sie]

Yeah I understand that probably no one has actual expereince with 11.1v, but there are probably afew electrical guru's here on the forum :D

Just interested to hear the theory behind peoples thoughts really :)

[Ian s]

Lets says an ESC/motor combo is designed to make X power at 7.4v

another combo makes the same X power at 11.1v then the 11.1v system will run cooler.

If you run the same system/combo at 7.4v and then 11.1v it will get hotter on 11.1v.

So systems can both be hotter and cooler at 11.1v depending on what your comparing it against.

[c0sie]

Ian,
Just so I understand your post, how are you working out your power generated?

I think my main reason for posting this is to see if others (just hyperthetically) would consider running 11.1V's in their cars if they were allowed as is the scenario with us micro lovers.

[Ian s]

Im not working out the power at all, you could say "equivilent speed" or something along those lines.

In your case none of this matters, if you are trying to convince people who currently run 7.4v to run 11.1v they are right in saying everything will get hotter.

[kevin bissell]

Ian

Can you please explain to a dimwit like me how a system that is designed to output X Watts at a given voltage would somehow run cooler if it was designed to output the same wattage at a higher voltage.

Does this mean that if I went to a country where the domestic electricity supply was 110V, all of the electrical equipment (which would be designed to operate at 110V) would run hotter than it does in this country where the voltage is at least double?

[Mark Christopher]

the higher voltage will run cooler, spoke to trakpower about this, i carnt remeber the reason though!

[c0sie]

How do you guys see the arguement that 11.1V gives the chance of not very experienced drivers going even faster??

[big-chris]

high voltage low current is more efficient i remember too , cant remember why to be honest but thats part of the reason they do the national grid at 250kV or something like that slow the motor down and up the voltage would be the ideal i think

[terry.sc]

Ian

Can you please explain to a dimwit like me how a system that is designed to output X Watts at a given voltage would somehow run cooler if it was designed to output the same wattage at a higher voltage.

Does this mean that if I went to a country where the domestic electricity supply was 110V, all of the electrical equipment (which would be designed to operate at 110V) would run hotter than it does in this country where the voltage is at least double?
The math is simple.

Power = Voltage X Current

So if you double the voltage you only need half the current to get the same power output. As there is half the current flowing there is less heat generated by the resistance in the wiring so everything runs cooler.

For example in the UK the 240v we have means you can buy 3kW kettles, yet in the US with 110v the maximum is 2kW because the electrical wiring can't handle the extra current needed as at 110v a 3kW kettle would need 27A of current compared with 12.5A.



The only problem with allowing 11.1v is that people won't be trading in their motors for slower ones, so keeping the same speed with more efficiency. No, they'll be running the same motors on the higher voltage for even more speed.

[kevin bissell]

Terry.

I agree, the maths is simple, but the theory is not. From my (admittedly dim and distant) recollection of electric motors and speed control theory, there are many more factors that come into play. I don't believe that just upping the voltage increases the efficiency of a system and the heat that is being given off is an inefficiency... energy that is not being transformed into useable power.

Anyway, miles off topic, and you're absolutely spot on in saying that increasing the voltage would inevitably mean more power. I've got more than enough power without 11.1 volts than you very much.

[mlitton]

Anorak on.... ;)

As already mentioned, Power = IV (Current x Voltage).
Power lost in a typical electrical system is mainly due to I*I*R (Current squared x resistance).

This means that if we double the voltage of a system while maintaining the SAME power output (THAT is the crucial bit!) we will have halved the current and potentially quartered the losses!

In theory this sounds brilliant but there are problems when the world of electric racing is considered:

The crucial bit of maintaining current power levels is the problem. If the cells have the capacity to not dump then you will see people attempt to run say a 3.5BL at 11.1V which is likely to result in premature eqipment failure.

Silly power combinations could be eliminated by two methods; both with problems:
1/Motor wind limits. Consider the 12T limit and it's problems. A turn limit would introduce BR/BL equivalency arguments into the mod class.

2/Cell capacity limits. This would limit speed as an ultra hot setup simply wouldn't last 5 minutes. However this would then introduce a cell capacity war which would be expensive, not to mention the expense of dead LiPo's if a cutout wasn't used....

Anorak off. :p

HTH.

Matt

[c0sie]

Hey lads, thanks for the helpful replies :)

As ive said, the inclusion of 11.1v has been proposed at the Micro Scale EGM and we are still thrashing this one out.

My personal opinion is that everyday people will not buy new motors and will slap 11.1v across their already super fast motors.
The arguement for is that it will make electrics run cooler, although only 3 people ive heard of have ever thermalled a micro brushless system.

Great replies though, thanks :)

[kevin bissell]

Matthew

Just borrowing the anorak...


As already mentioned, Power = IV (Current x Voltage).
Power lost in a typical electrical system is mainly due to I*I*R (Current squared x resistance).


I understand copper losses (I2R) but to say the they are the main loss in any electrical system is debatable.

Now I'm not claiming to be an expert on this subject but I did study AC and DC motor theory at college many years ago and all this talk of P = I x V is an over simplification of a complex system. The bit that is probably the most inefficient part is the motor which is an inductive load. There are other losses to take into consideration with an electro magnetic/inductive system such as iron losses. So to say that doubling the voltage reduces the losses by a factor of 4 (I2R) is not strictly true... no such thing as a free lunch.

A motor gives out ?? watts and because it will never be 100% efficient it will require more power in than it gives out. Some of the input energy that is not converted to useful power will be given off as heat. So, if you want the motor to run cooler you have to make it more efficient, and simply increasing the voltage is not going to achieve this.

People spend years learning this stuff and big companies will cough up millions trying to achieve that extra couple of percent efficiency. If it really was as easy bumping up the voltage it would have been sorted out a long time ago and we'd have 200 Megawatt motors running on 100 megavolts, drawing a couple of milliamps and running at 99.9 % efficiency.

Consider this... modified changed from 6 cell to 5 cell. Why? Because everything was burning out and by reducing the voltage they reduced the temperatures. Are the 5 cell mods of 2007 slower than the 6 cell variety of 2006?

Sorry Cris for hijacking your post :o

[MikeS]

Are the 5 cell mods of 2007 slower than the 6 cell variety of 2006?Don't you think the lower weight limit might have more than a little to do with this :confused:

[kevin bissell]

Nope

I was scrutineering the 5 cell mods at the Aldershot national, and not one of them got anywhere near the minimum weight. Many were around the 1450g mark and some were as heavy as the 6 cell stock cars.

[c0sie]

You carry on guys! Im interested by all the theories here :)

[MikeS]

Well I certainly have difficulty reconciling the implication that running a motor on 7.2V nominal does not produce more wattage than runnning the same motor at 6V nominal......even my rudimentary electrical knowledge and understanding knows there is a direct relationship. So if you reduce the supply voltage by 16%, the potential power output of the motor is going to reduce by a similar amount.....yes the actual electrical losses will reduce, but not by 16% and is only because the outright potential level is proportionately lower to start with. So the motor is not going to operate more efficiently to any significance, so it leaves the question, where does this apparent "lack of speed" come from ;)

[kevin bissell]

Sorry Mike, didn't make myself very clear.

All I was trying to say was than increasing the voltage to 11.1V but pegging the power output of the motor is not necessarily going to result in everything running cooler. This is because the efficiency of the system is not only voltage dependant.

Then just to try and make my point (and TBH play devils advocate) I gave an example where reducing the voltage had in fact resulted in lower temperatures without sacrificing speed. I didn't mean to intimate that the lower voltage of 5 cells produced a more efficient system.

I'm not saying that the reduction in voltage is the single defining reason for these lower temperatures because as you rightly say there are many other factors.

What I'm trying to do is to make people realise that they are not necessarily going to end up with a system that runs cooler by upping the voltage to 11.1V. The argument people are using is that this will result in lower currents for a given power output and because copper losses are proportional to the square of the current... hey presto suddenly everything is more efficient and the world is a better place. It 'aint that simple, if it were then the reduction from 6 to 5 cells would have resulted in everything running hotter which clearly isn't the case.

It's a very interesting debate and I don't have the answers. However, if nothing else it demonstrates that without full and expert knowledge in a particular field it is possible to over simplify and come up with an answer that doesn't stand up to scrutiny in the real world.

[Ian s]

Kevin - you are right in saying that the reduced voltage of the 5 cell class has resulted in lower temperatures, this is because we are running the same motors on 5 cell as we did on 6 cell.

The point everyone is trying to make is that if you ran a motor on 11.1v that was designed to give the same power output as a 5 cell (6v) 3.5 turn motor then the higher voltage car would run cooler.

I know your going to want evidence for this so as my boss isnt in im off to google to do some searching :D

Right - just had a quick look and i cant find anything directly related to our disscussion.

You might want to look into joule's law which relates heat generated in a conductor to the current passing through it (and time so it might be useful in a "we want longer races" debate :D)

A found a paper about running a motor above and below the stated operating voltage (which isnt really what were talking about but it does relate to the 5 cell business) The gist of it is running a motor below operating voltage but keeping it producing the same power will increase current and heat.

And lets not forget that everything we read on the internet is true :rolleyes:

oh and i found some motors that run on 13800v :cool:

[c0sie]

So....hyperthetically, if a proposal came into the 10th TC AGM to open the cell limit to 11.1v maximum, and the reason was to reduce heat/electrical wear/increase efficiency do you think it would pass a vote??

[mlitton]

Kevin,

You are absolutely right when you say "it ain't that simple". That is why I used words such as "mainly" and "potentially"! The full implications of all electrical issues are way beyond the scope of this forum, but I will summerise a few:

1/P=IV is perfectly valid to measure electrical power consumed by the motor. Mechanical power out is another issue entirely.....

2/I still maintain that I*I*R loses are the most significant, ESCPECIALLY in the ESC and batteries, a little less so in the motor due to the reasons you mention.

3/Iron losses or inductive core losses are proportional to the change in current in the coil. If we are running at lower currents then the change in current is lower and therefore these losses are also lower! (This is why a higer PWM frequency increases motor efficiency, but can sometimes decrease ESC efficiency due to switching losses!)

The reason mod went to 5-cell I believe is two fold:
1/Very few attendants of the AGM understand the full electrical implications.
2/By decreasing the voltage, we only have to change the cells. To increase it, however, would require a cell change, a motor change and possible an ESC change and you would still have the issues I mentioned earlier with speed.

So - who want's the anorak next? :p

Matt.

[mlitton]

If we forget MOD for a moment, an 11.1V powered 17.5BL system might make an efficient alternative to 19T!

Meanwhile, back on planet earth...... :p

Matt

[kevin bissell]

Matthew.

That anorak looks so much better on you than it does on me :p

Thanks for the very informative debate.