View Full Version : So what is better for top speed?

EVcycle

01 January 2011, 1527

Max Volts or amps.

For example (not defined as the parameters of the question) 72 volts and 800 amps or 96 volts at 600 amps given the SAME AH on the batteries.

Remember, as you go faster, the aerodynamic load is greater.

ALL opinions are welcome.

:)

DaveAK

01 January 2011, 1548

Well isn't the motor the variable in this scenario? For a given motor more volts = greater speed. If you're selecting a particular motor for a given voltage, then I'd go with more volts, less amps and pick the motor for it.

EVcycle

01 January 2011, 1624

I was thinking for any particular motor, but for this example we can use the Mars 1003 motor as the baseline.

http://www.marselectricllc.com/me1003.html

DaveAK

01 January 2011, 1631

Lets use the Mars 1003 motor as the variable.

Well you can only get more speed out of it by more volts can't you? Sure you can change the gearing between a 72V and a 96V to get the same rear wheel speed, but why would you want to do that?

What's going through your head that makes you ask this question? Are you concerned about pushing the motor to its limit RPMwise, so want to reduce the volts?

I find the fact that you raised this question more intriguing than the question itself! :D

Edit: If we're sticking to the 1003, that makes it a constant. :D

EVcycle

01 January 2011, 1655

"I find the fact that you raised this question more intriguing than the question itself!"

Just stirring the pot....um ....creative juices!!!! :)

Volts is fine for unrestricted top speed, but once you get to a certain point aerodynamics will command additional amps to the keep the speed.

DaveAK

01 January 2011, 1716

"I find the fact that you raised this question more intriguing than the question itself!"

Just stirring the pot....um ....creative juices!!!! :)

Volts is fine for unrestricted top speed, but once you get to a certain point aerodynamics will command additional amps to the keep the speed.

But won't that just dictate the amps and have little bearing on the voltage? Your original question is just volts vs amps, so we'd assume the same motor and aerodynamic properties in both scenarios. So now we're just talking power requirements to go each extra mile per hour. Once you've pegged your voltage you'll be increasing your amps only.

Unless of course I don't know what I'm talking about, which is definitely the case here. :)

I like having my creative juices stirred. Just trying to figure out the direction this is heading. :D

Skeezmour

01 January 2011, 1731

I bet your not going far if you really need 800amps at what ever speed your going for :)

Only2Jakes

01 January 2011, 1741

Voltage = speed

Amps = Torque

So since you are wanting high speed, the higher voltage would be best, but when you go faster there would be more air resistance so you would also need fairly high amps.

Considering the voltage and amps given I would say exactly in between :P

84V 700amps

Or just 85v nominal lipo??

But only if the motor, controller and batteries cope and hold up at those ratings

And thing is, there are only really 72v or 96v controllers as well as only 600 or 800 amps.

So If I HAD to pick, I would say 72v at 800 amps, Will get you fast and keep you fast.

Or 96v for even faster but not sustained.

NOTE: didn't read question fully LOL

(not defined as the parameters of the question)

magicsmoke

01 January 2011, 1831

The way I'd approach this Ed is to find out what the max safe rpm is for the given motor.

This will relate to a drive voltage but crucially this is the 'no load' condition.

By definition, a load such as drag, hill climb, rolling resistance etc. must cause the motor to slow down and so for any given rpm, the off load voltage will have to be increased to maintain that rpm when loaded.

So in a nut shell, you ideally want your pack voltage to be enough such that the motor can still be supplied with at least the max continuous current at max rpm (loaded) and might as well make it enough to cover the short burst current capability of the motor, again at max rpm.

A caveat on this is that under light loads, the increased pack voltage would be capable of over revving the motor so a rev counter or automatic rev limiter is a must!

As for the max pack current .. as much as is possible to be sucked by the motor .. i.e. increased acceleration, gets you to your increased top speed quicker.

Rob

lugnut

01 January 2011, 1849

Max Volts or amps.

ALL opinions are welcome.

Top speed is determined by power. And energy determines range.

The power is the product of volts and amps. So the choice is not one or the other, but what ratio of volts and amps suit your drive train component choices best.

EVcycle

01 January 2011, 1850

Top speed is determined by power. And energy determines range.

The power is the product of volts and amps. So the choice is not one or the other, but what ratio of volts and amps suit your drive train component choices best.

Getting closer......

Tony Coiro

01 January 2011, 1855

Trick question, if you let me pick the reference frame, I can make anything go any speed regardless of voltage or amperage. Was that right Ed? :D

EVcycle

01 January 2011, 1902

"No such thing as a trick question if you know the answer. " :)

Most of the folks here have used the same idea of volts is speed and Amps as Torque.

But....there is cross over where amps may take over volts when a load is present as in high speed and aerodynamic drag.

ICE motors have had their way as with high speed they are at their max HP and torque ( if built correctly)

Soooooooo

Do you "tune" for max amps, volts, or the mix of both at the right RPM.... (Aren't controllers a nice thing?)

Only2Jakes

01 January 2011, 1921

No huh you just buy the BIGGEST AND BADDEST controller available.

What is the answer MASTER?

Only2Jakes

01 January 2011, 1923

You get a controller to change from high volts to high amps at a certain point and thus even more speed?

EDITL: Wow work is affecting my brain and behaviour... Do I sound like Ted?

I IS TEDS MINION

DaveAK

01 January 2011, 1931

What kind of bike is this exercise for? Like Ted always says, build for what you need.

Tony Coiro

01 January 2011, 1933

No huh you just buy the BIGGEST AND BADDEST controller available.

What is the answer MASTER?

Haha, reminds me of two sayings we have at the EV Club here. First, maximum motor current is inversely proportional to motor cost. Second, if you're not having temperature problems, you need a bigger controller.

Only2Jakes

01 January 2011, 1952

Sounds VERY true actually :)

magicsmoke

01 January 2011, 1958

Most of the folks here have used the same idea of volts is speed and Amps as Torque.

But....there is cross over where amps may take over volts when a load is present as in high speed and aerodynamic drag.

ICE motors have had their way as with high speed they are at their max HP and torque ( if built correctly)

Can you expand your thinking on this Ed. I'm not sure what you're saying.

Only2Jakes

01 January 2011, 2031

I get what your saying Ed... I think.

When ICE's go to their max speed they achieve their max/rated HP.

But with electric motors we do not get to that point yet.

Imagine it like a graph; Linear.

At constant acceleration to get max speed, amps/volts must meet somewhere. And at this point they cross and Amps then take over?

And where they meet I guess we achieve the Motors rated HP at that voltage. And anywhere above that we over achieve it and this is why they tend to melt or go even faster, I guess.

So what Ed is saying is, We need to figure a way in which when they cross, we still get maximum efficiency and our speed would be even higher.

BY they way, I'm not sure how to describe the crosing.

Good explanation?? Very simple and I didnt even explain anything to do with aerodynamics:)

magicsmoke

01 January 2011, 2102

When ICE's go to their max speed they achieve their max/rated HP.

But with electric motors we do not get to that point yet.

Imagine it like a graph; Linear.

At constant acceleration to get max speed, amps/volts must meet somewhere. And at this point they cross and Amps then take over?

amps and volts can never meet anywhere on a graph. They're separate entities.

And where they meet I guess we achieve the Motors rated HP at that voltage. And anywhere above that we over achieve it and this is why they tend to melt or go even faster, I guess.

The motor is oblivious to the voltage applied to it except that it will rev to a point where the back emf equals the applied emf if it is unloaded.

If it is loaded then it must slow down and it will do so to such an extent that the difference between the applied voltage and the back emf results in a current flow that in turn creates a torque sufficient to drive that load.

Anyway, the point is that it is a misconception that the torque of a dc motor falls off with increasing rpm. Well sort of.

It is the starving of applied voltage which is responsible for the fall off in torque.

i.e as the rpm increases, the back emf increases and so the effective applied voltage is insufficient to drive full current into the motor. This is what I was saying in post #9, to achieve maximum rpm at maximum torque (i.e. maximum power) the pack voltage must be greater than that which would drive the motor to max rpm unloaded.

With the appropriate supply voltage, the dc motor will achieve maximum power at maximum rpm!

Only2Jakes

01 January 2011, 2110

Although I did say 'imagine it like a graph'

But yes, I guess you are right? And it does make sense.

LOL I learn new things every day.

cycleguy

01 January 2011, 2120

Provided you are not controller RPM limited, higher voltage will allow higher motor speed as well as more power throughout the motors range. A motor actually generates an opposite force called BEMF (back electromotive force) which increases with motor speed to the point that it pushes back on the incoming voltage, preventing the motor from spinning any faster. This force is proportional to RPM, which is why electric motor torque falls off rapidly after a certain point as RPM increases.

Higher voltage pushes harder against BMEF and increases the RPM point at which motor torque falls off.

Amperage is a function of motor load (resistance). Therefore a motor will draw maximum amps under maximum load, which is almost always at slower speeds.

A motor nearing it's maximum RPM will never draw as much current as it would at slower high load conditions, so the maximum amperage rating of your controller will have no effect on motor speed since the motor wouldn't be drawing anywhere near the maximum rating at high speeds anyway.

I was typing while magicsmoke posted, well said magicsmoke!

Only2Jakes

01 January 2011, 2128

WOW, I am saving your posts if that is ok. For future reference.

Anyways back to topic.

You are right then. More voltage would be more speed because it will combat the BMEF and rpm would increse further than a lower voltage, till the point torque drops off. :)

So Ed has got us going on the wrong road :S

Thanks for that, seriously, magicsmoke and cycleguy.

My head is clear :P

EVcycle

02 January 2011, 0229

Learning is never on the wrong road.... which was the intent of the original post.

Only2Jakes

02 January 2011, 0259

I had hoped originally that I could learn something from this thread.

So is there an actual answer or are we just discussing.

An Idea:

72v pack gets you up to max speed then kick in the small 96v capacitor bank for a speed increase/boost only lasting a small while. Then when you regen the power goes back into the cap bank. Many people have thought about this already, but it is a way to momentarily get a higher top speed :)

...................You know what would be a good idea... Someone starts off a site, devoted to teaching all this stuff.

EV stuff.

Theories etc.

And when Ev's kick on, we can supply manuals or why does my nissan leaf do this or etc.

I know lots of people who would want to learn this. Even my self.

And the knowledgable few teach/create articles and discussions.

Just an idea anyways. Back to topic!!

PM me if your interested. I would be interested in setting up the site etc.

teddillard

02 January 2011, 0402

EDITL: Wow work is affecting my brain and behaviour... Do I sound like Ted?

I IS TEDS MINION

...it was only a matter of time. muaHAHAHaahahaaa!

Sorry, I'm not sure this question is really clear. I think the question you're asking is, given the same system, for max top speed should you tune for amps or volts... is that it? I first read it to be asking which is better to build for. If that is the question, then I don't really think you get to choose.

It seems pretty simple. Your load curve (basically, I think you can use a simple aerodynamic drag curve here, for simplicity) is going to get steeper as you go faster, until it gets to a "terminal" speed, right? As you approach that terminal speed, the load is going up geometrically, so the current will too.

So, for your bike with it's specific drag and weight, you have a terminal velocity. You also have a speed that is going to start sucking huge amps with little gain. You have to tune for the RPMs (voltage) and gear the motor so you come as close as you can to this speed, and be able to feed as many amps as you can there.

Now, here's the catch. If you get to program the volts vs the amps, and you have to trade that off, (which is what I think you're saying when you say the curves cross, and I'm assuming there's a volts/amps tradeoff), then it's a simple combination of 3 graphs, right? Compare the speed/load to the voltage (which gives you the RPM) and the amps. Figure out where the amps start getting maxed out for the given load at speed, and you've got your optimum tune.

Here's your voltage/amps (wild guess on my part) over the drag curve. This isn't optimal, since the place that they cross falls short of the curve.

686

This is better, you increase the gearing so where the voltage and the amperage cross (which is what you tune for) intersect the load curve... and that's how fast you get to go.

685

...so that begs the question, why would you tune for anything else but where the two curves cross at the highest RPM? That would be specific cases where you can sacrifice RPM for amps, (giving you acceleration but not top speed) or just plain RPM (can't think of a case where low current high RPM would be good outside of a dentist's office...), but for an EV it seems like you'd always want to simply tune for that intersection, since you're always trying to optimize acceleration and top speed. It's the gearing that gives you that choice- acceleration vs. top speed, right?

Only2Jakes

02 January 2011, 0619

Exactly what she said ^^^^^^^^

Couldn't have said it any better, another for the records.

teddillard

02 January 2011, 0653

OK, I think I understand why you're asking the question now. It's a little more complicated, because you have the total power output for each of your programming options. So you have another curve, in theory, for a max voltage vs low amp setting and min voltage/max amp setting, and everything in between. That probably looks like a simple arc or nearly a straight line, though, right? Ohms Law and all that?

lugnut

02 January 2011, 0720

Here's your voltage/amps (wild guess on my part) over the drag curve. This isn't optimal, since the place that they cross falls short of the curve.

686

...........right?

You need to plot both the load curve (which you show in red) and the motor power (hp) curve to find the crossing (equilibrium). The motor power curve must be reflected thru the gear ratio and wheel radius to be in the same units as the load curve, hp vs mph. Plotting volts and amps on an hp/rpm grid tells you nothing. Now multiply volts times amps (and convert watts to hp) and plot that. Now you have two power curves. But you will still be missing the system efficiency. What you really need is the power curve for the rear wheel.

Motor power curves will be somewhat parabolic in shape. Zero power at zero torque and zero power at zero RPM and have a peak somewhere near the middle. The tuning which needs to be done is the selection of the gears to get the load curve to intersect that power curve near its peak.

Of course that is theoretical and at that peak motor power, the motor may not survive long enough for you to really go that fast. So that motor power curve has to be tempered with rating and controller limitation information.

teddillard

02 January 2011, 0755

um... out of my depth here, but since the vehicle speed does relate to the RPM of the motor, volts (or RPM) and amps in relation to the vehicle speed does tell you something. It tells you how your tuning of the volts/amps tuning of the controller will fall on the vehicle speed curve. That is, what your voltage needs to be, and available current will be at a target speed, say 100mph.

It's certainly not a complete picture, I understand that, and may offend the engineers in the group, but it does serve to illustrate the point that Ed was (I think) asking- the optimum tuning of a controller is that which gives you the most current at the required voltage for the target (highest) RPM.

(edit- if the units of the graph are what bothers you, ignore them. I just pulled that graph because it showed the relative load/speed relationship).

EVcycle

02 January 2011, 0906

"illustrate the point that Ed was (I think) asking"

Yes.

All of the info so far is what I was looking for. And no I did not has "the" answer. A good idea of what it might be, but the question

was to generate a conversion on something I am not sure we had addressed yet. I bet some of the TT folks have looked at it. :)

mpipes

02 January 2011, 1029

Increases in speed increase load, and increasing load increases total power requirement.

Ohm's law dictates that increasing power by increasing voltage also increases current.

So, to get higher speed you need BOTH higher volts and higher amps - an exponential increase in power at that.

If you were wanting to maintain current speed you could increase voltage and get by with less amps (ie same total power)

DaveAK

02 January 2011, 1110

I'm still not sure I understand the concept of tuning for volts or amps for a maximum top speed. I can't think why top speed for a given motor, fixed gearing and given cd wouldn't dictate the power required. Maximum volts would be determined by the motor specs, anything less and the motor wouldn't be spinning as fast as it could. The rest of the equation is how many amps you can deliver, which is down to batteries and controller. Maybe I'm oversimplifying this, (or very possibly just don't have a clue), but it seems like a non-question to me.

larryrose11

02 January 2011, 1113

Any system, is ultimately power limited.

In motors, generally the power limit it is the inability to supply (the battery) a voltage to sufficient to overcome the back emf voltage, and load, to continue accelerating.

So, at max speed, neglecting gearing losses and motor max RPM:

motor shaft power = vehicle loss power (aero, rolling loss and such)

You need to plot both the load curve (which you show in red) and the motor power (hp) curve to find the crossing (equilibrium).

Exactly!

BTW, gearing has no effect on power. It only is a loss.

Also, a motor has a current limit, but the magnetic field will reach saturation at a lower current. This is the motors stall torque. Once this point is reached, shoving more current in doesn't produce more torque, it just produces heat. But Im off topic, as this happens @ low speed.

Increases in speed increase load, and increasing load increases total power requirement.

Ohm's law dictates that increasing power by increasing voltage also increases current.

So, to get higher speed you need BOTH higher volts and higher amps - an exponential increase in power at that.

Not quite.

You will only be current limited when the motor speed is low.

Take a look at this plot from an RC site.

http://www.fastelectrics.com/helis/motordyno2.gif

don't pay attention to the specifics of this plot, dotted lines, but the trends. Current falls off its max (120 A) as the speed increases above 5k RPM. This corner is called Base Speed. It is the transition between current limit and voltage limit. To drive more current into the motor above base speed, (up to the controllers current limit, 120A in this case) and hence produce more torque, you must increase the system voltage.

I get the impression you are not just looking for top speed, such as the salt flats, but acceleration as well. If this is the case, then the bike should be geared so that you reach motor RPM limit for what you would like your top speed to be, and crank the voltage as high as you can, but you must employ a motor rev limit. In this case if the motor sees full voltage, it WILL hit the mechanical red line.

This is probably what Yates is doing, as his pack is 420V

http://www.chipyates.com/electric_faq

DaveAK

02 January 2011, 1116

Max Volts or amps.

Going back to the original question.

My answer is volts AND amps, not one or the other.

EVcycle

02 January 2011, 1239

Going back to the original question.

My answer is volts AND amps, not one or the other.

50/50? or a 40/60? or?

In drag racing you want MAX amps for the hard launch to go as fast as possible in a 1/4 mile. What about just top speed with no distance limitation.

Can-O-worms Ed

larryrose11

02 January 2011, 1304

50/50? or a 40/60? or?

In drag racing you want MAX amps for the hard launch to go as fast as possible in a 1/4 mile. What about just top speed with no distance limitation.

Within limits right? You don't want so much torque that you break traction.

Go with Volts for speed

Nuts & Volts

02 January 2011, 1310

I say higher voltage battery pack. The motor is only rated for a certain peak current, otherwise it will burn-up/melt. So you are limited there. So if you increase pack voltage your controller can push max current at a higher motor speed (higher BEMF). This actually increases the power of the system. It also allows you to gear lower (smaller reduction=less torque multiplier) and end up with a higher vehicle speed at max motor RPM. OR you can gear taller and have more torque/more power available at a higher motor speed.

Just trying to sum up the ideas stated. It all then comes down to what your controller can handle, bc the motors and batteries are both more flexible volts and current wise.

Kyle

magicsmoke

02 January 2011, 1354

I saw higher voltage battery pack. The motor is only rated for a certain peak current, otherwise it will burn-up/melt. So you are limited there. So if you increase pack voltage your controller can push max current at a higher motor speed (higher BEMF). This actually increases the power of the system. It also allows you to gear lower (smaller reduction=less torque multiplier) and end up with a higher vehicle speed at max motor RPM. OR you can gear taller and have more torque/more power available at a higher motor speed.

Just trying to sum up the ideas stated. It all then comes down to what your controller can handle, bc the motors and batteries are both more flexible volts and current wise.

Kyle

Exactly!

And just to put numbers on the pack volts increase .. 5 to 10 volts for the typical elmoto motor.

That's 5 to 10V above that voltage which would make the motor spin to its max safe rpm unloaded.

chdfarl

02 January 2011, 1646

So kinda off subject but what would be the likely scenario of a motor " melting " would the windings the solder or the enamel melt first or do motors commonly seize like at the bearings or do they break up and explode internally. Whose seen something like that.

teddillard

02 January 2011, 1705

Well, this is what happens when you give a brushed motor too many volts:

http://evmc2.files.wordpress.com/2010/08/5891_1204555753553_1218396040_30578487_1665598_n.j pg?w=604&h=455

chdfarl

02 January 2011, 1719

MMMM!!!!! What your cookin looks like it smells lovely Ted! Was that the hub motor that you fried. Ive seen starter motors that burnt out like that or sometimes the insulation at the posts break down and short but is that it do they ever melt or fly apart.

Tony Coiro

02 January 2011, 1736

Well, copper (depending on the alloy) will melt at around 2000 degrees F, so that seems unlikely, even for Ted. :D I'm nearly sure its the enamel/insulation that fails, since motors state the rating of the insulation in the motor specs. Most motors I've seen use H rated insulation, with is good up to 180C or 356F.

DaveAK

02 January 2011, 1754

50/50? or a 40/60? or?

In drag racing you want MAX amps for the hard launch to go as fast as possible in a 1/4 mile. What about just top speed with no distance limitation.

Can-O-worms Ed

I don't think you can split it like that. I'd go 100/100. All the volts you can with as many amps you've got.

EVcycle

02 January 2011, 1808

There is nothing like the smell of burnt windings in the morning...... :)

EVcycle

02 January 2011, 1810

I don't think you can split it like that. I'd go 100/100. All the volts you can with as many amps you've got.

Some of us do not have that kind of money :( ....... plus that may add (possibly) unwanted weight.

I have the options of

72 volts at 450 amps (Motor can take that)

96 volts at 300 amps (motor can take that too)

0r

96 volts at 800 amps, (and yes the motor will survive that for a while), but that is a whole different system, and I do not think the batteries can take that for too long.

DaveAK

02 January 2011, 1822

Some of us do not have that kind of money :( ....... plus that may add (possibly) unwanted weight.

I have the options of

72 volts at 450 amps (Motor can take that)

96 volts at 300 amps (motor can take that too)

0r

96 volts at 800 amps, (and yes the motor will survive that for a while), but that is a whole different system, and I do not think the batteries can take that for too long.

Yeah you do have that money. I'm talking about what you've got. 96V is all the volts you can and probably 300 amps. You have to put some kind of constraints on any system, be it money or the laws physics. I assumed all along this was something that you were working with and hence not an unlimited budget.

mpipes

02 January 2011, 1845

So kinda off subject but what would be the likely scenario of a motor " melting " would the windings the solder or the enamel melt first or do motors commonly seize like at the bearings or do they break up and explode internally. Whose seen something like that.

I wish I had a picture of it but I had a Magmotor running at 48v and the commutator bars broke free from the armature and became frozen inside the end of the motor can. It seized up instantly and unexpectedly while riding.

EVcycle

02 January 2011, 1854

Yeah you do have that money. I'm talking about what you've got. 96V is all the volts you can and probably 300 amps. You have to put some kind of constraints on any system, be it money or the laws physics. I assumed all along this was something that you were working with and hence not an unlimited budget.

It is what I have on hand.

How about 90 volts at 450 amps?

magicsmoke

02 January 2011, 1912

I have the options of

72 volts at 450 amps (Motor can take that)

96 volts at 300 amps (motor can take that too)

0r

96 volts at 800 amps, (and yes the motor will survive that for a while), but that is a whole different system, and I do not think the batteries can take that for too long.

c'mon Ed, give us some parameters here.

What are you talking, 1/8 .. 1/4 mile drag, rolling start, prolonged top speed ??

EVcycle

02 January 2011, 1920

Now you want parameters? I was just trying to start a discussions and you all want rules..... :) :)

If I do that all I get is info on ONE project. I really wanted a broader chat...which we have had.

The discussions was for top speed. 1/4 mile would be acceleration. Top speed would be Bonneville stuff. (No I am not going there)

Top speed with a rolling start. No MPC required. After a certain speed drag will play a role. (actually Drag plays a role at any speed)

Once you get past 80 MPH (128KPH), the effect will put a greater load on the motor/battery.

DaveAK

02 January 2011, 1933

If there were no rules then there would be no difference between a 1/4 miler and a street bike, and the budget would be limitless.

Without parameters the question becomes how long is a piece of string.

magicsmoke

02 January 2011, 1943

The discussions was for top speed. 1/4 mile would be acceleration. Top speed would be Bonneville stuff. (No I am not going there)

Exactly. So for prolonged top speed, an 800A capable supply is pointless for a 200A continuous motor as would be a 300A limited supply in a drag scenario where the 800A could be used briefly. TTXGP circuit racing and TT Zero scenarios different again.

Similarly 96V may rev the boll**ks off a particular motor while 72V might not achieve max.

It's like asking what's the best camouflage colour!

EVcycle

02 January 2011, 2006

There is that box again...... :(

Everyone keeps putting parameters in.

Why would I put 800 amps into a motor that would only take 200?

I think we have killed this subject.

Class ....dismissed.

DaveAK

02 January 2011, 2026

I think it's a little unfair to say that some of us aren't thinking outside the box. I'm sorry if that's how it comes across.

magicsmoke

02 January 2011, 2030

Why would I put 800 amps into a motor that would only take 200?

Because in post #2 you said "for this example we can use the Mars 1003 motor as the baseline."

Anyway, before the end of class bell stops ringing, a definitive answer to your question is ..

Pack Volts = Volts to achieve max safe unloaded rpm + (Current desired at max rpm x Armature resistance)

In the above, choose 'current desired' to be peak / continuous / whatever.

teddillard

03 January 2011, 0700

Yeah, I'm sorry, but this question still makes no sense to me. I can't figure out what you're asking or what you're trying to get at. If you're looking for the best configuration or tuning within a system you have on hand, fine, what are the choices- we need the parameters- and if not, then it's not really much of a question. Even I know the answer... ;)

I still can't figure out if you can actually program the volts/amps on the controller you're using, and if that was one of the things you were trying to get at... would you enlighten me?

frodus

03 January 2011, 1015

I have the options of

72 volts at 450 amps (Motor can take that)

96 volts at 300 amps (motor can take that too)

0r

96 volts at 800 amps, (and yes the motor will survive that for a while), but that is a whole different system, and I do not think the batteries can take that for too long.

pick the one with the highest power then. None of those are equivalent power, but that is going against your first question. You need to have some constants, i.e. power is constant, which is better, higher voltage lower amps, or higher amps lower voltage. If you have the same exact battery pack, same controller, same motor but its a question of configuration, then I think that you've seen that a higher voltage would be best for getting that higher RPM.

96V and 800A (77kw) is obviously the higher power one, but after that, 72V and 450A (32.4kw) is the next highest power. The lowest power of those three is 96V and 300A (28.8kw).

Now.... back to the original question:

All that has been said about power being the limiting factor and the back emf controlling RPM....etc are all valid. In order to overcome the back EMF once RPM increases you need to increase the voltage to get any more RPM.

But lets look at a different motor for instance (because Mars, Agni curves are over a really narrow range and i can't stand their graphs), where the torque curve is something we can see over a wider RPM range.

http://www.evfr.net/synkromotive/components/motors/K91.pdf

(sorry its hard to read)

look at page 2. Note that there are 4 different RPM curves. This shows the effect of increasing voltage on RPM. The HP on the graph is for 75V only, but you can calculate the HP and see how it would be effected (TxN/5252 = HP). Watch RPM decrease as the load on the motor (torque) is increased. Now, the HP does keep going, but it will trail off and start decreasing. The Torque is a function of current, but as you increase RPM, your Back EMF comes into play..... but its still directly proportional to input current.

So the trick is, if you're working within the parameters of the motor, to maximize the HP, since in the end, that is what controls your ability to overcome air resistance and thus effect your top speed. Anything over about 40mph and air resistance really starts to effect your HP requirements.

Now lets use that K91 motor and 2 scenarios, and a few constants, since I like comparing apples and apples and I know it drives you &%$#ING nuts ;)

lets say same controller set to different limits. Same batteries in two configurations (72V and 96V). They're the same power, just different voltage and amp configurations. Same motor (my K91). Lets say the power is 21.6kw. At 96V that's ~225A. At 72V that's 300A.

Look at the power curve, and draw horizontal lines at two points. One in red at 225A and another in blue at 300. Now, see where that red line hits 225A, lets say its about 32ftlbs of torque. Now do the same for the blue line, and it apears to be about 45ftlbs. At 225A and 32ftlbs, the RPM for 96V is ~3500rpm. At 300A and 45ftlbs, the RPM for 72V is ~2250rpm.

So:

HP at 72V 300A = (45ftlbs x 2250)/5252 = 19.27HP (which roughly lines up with the graph)

HP at 96V 225A = (32ftlbs x 3500)/5252 = 21.32HP

So for the same input parameters (input power), the higher voltage lower current scenario would give you a higher output power.

EVcycle

03 January 2011, 1432

Thank you Travis, (and every one else that added to this thread!).

That was a good way for us mere mortals to understand it.

:):):)

Now back to our regularly scheduled programming.

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