[IamIan]

Interesting thread... sorry I came in late and took a while to read through it all.

I don't think it is a black and white either way.

but there are a few things that occurred to me that I didn't see mentioned I thought I would through out to get the point of view of others.

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#1>
Equations represent relationships.

F=MA is just as correct as M=F/A... it is deceptive to say power comes from torque when that is only true in one form of an equation... the equation can be re-written still be 100% accurate and make torque come from power .... etc.

In short The whole equation is expressing a relationship ... pulling out pieces of it makes that piece meaningless on its own.

Keep in mind the context of the relationship the equation represents... M=F/A does not mean that your mass disappears when the force applied goes to zero... or becomes infinite as acceleration goes to zero.

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#2>
There is power at 0 RPMs.

It is just not expressed in the simple power torque rpm formula.
Just like your mass doesn't become infinite at 0 acceleration or go to zero at zero applied force.
Given the power applied to the motor... V*A... and the Power being dissipated by the motor as its less than 100% efficient ... most of the dissipated power goes out as heat... a bit as light , radio waves, sound , etc... but the remainder of the power is applied at 0 RPMs...

The sum of the power losses from heat, sound, radio waves, etc... is less at 0RPMS than the total power applied to the electric motor... thus even at 0RPMs there is power.

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#3>
While a a 1 HP or 1 kW motor with an ideal transmission can provide any amount of torque you want even over 2,000,000,000,000,000 Ft/Lb ... it is the real world limitations on parts, performance etc... thus power and torque can both be useful to varying amounts depending on the specifics of the application... specifics may effect priorities more toward one or the other but all of those things are still there in real world applications.

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just my 2 bits / random thoughts.

Ian.!

[major]

Originally Posted by IamIan

Interesting thread... sorry I came in late and took a while to read through it all.

Hi Iam,

Thanks for your input and thoughts. So what is your vote? Torque is irrelevant of relevant?

There is power at 0 RPMs.

Gotta disagree with you here. On the shaft of a motor, even if there is torque, if it is stationary (zero RPM), the mechanical power is zero. Now there may be electrical power into the motor which produces torque, but the power output is zero. But there could also be a spring which produces that torque. Or a weight on a string around a pulley. Force (or torque) can exist without power.

Regards,

major

[PhantomPholly]

Originally Posted by major

Gotta disagree with you here. On the shaft of a motor, even if there is torque, if it is stationary (zero RPM), the mechanical power is zero.

I don't think "power" is a physics term, so whether you are right or wrong on this depends on your subjective meaning of "power."

Torque is a force. At zero rpm, no WORK is done (horsepower is a measure of "work") but there is certainly a force. If the force is 10 ft/lbs and you balance a 10lb weight on the end of a 1 foot pole (of zero mass, for you perfectionists) with the pole horizontal to the ground and the plane of rotation vertical, no work gets done because the force of the motor exactly matches the force of gravity. Lighten the load by a few photons and it starts to move, accelerating away from the earth, and then you may calculate work done over some period of time "X". Make it heavier, and it will begin to descend, but more slowly than if it were in free-fall, and NEGATIVE work would be done (you give up your POTENTIAL energy stored in the form of altitude).

Now there may be electrical power into the motor which produces torque, but the power output is zero. But there could also be a spring which produces that torque. Or a weight on a string around a pulley. Force (or torque) can exist without power.

Regards,

major

So, I think your intent was to say that the instantaneous hp rating is zero, which is true because hp (like "WORK") can only be measured over time.

The most confusing part of all of this is that in the real world our motors are hardly ever making a constant amount of power, unless we are at cruise and have balanced the forces (the instantaneous torque applied to the wheels at each instant is exactly equal to the opposing forces of wind and Rolling Resistance).

In MY personal vocabulary, "the power you are making" would mean the instantaneous measurement of force with the assumption that if you continued that exact force over a finite period of time you could then translate into horsepower. I tend to think in terms of thrust as power, which would be different from what Major is saying (where he is equating power to "work"). Neither is right nor wrong because they are simply English expressions, not physics terms.

[IamIan]

Originally Posted by major

Thanks for your input and thoughts. So what is your vote? Torque is irrelevant of relevant?

I don't think it is a universal black and white relevant or irrelevant kind of thing ... I think it is a sliding scale depending on the application... where the value never completely reaches zero.... even if for some applications it can be so close to zero to be virtually the same thing... but depending on the application it becomes more or less relevant... kind of like Aerodynamics on a large cement bridge, might be very minor concern at one application but in another they can become a major dominating concern.

Originally Posted by major

Gotta disagree with you here. On the shaft of a motor, even if there is torque, if it is stationary (zero RPM), the mechanical power is zero. Now there may be electrical power into the motor which produces torque, but the power output is zero. But there could also be a spring which produces that torque. Or a weight on a string around a pulley. Force (or torque) can exist without power.

I agree force even by another name of torque can exist without power...

But I suspect there is more to it than one and only one simple equation.

__ Power goes in - ___ Power in heat , sound, radio waves, etc = __ Power remaining.

Just like F = MA ... M = F/A ... your Mass does not go to zero as the applied force goes to zero... the equation is just an expression of a relationship.

The relationship of the equation that relates power to torque and RPM is only one variation of that equation... there are others which are 100% equally as correct and accurate that do not include a RPM term at all... one such example is electrical power, among others.

The mechanical power out can not exceed the electrical power in... there is a direct relationship between them.

Although I guess that side of it is more academic and deviates a fair bit from the useful common application of the term power... so maybe it is best left for thought experiments just for fun.

[major]

Originally Posted by PhantomPholly

I don't think "power" is a physics term, so whether you are right or wrong on this depends on your subjective meaning of "power."

Hi Phant,

In physics, power is the rate at which work is performed or energy is converted.

from:

http://en.wikipedia.org/wiki/Power_(physics)

That is what we are talking about

major

[PhantomPholly]

Originally Posted by major

Hi Phant,

In physics, power is the rate at which work is performed or energy is converted.

from:

http://en.wikipedia.org/wiki/Power_(physics)

That is what we are talking about

major

Ah, I didn't remember that - I stand corrected.

At zero rpm, power is zero.

[IamIan]

Originally Posted by major

Hi Phant,

In physics, power is the rate at which work is performed or energy is converted.

from:

http://en.wikipedia.org/wiki/Power_(physics)

That is what we are talking about

major

Although I will agree in the common usage of the term as it has been used... and in that common usage sense the vast majority of people consider it to be 0 Power at 0 RPMs...

but ... while it might be academic ... there is more to it...

Even in your own quote don't forget the 'or' part of that statement.

The next sentence you did not include from that Wikipedia article you quoted clarifies my meaning.

In physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time.

as long as the 'or' statement of ,

or energy is converted.

is included and as long as statements like ...

It is an energy per unit time.

Statements are made

There very well can be power at 0 RPMs... even if that is not how most people have been taught to think of power.

Although I will agree that is a much more academic discussion on the nature of Power ... I will also continue to agree that the common usage of the term Power it would be zero... but there is more to it, if one wishes to delve into it... Although from an application point of view I doubt there would be any benefit in doing so.

Kind of like ... It is the electrons that move ... from the - or ground state up to the higher potential + state... the + protons do not move in electrical current ... it is only the - electrons that move... but while that is correct it doesn't change how we connect our batteries or motors ... and it doesn't change how people are taught electronics.

so there are many things that we could discuss ... like the nature of power... weather light is a particle a wave or both, etc ... but weather there is or is not power at 0 RPMs does not significantly change any ones application ... so it is a much more academic discussion.

[DavidDymaxion]

I'd say it is not completely academic. If you know that you have zero output power at 0 rpm you won't use battery power to hold the car on a hill. You'd realize all the input power is going 100% into heat, and not overheat your comm bars and waste energy. You'll use your brakes instead to hold the car still on a hill.

Another practical consequence is you realize your efficiency is 0% at 0 rpm, even if it quickly climbs to 80+% at 1000 rpm, and that efficiency is poor at very low rpm. You'll avoid designing a big heavy direct drive truck, geared for 100+ mph, for slowly climbing steep hills -- you'd keep the tranny to minimize the near 0 rpm region.

Originally Posted by IamIan

... ... but whether there is or is not power at 0 RPMs does not significantly change any ones application ... so it is a much more academic discussion.

[IamIan]

Originally Posted by DavidDymaxion

I'd say it is not completely academic. If you know that you have zero output power at 0 rpm you won't use battery power to hold the car on a hill. You'd realize all the input power is going 100% into heat, and not overheat your comm bars and waste energy. You'll use your brakes instead to hold the car still on a hill.

Another practical consequence is you realize your efficiency is 0% at 0 rpm, even if it quickly climbs to 80+% at 1000 rpm, and that efficiency is poor at very low rpm. You'll avoid designing a big heavy direct drive truck, geared for 100+ mph, for slowly climbing steep hills -- you'd keep the tranny to minimize the near 0 rpm region.

The heat power produced by a electrical motor can be determined from the change in temperature of that motor over a defined period of time in a defined atmosphere such as air density, humidity, temperature, etc...

If you put a EV motor on a hill ... you can measure the electrical power going to the motor even if it is just enough to balance the EV so that it doesn't move up or down the hill... of againt a cement wall... of course one should be careful not to damage ones EV or motor.

The difference between the power applied to the motor and the dissipated / waste power to heat will show you what is left... it is a bit harder to quantify the radio waste power ,etc ... if it is zero there is no power at 0 RPMs... if there is still power unaccounted for there is still power at 0 RPMs.

Keep the power to the motor the same but vary the current ... then the voltage... the waste heat is produced by the current flow not the voltage... so when you half the voltage but double the current you have kept the power the same but you still might expect to get 4 times as much heat produced because you doubled the current even at the same power.... but if efficiency is 0% and all power is being converted to heat... than you would expect to get the same heat generated at any current level as long as the applied power was the same... and it would not change the amount of heat to have 10 amps and 1 V or 10V and 1 Amp... both are 10Watts of power and if there is no power than all power must be converted and we would expect the same heat generated either way... even though we already know that heat generated is only based on current or Amp flow and is independent of Voltage... which seems to contradict a 0% efficiency or 0 Power at 0 RPMs.

but feel free to do the experiment... carefully.

Either way there is energy converted ... which is a definition of power... and there is energy over time... which is another definition of power.

But I still think it is ... 99% just an academic discussion on the nature of power... I still don't see how it would effect actual application either way of a real world EV build.

[DavidDymaxion]

Nah, there's a much easier test. First a little energy equation:

Ebattery = Eheat + Elinear motion + Erotationalmotion

Here's a power equation:

Pbattery = Pheat + Plinearmotion + Protationalmotion

All I have to do is draw some power from the batteries while holding the brakes, and observe the car didn't move -- therefore there can't be any energy put into linear or rotational motion; it is similar for power.

While it is correct that you could measure the heat and assume any missing heat goes to some other form of energy, in practice that is hard to do accurately. In this case it is far easier to observe there is no useful output power (motion).

Maybe there was confusion in saying there was no power. To clarify, you have electrical power going in, and power going out as heat, but no useful power in terms of motion.

If you vary the current, or voltage, or frequency, different things might get hot (possibly including air itself from arcing), but all that battery power is going to waste if the vehicle is not moving. Almost all of this loss is heat, a little bit might go into sound or flexing things.

Originally Posted by IamIan

The heat power produced by a electrical motor can be determined from the change in temperature of that motor over a defined period of time in a defined atmosphere such as air density, humidity, temperature, etc...

If you put a EV motor on a hill ... you can measure the electrical power going to the motor even if it is just enough to balance the EV so that it doesn't move up or down the hill... of againt a cement wall... of course one should be careful not to damage ones EV or motor.

The difference between the power applied to the motor and the dissipated / waste power to heat will show you what is left... it is a bit harder to quantify the radio waste power ,etc ... if it is zero there is no power at 0 RPMs... if there is still power unaccounted for there is still power at 0 RPMs.

Keep the power to the motor the same but vary the current ... then the voltage... the waste heat is produced by the current flow not the voltage... so when you half the voltage but double the current you have kept the power the same but you still might expect to get 4 times as much heat produced because you doubled the current even at the same power.... but if efficiency is 0% and all power is being converted to heat... than you would expect to get the same heat generated at any current level as long as the applied power was the same... and it would not change the amount of heat to have 10 amps and 1 V or 10V and 1 Amp... both are 10Watts of power and if there is no power than all power must be converted and we would expect the same heat generated either way... even though we already know that heat generated is only based on current or Amp flow and is independent of Voltage... which seems to contradict a 0% efficiency or 0 Power at 0 RPMs.

but feel free to do the experiment... carefully.

Either way there is energy converted ... which is a definition of power... and there is energy over time... which is another definition of power.
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