THE PHYSICS OF... POWER

[mike_kilroy]

here's one for Wayne and GoldDigger

R is a term related to pure resistance. amps through R makes nothing but heat.

ok, if we say its pure R then 100watts being converted into heat, but not any useful output work at all, its all just heat that we cannot use for any meaningful purpose. do you agree (i suspect yes).

if R changes electrical potential into useless heat then why do you say R is the real power when heat cannot do work?

now, an induction motor has a R component, so we can say the amps running through that R of the motor does nothing more than create heat, a useless conversion of electrical potential energy. do you agree (i suspect yes).

so an ideal induction motor would have no R value. do you agree (i suspect yes).

so what you have is a motor that has nothing but Q. do you agree (i suspect yes).

this ideal induction motor can still do work? do you agree (i suspect yes).

but you say Q cant do any work, so how can this ideal motor be doing work?

if the ideal induction motor has PF=0 and can produce enough torque to do 1kW worth of work, how many amps are needed? how much input work to gen is required?

FV, I strongly suggest you keep your discussion to non-induction motors as your understanding of how they work is seriously flawed.

 

[mike_kilroy]

An induction motor even ideal one with PF=0 is possible provided it has no load, operates in vacuum and no internal losses. But PF=1 is an impossibility, because the motor requires a magnetic field and so its associated reactive power to transfer power to load.

Very perceptive comment.

 

[Besoeker]

We are talking about A.C.

Might I suggest that you read the title of the thread?

 

[Sahib]

FV, I strongly suggest you keep your discussion to non-induction motors as your understanding of how they work is seriously flawed.

He is just brainstorming, IMO.

 

[mike_kilroy]

I don't agree. Motor and panel anti-condensation have a meaningful purpose.

If I don't stuff my outside wood boiler with wood, I will soon freeze come winter without that useless 6kw resistive electric heating element in my furnace. I could not recover by dipping into the cold water in my hottub without its equally useless 6kw resistive heating element.

 

[Besoeker]

He is just brainstorming, IMO.

I think that "he" is a "she".

 

[mike_kilroy]

Permanent magnet brushless DC motors don't.

In Sahib's defense, he DID clearly say "induction motor," not PM motor.

 

[Besoeker]

In Sahib's defense, he DID clearly say "induction motor," not PM motor.

Fairy 'nuff.

 

[Sahib]

I think that "he" is a "she".

Oh! She is He. Marvelous! My respect to her.

 

[wwhitney]

FV, I strongly suggest you keep your discussion to non-induction motors as your understanding of how they work is seriously flawed.

Hey, everybody is allowed to ask questions.

Cheers, Wayne

 

[FionaZuppa]

post #656, why did you change the name on the quote?

PF=0 not real, why's that, all of the copper in the motor is super-conducting, no R and 100% reactive.

we know that it takes work to create the heat (amps through R), we also know heat itself can do work (water heater burning gas to transfer energy into water).

so if I²R in general turns potential energy into kinetic heat, why do you rate output based on the energy the object wastes? if the motor had Z components of R=100 and mHenry=0.000000001 and V=50, would it make a good motor?

 

[wwhitney]

post #656, why did you change the name on the quote?

Multi-quote user error on my part, sorry. Perhaps a mod could fix.

Cheers, Wayne

 

[Carultch]

R is a term related to pure resistance. amps through R makes nothing but heat.

I don't remember saying that.

OK, how do you craft a formal definition that makes that distinction?

I guess my question boils down to this: in the context of electrical engineering, how and why is it useful to distinguish some ΔE as work as some as not? Or what is the issue with just defining power as P as dE/dt?

Cheers, Wayne

If you break it down to the first principles of the electrostatic Coulomb force, you can see that volts * current* time is the same thing as mechanical work in general. It is a force (electric field) that moves an object (charge) through a distance.

Distinguishing the concept of work, is more important in thermodynamics, than it is in electrical engineering, where the purpose of which is to distinguish ordered energy from chaotic and entropy-prone energy.

 

[FionaZuppa]

Multi-quote user error on my part, sorry. Perhaps a mod could fix.

Cheers, Wayne

what about my other questions in #671?

 

[mike_kilroy]

Hey, everybody is allowed to ask questions.

Cheers, Wayne

Fairy 'nuff.​

 

[wwhitney]

I don't remember saying that.

Yes, sorry for the misattributed quote, I made a multiquote error but can't edit it now.

If you break it down to the first principles of the electrostatic Coulomb force, you can see that volts * current* time is the same thing as mechanical work in general.

I thought voltage was "potential energy/coulomb", so I get volts * amps * time = energy. Which could be used to do mechanical work, or could be dissipated as heat, or transformed into some other sort of energy.

Anyway, I'm happy enough with a definition of power as dE/dt, and it's not too important to me to nail down which types of delta E in this context should be considered work. Unless that definition is going to lead me astray somehow?

Cheers, Wayne

 

[wwhitney]

we know that it takes work to create the heat (amps through R) we also know heat itself can do work (water heater burning gas to transfer energy into water).

I would say it takes energy to create heat, partly because I don't have a precise definition of work in this context.

we also know heat itself can do work (water heater burning gas to transfer energy into water).

Only through a heat engine, so any mechanism that takes electrical energy to heat to work will be rather inefficient.

so if I²R in general turns potential energy into kinetic heat

I wouldn't say that. My (limited) understanding is that, for example, a permanent magnet synchronous motor can convert electrical energy into mechanical work with power factor 1. Any heat produced is incidental, in that the conductors aren't superconducting.

why do you rate output based on the energy the object wastes?

I don't understand the question.

if the motor had Z components of R=100 and mHenry=0.000000001 and V=50, would it make a good motor?

Just knowing R and Z doesn't tell you how efficient the motor is. Energy converted to heat and energy converted to mechanical work both show up as in phase current, or in the "R" term, to speak loosely.

Cheers, Wayne

 

[Carultch]

Yes, sorry for the misattributed quote, I made a multiquote error but can't edit it now.


I thought voltage was "potential energy/coulomb".

Indeed it is.

But another way to look at it, is the path integral of electric field. Electric field being force per unit charge.

When no magnetic induced EMF is present, the closed path integral of electric field is zero, which is where the concept of potential energy is useful for keeping track of the net effects of the electric field accumulated over any path between two points.

 

[wwhitney]

But another way to look at it, is the path integral of electric field. Electric field being force per unit charge.

OK, I get that E (the electric field) = - grad V, so if a charge Q moves across a given delta V, the F dot d on that charge will equal Q delta V. That helps, thanks.

So I agree now, it makes sense to say that a voltage source is doing work on a resistor. And since the resistor has no way to store that energy internally, or do work on the outside world, it is all dissipated as heat, per the first law of thermodynamics.

From this point of view, power = work / time is a functional definition. I imagine in other contexts it may be useful to consider thermal power = heat transported / time. Both definitions are examples of power = delta energy / time. Is there any problem with using this last definition?

Cheers, Wayne

 

[FionaZuppa]

Energy converted to heat and energy converted to mechanical work both show up as in phase current, or in the "R" term, to speak loosely.

Cheers, Wayne

interesting. what real world electrical device has in-phase current? you might model the process using power in terms of pure R and pure reactance................ oh wait a sec, thats what we are doing when inking the paper with S=P+Q, in in reality there is no load that is a pure R. small enough Q to discard? maybe, but its still there.

so i have a "motor" that has 100ohm of R from copper wire, and the Q component is 10^-10 times smaller than R (X from 10^-10 henries). @ Hz 100Vac-rms. will this make a good induction motor.

i then redesign my motor, it now has 100ohm of Q from copper coils (X from 5 henries), and the R component is 10^-10 times smaller than Q. @Hz 100Vac-rms. will this make a good induction motor? is it at least a better motor than the other?

with R in Z of induction motor, where does all the watts associated with R go? how is it used?