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Mess with your minds: A wind powered craft going faster than a tail wind speed.
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fourfathom:

--- Quote from: IanB on December 31, 2021, 11:52:41 pm ---
--- Quote from: fourfathom on December 31, 2021, 10:09:20 pm ---Yes and no. 

My thought experiment posits a motor directly connected to the wheel, via gears if you like,  but in a completely linear fashion where the wind can cause an un-powered motor to rotate in either direction.

But even with your one-way gearing, you will need more than "any tiny bit of power" to get the motor to turn.  The motor needs to generate sufficient torque to overcome the (head wind) wind-force.  Whatever your gear-ratio, there will be some minimum amount of power needed to make the motor spin.  Otherwise the motor is stalled and power turns into heat.

--- End quote ---

The distinction between torque (rotation force) and power is where this thread was coming unstuck before.

Suppose, for a moment, that we have ideal, frictionless gears. It's not going to happen in the real world, but suppose we have Teflon gears and roller bearings and whatever.

Now, suppose we wish to move our vehicle forward at 0.1 m/s against a 100 N force of headwind. We can calculate the required power as 0.1 x 100 = 10 W. So, if there are no losses in our ideal gear train, then the motor needs to output 10 W to achieve this rate of forward progress.

If, maybe, we only have a 1 W motor, then we cannot go this fast. However, we could go at 0.01 m/s, since 0.01 x 100 = 1 W.

How big the motor is determines how fast we can go, but if we just want to go at any speed at all, then we can introduce ludicrous gear ratios and make the motor as tiny as we like.

This is why I say "any tiny bit of power". In the real world, of course, some power is required to overcome the friction in the gears, and more gears will have more friction, so there is a law of diminishing returns. However, in principle, a wind up clock mechanism could make a vehicle move against a gale force headwind, albeit at a glacial pace.

--- End quote ---

Yes, 100% yes.  With an arbitrary amount of electrical power applied to the motor, and an appropriate gear ratio, the mechanism will move forward at some speed.

But I think we're talking past each other.  ALL I AM SAYING IS THIS:


* There is a force acting on the mechanism.  Let it be a head wind.  With no brakes and no friction, the mechanism will roll backwards at wind speed.
* An electric motor is used to counteract this force.  Just enough power is applied to the motor (volts * amps) to hold the armature in one place.  This is a locked rotor.
* Since the mechanism is not moving, there is no mechanical work being done.
* But the motor is still consuming power, turning it into heat.  The motor is operating at 0% efficiency, but still performing a useful function.
This -- electrical power being used by the motor while no mechanical work is being done -- may be one of the confusing factors when discussing the system.  If the electrical motor were replaced by a wind-up spring-powered motor, and the motor torque were (through gears) equal to the opposing wind force, then I think there would be less confusion.  The motor would be stalled, nothing would consume power, and no work would be done.
IanB:

--- Quote from: fourfathom on January 01, 2022, 01:05:41 am ---
* There is a force acting on the mechanism.  Let it be a head wind.  With no brakes and no friction, the mechanism will roll backwards at wind speed.
* An electric motor is used to counteract this force.  Just enough power is applied to the motor (volts * amps) to hold the armature in one place.  This is a locked rotor.
* Since the mechanism is not moving, there is no mechanical work being done.
* But the motor is still consuming power, turning it into heat.  The motor is operating at 0% efficiency, but still performing a useful function.
This -- electrical power being used by the motor while no mechanical work is being done -- may be one of the confusing factors when discussing the system. 
--- End quote ---

OK, that's clear. I understand where you are coming from. I am also trying to make sure the information is clear for anyone else reading the thread.

This is a bit like the situation where you are holding up a heavy weight with your arm, without moving. In a physical sense, no work is being done because nothing is moving, but your arm is still doing muscle work and getting tired.

Situations like this are the correct use of energy balances. In this case there is a steady state balance that says:

   (total power in) = (mechanical power out) + (power dissipated as heat)

When talking about power, it is of course important to be clear about which measure of power is being discussed.
gnuarm:

--- Quote from: fourfathom on December 31, 2021, 06:50:46 pm ---Here's something that had me confused:

Bicycle stopped in the wind.  Wheels are not turning.  Brakes are locked (or wheels are bolted to the ground -- same thing).  There is force, but no movement, so no power.
--- End quote ---

Absolutely correct.  Same as if the bike were laying on the ground or a car was laying upside down.



--- Quote ---Now, replace the brakes with a stalled motor.  There is still no movement, just two opposing forces (just as in the previous situation).  But the motor requires power (V*A) to remain stalled.  This power is being turned into heat.  I assume that we just consider that the motor is operating at 0% efficiency?  But it is providing torque. 
--- End quote ---

That's one reason why I didn't want to involve a mechanism.  Mechanisms always have inefficiencies or other complications that muck up corner cases.  But yes, the power flowing into the motor is only to overcome inefficiencies in the mechanism.  Motors get max torque at zero RPM, so the current would not need to be high unless the force required were high.  In essence the motor has become an electromagnet.  It could be replaced with a permanent magnet.  In fact, I believe stepper motors have this effect, they have spots where the motor comes to rest and without any current flowing a force must be applied to move them from that spot.


--- Quote ---Or replace the motor with an electromagnetic solenoid that holds the bicycle in place against the wind -- really the same situation as the stalled motor.  In both cases power is being consumed, but no work is being done other than heating the air?
--- End quote ---

Ah, great minds think alike. 



--- Quote ---It seems to me that some of ED's confusion is related to this.  I know I still struggle with the terms, since I always go back to the volts and amps.  ED's models using generator and motor also add to the potential (no pun intended) confusion.

--- End quote ---

I can't imagine what ED's confusion is.  He has trouble understanding the energy is not related to time but power is.  He keeps posting the equation relating power to energy and time as if that proved something.  That's why I posted the true story of the guy who didn't know how to calculate the square footage of a room.  "How many quarters in two dollars?  See?"  This is ED's thinking.

Actually his issue is not about the waste heat of a motor or solenoid.  His problem is he doesn't want to be proven wrong so he continues to insist on applying a formula in a case where it is irrelevant... among many other mistakes.  The lady doth protest too much, methinks.

Feliz año nuevo
IanB:
We should probably just let this thread rest at this point. It seems to have run its course.
MikeP:
  Dear friends, thank you for a little adventure. Very funny!

   To electrodacus. Give up! opposition is useless! This device really works. Although the principle is not obvious. :)

 Yes, I have not read everything here. Perhaps I will repeat someone's words, sorry.
  Quite a long time ago, my friend voiced a banal thing - the mathematical apparatus is secondary. Understanding the principle is primary. In this case (electrdoacus) understanding did not happen. Therefore, all formulas are useless.
   And now, in essence. Perhaps this will help. The entire mechanism with a transmission and a propeller makes it possible to perceive wind energy at almost any speed. The basic ratio of wheel circumference to propeller pitch is 0.7. In other words, the propeller always "advances" the movement of the cart. Everything is very simple.
 
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