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Originally Posted by Samurai You know, you reminded me of something. When I was a teenager, I read these books on Physics. These books were written in 1913, and had numerous example of inventors trying to create perpetual motion machines....
Not really, not unless you are assuming very theoretical situation like no friction, no gravity, etc. |
I really dont understand why this talk of perpetual motion and newtons second and first law coming into picture. When I have already said that 10 Nm is the torque required to overcome system resistance, this should answer your above point.
Ok lets modify the example, that the shaft at one end has a fan blade attached to it and is dipped in a viscous fluid (resistance). So once the clamp is released there is still resistance of 10 Nm after the clamps are released.
So now does it makes sense...
Just for you to get a feel I will give another analogy (I have learnt something from you!)
You have a bicycle and you are pedalling hard read standing on the pedals (torque) to climb a hill (resistance) and suddenly the chain comes off the sprocket.
What happens?
Your leg were applying torque and suddenly the load is removed which makes your legs go down with violent speed (read increase in rpms).
After that depending on your body you will keep on pedaling at increased rpms which suit your body (system design/inertia)
Well ultimately the person riding the bicycle will fall down and have a nasty bump between the legs. Telling from my own experience
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It will induce acceleration, but not steady, in fact the acceleration will be decreasing until the equilibrium is reached, where acceleration is zero.
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No not as per your definition of constant torque drives.
Again I repeat for maintaining rpms power is required not torque. Torque is required only to overcome the resistance.
If torque was required to maintain rpms then we should have rpms in definition of torque which is not there anywhere.
Its already mentioned in my earlier posts that
T=F*r formula to be used for static equilibrium (that also applies to steady state ie a shaft rotating at constant rpm with constant load)
T=I*alpha (For transient state)
and P=K*N*T applies for steady state again.
Let me explain the T=I*alpha with another analogy..
Lets take a shaft without any load, and suddenly you apply torque to it. What happens it starts rotating agreed, but if you observe at a microscopic level the portion of the shaft (fibres)near the torque source twists (due to inertia even without load) and the other end of the same shaft lags behind by a very minute amount. This twist leads to the increase in torque because torque is measured by strain or twisting of the shaft.
Another example in an accident your seat belts hold your body but your neck gets a whiplash beating right. The same thing happens with a shaft microscopically(as the shaft is not perfectly rigid practically in real world).
The more the impact speed more the de-acceleration (g forces) the more the damage to the neck, same for a shaft.
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Now you have rightfully said power is required to maintain that 1000rpm. But if you reduce T, power will go down. To keep that power, you need the torque that got you there. There you go, I am using your formula.
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How easily you forgot the rpms and kept it constant.
If you reduce torque the power will go down, agreed but then it doesnot because there is a increase in rpms to balance the power.Thats what we see in the bicycle example right?
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Let's take your scenario. You say the rpm will go on increasing if we sustain the 50Nm of torque, that means P will also go on increasing... possible? Not possible, the drive can't possibly provide infinite power, while we know constant torque drives are real.
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To keep torque constant the Power will go on increasing. Yes, the drive should provide infinite power (as per the definition of constant torque drives which would maintain constant torque on shaft irrespective of load). This tendency to reach infinite power should destroy the system.
If you have seen Constant Torque drives in real then it should have a speed limiting mechanism also.
I think ampere has also talked about the same.