Quote:
Originally Posted by Samurai But constant torque drive will mostly be a electric motor, which takes negative feedback. That means a closed loop system. Let's say the motor is delivering 100Nm. Say at load X, the rpm is 1000. If the load suddenly drops, the rpm will pickup instantly to bring the torque back to 100Nm. |
Can it really do so? No it can not. By picking up rpm will not bring torque back.
T=I*
Angular acceleration
Torque is dependent on acceleration and not angular velocity (rpm) so that means to get the torque of 100 Nm the angular acceleration will have to be constant and not the angular velocity (rpm).
So in this example, lets say the angular acceleration needed is 10 rpm/second square to reach a torque reading of 100 Nm.
Once 10 rpm/s2 is reached the torque levels should fall, but as per the definition its a constant torque source so to maintain constant torque on the shaft the motor will make the shaft to accelerate constantly at 10 rpm/s2.
So after a period of time the shaft would have reached speeds (rpm) crazy enough to break the whole apparatus.
If you see the graphs, I always say that once the rpms come into picture the torque domain ends and Power starts.
And we should use the Power formula of:
P= 2*Pi*N*T/60 where N is rpm and T is torque so simplified equation is
P= K*N*T where K is constant. This formula can be used for the steady state condition.
So with more rpms means more power.
Quote:
Therefore, if the wheel was stationary and then released, the torque will remain same, even if the speed goes from 0 to 1000rpm.
|
It cannot be the same. When the wheel was stationary the torque on the shaft was the max. torque what the source can apply say 100 Nm
Once you unclamp the wheel (say remove resistance) the torque level should fall down. And once a steady-state is reached the torque will be at 10 Nm (say this is the torque required to just keep the wheel rotating read system resistance). Please note here we are not using a constant torque source which will itself ensure that 100 Nm is maintained on the shaft. I am talking about a source which was applying 100Nm when the clamps were applied.
Quote:
I have no practical experience of constant torque motors. Won't it be dangerous to have these motors at no load? I guess the motor circuit will have breakers (Zener diodes) to avoid such situations. May be those motors always have load.
|
Same for me about closed loop and open loop. But what I understand is that if you have a constant torque source (which somehow tries to maintain constant torque on the shaft irrespective of the load) it has to have a safety feature, otherwise it will blowup itself to pieces.
In my earlier examples (previous to 4-5 posts) I was trying to consider a theoretical source which will apply a max torque of 100 Nm and try to stay at a constant rpm of say 500 rpm.
I had explained the same thing earlier...
Quote:
Originally Posted by amit_purohit20 Uptill now I was not thinking of the torque source. So all the explanations in my last 3-4 posts were assuming constant torque and constant rpm source and not the constant torque source which will accelerate the shaft till the earlier torque levels are reached.
Say 50 Nm was the torque before clamps were released, after releasing, the torque source accelerated the shaft to achieve 50 Nm. Fine but remember here its the acceleration not the angular velocity.
So what happens next after 50 Nm is reached? It will fall down right?
If not because its a constant source torque then the shaft should maintain constant acceleration to match 50Nm. That means shaft would reach very high rpms till the whole thing breaks down.
So the notion of a source which maintains constant torque even after release of clamps is not right.
Thats why I assumed a source which will be able to apply a torque of 50 Nm considering resisting torque is there and wont go beyond a certain fixed rpm say 500
In such a case whatever I have written in previous posts should hold true. |
Quote:
Originally Posted by Samurai When the shaft is clamped, the torque is 50Nm. That is in equilibrium.
When the clamp is released, the shaft starts rotating fast enough to counter the 50Nm of applied torque. Slowly at first, for need of breakaway torque, then faster until a steady speed is reached. Again we have equilibrium. |
With decreased load and steady speed can we have the same torque? No right, because its not the speed its the steady acceleration we need for the same torque.
Quote:
let's look at the time lapse.
T0 second - 50Nm of torque is fighting the clamp
T+ second - 50Nm of torque is fighting the inertia of the just released shaft
T1 second - 50Nm of torque is accelerating [not just turning] the shaft at 50rpm/sec
T10 second - 50Nm of torque is accelerating the shaft at 25rpm/sec [accleration is reducing every second now]
T20 second - 50Nm of torque is turning the shaft at 1000rpm, acceleration is 0. Equilibrium is reached.
|
My version (Considering a source which can apply 50Nm of torque and not constant torque and is say limited to a finite rpm due to practicality:
T0 second - 50Nm of torque is fighting the clamp.
T+ second - 50Nm of torque is fighting the inertia of the just released shaft
T1 second - Applied 50Nm of torque is accelerating [not just turning] the shaft at 50rpm/sec, but the reading of torque will go down because the load has decreased so 50Nm can not be seen on the torque measurement device.
T10 second - We see a torque spike as the shaft accelerates to a certain level and then starts deaccelerating. The torque spike is surely below the 50 Nm torque and above the 10 Nm torque (which is the bare minimum torque required to rotate the wheel read resisting torque without the clamp)
T20 second - Torque level is now 10 Nm and the rpms stabilised to say 500/1000/1500 any rpm based on the nature of the source and the whole system and its inertia.
Quote:
Why should the torque drop? The 50Nm is required to maintain the shaft at 1000rpm.
|
No the torque required to maintain 500 or 1000 or 1500 rpm is the same and is dependent on the resistance of the system.(Steady State)
Only when the system is accelerating (unsteady state) the torque required will be different.
Quote:
Originally Posted by DirtyDan In an open loop constant torque system acceleration continues indefinitely. |
Totally agree.