Calculate Rotational Torque: 8700 kgm2 @ 18 RPMs

  • Context: Undergrad 
  • Thread starter Thread starter Pinon1977
  • Start date Start date
  • Tags Tags
    Rotational Torque
Click For Summary

Discussion Overview

The discussion revolves around calculating the torque produced by a flywheel with a moment of inertia (MOI) of 8700 kgm² rotating at 18 RPMs. Participants explore the relationship between torque, angular velocity, and the operational characteristics of the flywheel in the context of connecting it to a generator. The conversation includes theoretical considerations, practical applications, and the implications of various factors affecting torque output.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant questions the use of angular velocity instead of angular acceleration in the torque calculation, asking for clarification on the acceleration involved.
  • Another participant explains that torque is necessary to overcome friction or to accelerate the flywheel, not simply derived from the MOI and angular velocity.
  • A participant describes the flywheel's function as being powered by a prime mover and seeks to determine the torque available at the axle for generator sizing.
  • Concerns are raised about the flywheel's constant RPM and whether the motor is using all available power to overcome friction, leaving no spare power for the generator.
  • Participants discuss the relationship between the flywheel's kinetic energy, angular momentum, and the torque required to operate a generator under load conditions.
  • One participant emphasizes the need for specific load requirements to calculate how long the flywheel can sustain power output, noting that torque may not be relevant in this context.
  • Another participant suggests that the system's viability is questionable without clear engineering facts and that torque is unknowable without further information.
  • There is a reiteration of the equation relating input and output torque, emphasizing that without a generator connected, the output torque is zero.
  • A participant specifies the generator's requirements, indicating a need to know the output torque of the prime mover to select an appropriate generator.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of torque in the context of the flywheel's operation and its connection to the generator. There is no consensus on how to calculate or interpret the available torque, and multiple competing perspectives on the system's functionality remain unresolved.

Contextual Notes

Participants acknowledge various factors that may affect the calculations, including friction losses, the nature of the prime mover, and the load on the generator. The discussion highlights the complexity of the system and the need for additional information to arrive at a definitive answer.

  • #31
Pinon1977 said:
I feel like we are losing our focus here. Regardless of what's making the driver rotate, I just need to know how to calculate the available torque at the axle at 18rpms.

Problem is you are asking a question that's impossible to answer based on the info provided. If it stops accelerating at 18rpm then there is NO torque available. It's all being used to overcome losses somewhere. You even explained that centrifugal force somehow stops or reduces the power to stop it accelerating further.

That's why I suggested using a lower rpm in the calculation above.
 
Physics news on Phys.org
  • #32
CWatters said:
Ok so if it takes say 3 seconds to go from zero to 18 rpm you can estimate the torque and power as follows...

Torque = MOI * Change in angular velocity/Time

18 rpm = 0.1 rads/s

= 8700 * 0.1/3
= 290 Nm

However if it has stopped accelerating at 18rpm you won't get that torque at 18rpm. Better use a lower value say 9rpm (0.05rads/s) for safety..

Then..

Power = Torque * angular velocity
= 290 * 0.05
= 14W

I have assumed your figure for the MOI is correct.

While I do not know exactly what the torque is at the axel, I do know it's more than 290Nm. I used a 4000 ftlbs prony brake last week and it maxed it out in less than 2 rotations. So the torque is at least 4000 ft lbs at 18rpms.

What's wrong with this calculation:
8700kgm2 * (18 RPMS/9.8) OR
8700kgm2 * 1.8 rad/sec = 15660Nm
 
  • #33
14 watts? Really? Common sense would tell you that a flywheel with a MOI of 8700 kgm2 spinning at 18 rpms is going to produce more than 14 watts of power. Right? I can generate 100 watts of power with my bicycle generator.
 
  • #34
This driver or (prime mover) I have designed is 10' in diameter with an overall weight of 3900lbs. The entire assembly creates a MOI of 8700kgm2. The driver is rotating at a constant speed of 18 rpms. That being said, you're telling me one can only expect 14 watts of power out of that beast?
 
  • #35
Pinon1977 said:
Common sense would tell you that a flywheel with a MOI of 8700 kgm2 spinning at 18 rpms is going to produce more than 14 watts of power. Right?
Depends for how long.

Pinon1977 said:
The driver is rotating at a constant speed of 18 rpms.
If it doesn't slow down, you will get out the power you put in, minus losses.
 
  • #36
But it's still a state secret where the original Energy is coming from. As far as I'm concerned, the whole enterprise is suspect until that is known. Why should PF have any part of it? A machine that drives itself is just not in PF's terms, is it?
 
  • #37
Pinon1977 said:
What's wrong with this calculation:
8700kgm2 * (18 RPMS/9.8) OR
8700kgm2 * 1.8 rad/sec = 15660Nm

You are multiplying MOI * Angular Velocity. That gives you the angular momentum not the torque.
 
  • #38
Pinon1977 said:
gravity was probably never perceived as a source for creating power. It's always been the enemy
What sort of a statement is that? How can gravity be an "enemy"? In any case. gravity is not the "source" of hydro power. The SUN is the source of the power because it causes evaporation which gives us rain etc. etc.. A long case clock 'uses' gravity as the weights fall but the guy who wound it up was the source of the energy.
 
  • #39
Pinon1977 said:
14 watts? Really? Common sense would tell you that a flywheel with a MOI of 8700 kgm2 spinning at 18 rpms is going to produce more than 14 watts of power. Right?

Wrong. Flywheels spinning at a constant rpm do not produce any power. Zero. None.

Perhaps you misunderstand what I calculated...

I calculated the torque required to accelerate your flywheel from rest to 18rpm in 3 seconds. That only takes a constant 290Nm. So all I can say is that your prime mover is capable of delivering at least that much torque.

If you say your prime mover is capable of generating more torque then that's fine but you will have to measure it. It cannot be calculated from the MOI and rpm alone.
 
  • #40
Pinon1977 said:
I used a 4000 ftlbs prony brake last week and it maxed it out in less than 2 rotations. So the torque is at least 4000 ft lbs at 18rpms.

All that shows is that the torque is at least 4000 ft lbs for 2 revolutions (about 6 seconds).

Leave it connected for 10 mins and measure the rpm after that time.
 
  • #41
Pinon1977 said:
14 watts? Really? Common sense would tell you that a flywheel with a MOI of 8700 kgm2 spinning at 18 rpms is going to produce more than 14 watts of power. Right? I can generate 100 watts of power with my bicycle generator.
Yes, because you operate a generator in this example. It's not the wheel that produces the energy.
CWatters said:
Wrong. Flywheels spinning at a constant rpm do not produce any power. Zero. None.

PMM are not possible, regardless how well hidden the mechanism or yield is. We do not discuss magical tricks on PF. The question has been answered in many ways. All use fundamental physics.

Thread closed.
 
Last edited:
  • Like
Likes   Reactions: Nidum

Similar threads

Graduate Rotation of a roll by a driving wheel
  • · Replies 24 ·
Replies
24
Views
3K
Automotive Calculating Rotational Torque of Prime Mover
  • · Replies 5 ·
Replies
5
Views
2K
High School How to Calculate Power and Torque for a 15kg Load at 15 RPM?
  • · Replies 7 ·
Replies
7
Views
3K
Undergrad Conservation of angular momentum in the iceskater example
  • · Replies 10 ·
Replies
10
Views
2K
Graduate Order of rotations due to torque in 3DOF in simulations
  • · Replies 2 ·
Replies
2
Views
2K
High School The physics of flywheel launchers (like tennis ball shooters)
  • · Replies 60 ·
3
Replies
60
Views
7K
Undergrad How Does Torque Affect Balance on a Self-Balancing Electric Unicycle?
  • · Replies 10 ·
Replies
10
Views
4K
Undergrad Acceleration on an electric unicycle, how much does the rider have to lean?
  • · Replies 30 ·
2
Replies
30
Views
3K
Undergrad Calculating Torque for Rotating Wheels: Factors to Consider
  • · Replies 21 ·
Replies
21
Views
4K
Graduate Reaction torque in a traction system
  • · Replies 10 ·
Replies
10
Views
2K