Calculate Horsepower from a flywheel

[Onesnorkeler]

I have a flywheel (not connected to an engine, just on a shaft and bearings, so assuming zero resistance), that weights 60 lbs, 18" wide, at 3200 RPM's.

How can I calculate horsepower, brake, or torque?

Working on a project,

Thank you,

Larry

 

[anorlunda]

With zero resistance and constant RPM, zero,power is required. That is Newton's first law.

With nonzero resistance and constant RPM, the power needed is proportional to the resistance.

To accelerate from 0 to 3200 RPM requires power but you must specify how long it takes to accelerate.

 

[Onesnorkeler]

I only want to know the horsepower it has at the current rate of speed for the given weight of the flywheel. For this I don't need to know what it was that got it there, make sense?

For what I am doing I need to know what generator I can run off of this flywheel (yes, I know loads will depend, but this is my starting point). What is the potential of the load I could apply (is what I am trying to find out)

Larry

With zero resistance and constant RPM, zero,power is required. That is Newton's first law.

With nonzero resistance and constant RPM, the power needed is proportional to the resistance.

To accelerate from 0 to 3200 RPM requires power but you must specify how long it takes to accelerate.

 

[anorlunda]

Re-read #2. The answer is zero. You misunderstand why we need power. It is to overcome the resistance.

 

[berkeman]

or what I am doing I need to know what generator I can run off of this flywheel

Your flywheel has an amount of stored Energy. The power you extract depends on how long you take to deplete that Energy. Power is Energy/Time:

https://en.wikipedia.org/wiki/Flywheel_energy_storage

 

[russ_watters]

I only want to know the horsepower it has at the current rate of speed for the given weight of the flywheel. For this I don't need to know what it was that got it there, make sense?

You keep saying power, but it sounds like you are describing kinetic energy. Do you understand the difference?

When you connect it to a load at constant power it will slow down and eventually stop.

For what I am doing I need to know what generator I can run off of this flywheel (yes, I know loads will depend, but this is my starting point). What is the potential of the load I could apply (is what I am trying to find out)

That will depend on how long you want to run it. You should Google "flywheel ups".

 

[Onesnorkeler]

You are correct, I do not understand this (humbly why I am asking for help).

Assuming I can continue to maintain constant of 3200 RPMs, what horsepower would that represent? (For example, could it maintain a 100KW Generator, maybe an exaggeration, but it is what I am trying to get at.)

Think of it this way:
I am using water to turn a turbine, the turbine will maintain the 3200 RPMS of the flywheel, what can I convert that too?
(I am trying guys, sorry for my ignorance. If you have construction questions, I can provide the expertise in return)

Larry

 

[russ_watters]

You are correct, I do not understand this (humbly why I am asking for help).

Assuming I can continue to maintain constant of 3200 RPMs, what horsepower would that represent? (For example, could it maintain a 100KW Generator, maybe an exaggeration, but it is what I am trying to get at.)

Think of it this way:
I am using water to turn a turbine, the turbine will maintain the 3200 RPMS of the flywheel, what can I convert that too?
(I am trying guys, sorry for my ignorance. If you have construction questions, I can provide the expertise in return)

Larry

It's ok, this is actually a common misunderstanding.

The flywheel is actually irrelevant to the power generation (it will just smooth it out a little). What you really need to know is how much mechanical power (torque times rpm) your turbine is generating.

...and 100kW is a lot, so i suspect you are going to be disappointed. What can you tell us about the water turbine? How much flow and how high is the water falling from?

 

[Nugatory]

You are correct, I do not understand this (humbly why I am asking for help).
Assuming I can continue to maintain constant of 3200 RPMs, what horsepower would that represent? (For example, could it maintain a 100KW Generator, maybe an exaggeration, but it is what I am trying to get at.)

Think of it this way:
I am using water to turn a turbine, the turbine will maintain the 3200 RPMS of the flywheel, what can I convert that too?
(I am trying guys, sorry for my ignorance. If you have construction questions, I can provide the expertise in return)

The amount of power you will get out of the flywheel over any extended time is equal to the amount of power produced by the turbine - in fact, the flywheel is completely irrelevant to the power output (except to the extent that you'll lose a little bit of power to friction in the flywheel bearings).

This is because the flywheel isn't creating any energy, it's just storing energy created by the turbine, and of course you can't get any more energy out of the flywheel than you've put into it.

So why bother with a flywheel at all? It's easier to see why with a gasoline engine driving the generator, so I'll use that example (although the principle is the same as with a water turbine). Suppose that the load on the generator is 3 kilowatts, so the engine is happily chugging out 3KW, everything is spinning at 3200 RPM, 3 KW are coming out of the generator (less a tiny loss for friction in the bearings).

Then the load suddenly increases to 5 KW (maybe someone switches on a electric motor to increase the load). The load increases immediately, but it take a second or so for the engine's throttle to open up enough to allow it to generate 5KW - so the engine slows down and stalls and now we aren't getting any power at all. But with a flywheel, the flywheel slows down just a little bit, releasing just enough of its stored energy to deliver 2KW and keep everything spinning for the second that it takes for the engine to catch up.

 

[Onesnorkeler]

Ok Nugatory,

I am completely with you up to this point (I think).
So the flywheel has the stored energy, what is that potential stored energy? (assuming the flywheel can maintain the 3200 RPM speed?

Side Note: Russ Watters, I AM LMAO about the the flow and height of the water (such and engineer and loving it)...just a hypothetical.

But if it was water driving the flywheel/turbine or gas engine, all the same. What I am trying to gauge (as in figure out Russ-LoL), is could I drive a 50KW Generator at full load?
What if I was able to take the RPMs up to 6 or 8000, or a lot more? (yes, I know there are limits, not a concern in this case. The flywheel is a high grade SS, bearings will be the issue first, but that is a different conversation).

I also have to weight the flywheel to be sure, 60 lbs is a guess at the moment, but it is balanced and spins true.

 

[gleem]

To generate 100KW of electrical power you need the wheel to turn at a rotational speed equal to what the generator must turn to generate the 100KW..

 

[Nugatory]

So the flywheel has the stored energy, what is that potential stored energy? (assuming the flywheel can maintain the 3200 RPM speed?

(It's not potential energy, it's kinetic energy - this is a quibble if you just chose the wrong word but do understand the difference between the two).

The general formula you are looking for is $E_K=I\omega^2/2$ where $\omega$ is the rate of rotation and $I$ is a a property of the flywheel that depends on its shape and weight, called the "moment of inertia". Google will find formulas for the moment of inertia of most shapes that you would use for a flywheel, so I'll leave that for you to look up.

But if it was water driving the flywheel/turbine or gas engine, all the same. What I am trying to gauge (as in figure out Russ-LoL), is could I drive a 50KW Generator at full load

Only if your water wheel or gas engine is able to deliver 50 KW. If it's not, then you can't. The only thing the flywheel does for you is keep everything running if the load spikes above 50 KW for a moment.

What if I was able to take the RPMs up to 6 or 8000, or a lot more? (yes, I know there are limits, not a concern in this case. The flywheel is a high grade SS, bearings will be the issue first, but that is a different conversation).

That increases the amount of energy the flywheel will store, so that in theory you'll be able to get through a longer or higher spike in power demand. In practice, however, generators must be run at a particular fixed RPM so either that's the speed your flywheel spins at or you will need reduction gears (which are either insanely expensive, or lose too much energy to friction, or both). In fact, the easiest way to understand the purpose of the flywheel is that it keeps the rotation from fluctuating when the load fluctuates and the real power source hasn't had time to compensate yet.

 

[Onesnorkeler]

I understand but confused:

Lets say my water wheel is what is the shaft (be it a turbine/engine/etc), how do I know how many ponies I have?
The only thing I have to compare to at the moment is the flywheel. How am I able to calculate the horse power I have at the shaft?

 

[russ_watters]

I understand but confused:

Lets say my water wheel is what is the shaft (be it a turbine/engine/etc), how do I know how many ponies I have?
The only thing I have to compare to at the moment is the flywheel. How am I able to calculate the horse power I have at the shaft?

Power is force times speed. For a water wheel, that's flow rate times pressure.

So if you want 100kW, adding a 50% efficiency factor and converting units gives an input of 268 horsepower or 147,500 ft-lb/sec.

Water weighs 62.4 lb per cubic foot, so if you have a drop of 100ft(arbitrary) you'll need 1418 cubic feet per minute or about 100,000 gpm.

 

[CWatters]

I think this is the third time the OP has asked this question?

Edit: Perhaps I'm mistaken, it just looks familiar.

 

[CWatters]

A crude way would be to disconnect the flywheel when the running and measure how long it takes to come to a stop. Then divide the KE stored in the flywheel by the time. That would give you a rough estimate of the power it takes to turn the flywheel against fricton.

If you can measures the rpm you can improve the accuracy by measuring the time needed to slow down by say 10%. The maths is only slightly more complicated because the energy stored is proportional to rpm squared.

 

[anorlunda]

@Onesnorkeler ,

We are confused and not sure what you're really asking. It sounds like you have some incorrect idea about the relationship between RPM and power. Please try again to phrase your question without mention of flywheels or RPM.

 

[Onesnorkeler]

CWatters,

This is something I can work with (even if the info will be a bit vague, it gives me enough to go on for now).

I will weigh the flywheel in the future (I have to take if off the mount (not on a motor), but for the equation let's say 60 lbs. The time it spins from 3200 RPMs to stop is 10 minutes (I may be off a little bit, I will retime that, it has been a while since I tested it like that, but close enough for the moment).

A crude way would be to disconnect the flywheel when the running and measure how long it takes to come to a stop. Then divide the KE stored in the flywheel by the time. That would give you a rough estimate of the power it takes to turn the flywheel against fricton.

If you can measures the rpm you can improve the accuracy by measuring the time needed to slow down by say 10%. The maths is only slightly more complicated because the energy stored is proportional to rpm squared.

 

[Onesnorkeler]

Amorlunda,

I am trying to figure out what size generator I could power if I were able to maintain speed of the flywheel (I do not know the force/power of the turbine powering the flywheel (sorry).

@Onesnorkeler ,

We are confused and not sure what you're really asking. It sounds like you have some incorrect idea about the relationship between RPM and power. Please try again to phrase your question without mention of flywheels or RPM.

 

[sandy stone]

I think what people are trying to explain is that you can't pull more power from your flywheel than you put in. If you could maintain the RPM under any conditions, then there is no limit to the power you could draw. What's maintaining the RPM is your turbine, and if you don't know it's power then there is no answer to your question.

 

[Onesnorkeler]

Hummm,

I understand but am also confused. Take a Tesla turbine, he used steam to run his turbines (also water), how does that equate? What if you were to use a pressure washer on the same turbine (similar to steam for conversation)? Wouldn't the wider the disc create greater torque, thus more horsepower?
(yes, I admit I am ignorant, but trying to learn)

 

[Nugatory]

Wouldn't the wider the disc create greater torque, thus more horsepower?

If you can put a given amount of pressure on every square centimeter of the disk, than the wider disk will indeed develop more power at a given RPM. But if the disk is larger you have to throw more water or steam or whatever against it to maintain the same pressure across the larger area and keep it turning at that RPM.

To borrow from Russ's example in post #14: there's a certain amount of power that you'll get out of 100000 gpm dropping 100 feet. You can use this to drive a big turbine slowly or a smaller turbine faster but the power output is the same. To increase the power output you need to either increase the gpm or increase the height the water falls from so that you're putting more energy into the system per unit time.

 

[CWatters]

Can you give us the dimensions of the flywheel? Diameter? width? Is it a plain disc or is the weight concentrated at the rim?

 

[CWatters]

I have a flywheel (not connected to an engine, just on a shaft and bearings, so assuming zero resistance), that weights 60 lbs, 18" wide, at 3200 RPM's.

If we assume that's a plain cylinder of diameter 18" (0.46m) and mass 60lbs (27kg) then the moment of inertia is given by..

I = 0.5mR²
= 0.5*27*0.23²
= 0.714 kg.m²

3200 rpm = 335 Radians/sec

The energy stored in the flywheel is:

E = 0.5*I*ω²
= 0.5*0.714*335²
= 40,000 Joules

If it takes 10mins = 600 seconds to stop then a very rough estimate of the power required to overcome friction and other losses in the flywheel is

40,000/600 = 67W

In other words it takes <100W to keep the flywheel turning at a constant speed.

So if you disconnected the flywheel and used the motor to turn something else like a generator it should be capable of delivering around that much power to the generator.

I don't think this will be a very accurate estimate. If you can tell me how long it takes for the rpm to drop from 3200 to say 2800 rpm then I can make a better estimate.