HP Motor for Flywheel?

[ME_Guy2594]

Hey everyone! I am new here so I hope I can get some help! The question is...
I have a flywheel with an I = 2.8 slugs-ft^2 that needs to spin to 11,300 RPM in about 5 minutes. I need a motor to do this. what horsepower is needed?

I got roughly 764HP using these equations,
HP = (torque x RPM)/5252
Torque = I x a
v = v0 + at; v0 = 0

I just want to make sure and check to see if I used the right units and such. Thank you!

P.S. I have tried to googled such a case for help and came up with nothing. Also, I am not a student, however I am a new engineer who still has a lot to learn!

 

[JStephen]

I'll leave checking the units to you. If I visualize I = 2.8 slugs-ft^2 as being 3 weights of 32 lbs located 1' from the axis, that's not a giant assembly, maybe something like a truck wheel/tire or smaller, and I'm thinking 800 hp could get that up to speed in more like 5 seconds rather than 5 minutes.
Assuming you're not a student and this is a real problem (which seems unlikely, to be honest), you probably need to start contacting motor suppliers.
I'm assuming that motors don't have uniform torque or power output across their entire range of speeds, for example.
And I'm thinking 800 hp motors generally don't turn 11,000 RPM, so you're going to have some kind of gearing in there, maybe some kind of transmission or torque converter- anyway, some additional power loss in that system. Think locomotive traction motor for that 800 hp.
If you're starting at zero and accelerating over 5 minutes time, check for overheating in the motor under those conditions as well.
On your approach, you're assuming constant acceleration and constant torque, right? But constant torque at varying speed means varying power, so you don't have uniform power used?
You could use energy methods on the problem also.

 

[GregLocock]

As a new engineer what is your gut feeling about 764 hp as an answer?

I can't say whether your method is right or wrong, as you are making unstated assumptions.

I could make my assumptions but then I'd be solving my problem not yours. I don't come close to your answer.

As to units, you seem to be likely to have an error of 32.2ft/s/s

In future you might aswell post your entire working of s problem.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[TheTick]

I never touch these problems in imperial units any more.

 

[ME_Guy2594]

I had a feeling it is wrong and way overkill for that little flywheel and I did a recalculation and got around 24 Horsepower. I messed up a conversion in my calculations. And this a real world problem I was given. Thank you everyone for getting my brain kick started again though. definitely helped.

I am currently plugging in friction and windage losses also.

 

[drawoh]

ME_Guy2594,

I have no problem with English units, although I try to stay away from slugs.

I am getting a constant acceleration torque of 11ft.lb, and 24HP at maximum speed. Try working things out from first principles, and write out your calculations with a unit balance. 1HP=550ft.lb/sec. Your acceleration torque from zero to 11,300rpm almost certainly will not be constant, as I have assumed.

--
JHG

 

[gruntguru]

764/32 = 24 (Well done Greg)

As pointed out above, the "constant torque" case does not answer the question as posed ("I need a motor to do this. what horsepower is needed?") since your constant torque motor is producing zero hp at the start of the acceleration and 24 hp at the end. To find the minimum power required, best use an energy method - (final KE)/(acceleration time) = power required.

PS. Since your constant torque model has a linear, rising power requirement, the average power over 5 minutes is 24/2 = 12 hp = minimum motor power requirement. (unfortunately torque at zero speed will be infinite, although even with a finite torque limit, the minimum power requirement will be not much more than 12 hp)

je suis charlie

 

[dvd]

There are quite a few AC motors that don't like to reach full speed over such a long period of time. Is this something that is going to be accomplished with a vfd? How is the motor torque intended to be transmitted to the flywheel? Theoretical calculated HP may not be practical.

Some pertinent information here - thread404-167467

Also recommend to look at pages 37-39 of Cowern Papers.

 

[mikekilroy]

But there are many many more AC motors that are fine with reaching speed after a long period of accel. I have never run into any AC motor that would not accel this type of load over this 5 minute time. Ever. Consider: @ constant 11#-ft torque output on a 24hp motor (rated 70#-ft), it will be in almost unloaded state during the critical <1/3 rated rpm range; hence NO overheating.

Since the object of this excersize is to spin up a flywheel, I see no reason to not use the rated 70#-ft rated torque from 0-1800rpm (or 35#-ft 3600rpm if 2 pole design) and spin her up quicker.

That said, trying this with a 12HP motor will fail and NEVER reach speed in 5 minutes. Even your 24HP would likely fail to achieve even 11#-ft at the high speed end.

For the 12HP case, no motor will allow 200% overload (breakdown torque) way up in the field weakening area of 11,300 rpm.

For the 24HP case, you will not even obtain nameplate motor torque rating that high above base speed. The losses mount and you will eventually hit a speed where the motor torque rating will drop off - usually on this size spindle motor of these speeds of around 20% drop off in torque rating. Yes, we play games with the motor designs to allow wider constant HP range, but physics is physics - the games we play simply at the end of the day make it a larger HP rated motor anyway.

Assuming there is negligable friction, which there MUST be or the heat at 11,300rpm will destroy the system in minutes, I would pick a 30hp motor to stay out of overload at the high speed. Many manufactures build this size and speed motor for use on machine tool spindles.

http://www.KilroyWasHere<dot>com

 

[3DDave]

Why assume the motor is directly connected to the load?

Anyway, it is important to look at the area under the motor curve over the range of operations to determine the amount of energy available over the range of speed change. Since the motor is accelerating, the calculation should also include the rotor inertia and any multipliers and effects of intermediate transmission inertias.