Electrical motor selection

[74spider]

Hi All,
This is my first post to this forum so Hi!, and I'd like to ask a question please.
I need to select a DC electric motor to drive a flywheel. The connection between the motor and the flywheel can be direct or geared. The flywheel mass would ideally be in the order of 10KG and needs to be spun up to 500rpm in about 5 seconds, so the motor needs to have good torque qualities. The crunch is, I'd like to power this motor from a rechargeable battery (possibly 10, D size rechargeable batteries (each 1.2V, 2600mAh). Maybe the motor could provide a charging current to the battery when the flywheel is slowing down?
Other battery types could be used but the whole system may be mounted on a turntable running at up to 500rpm so the battery needs to be robust and sealed so as not to leak when spun.
Ideally the motor would be quite 'flat' geometrically, so the main body of the motor could have a large diameter of say 100 to 150mm, but a height of only about 50mm (plus drive shaft length).
Can anyone advise me if there is something out there that fits this bill please ?
If I've left anything unclear, please let me know and I''l give more details.
I would really appreciate any advice.
Many thanks in advance.
74Spider.

 

[MetalworkerMike]

You're going to have a 10KG flywheel spinning at 500rpm and the whole thing mounted on another platform spinning at 500rpm? That sounds like a scary device. Good luck balancing _that_ sucker.

Mike

 

[gepman]

74spider
You need to provide the moment of inertia of the mass. It will take less energy to spin the flywheel if the weight is concentrated at the center than at the edges. You should know the required moment of inertia by the amount of energy that you want to store. I am not going to take the time to go through a bunch of calculations but my gut feel is that there isn't enough energy stored in 10 D batteries to get a 22 pound mass moving to 500 rpm (just thinking of my rechargeable drills).

You should go through all the calculations and then state the the required average accelerating torque. Then someone can let you know if there is a motor available with the dimensions, power source voltage, and accelerating torque that you need.

 

[74spider]

Hi Mike and Gepman,

Firstly, thanks for your posts - it's much appreciated.

I'll run through your points in order, Mike's first:
Regarding balancing the whole system, all the rotating elements share a common axis and other elements such as the batteries will be positioned equally around the axis for balance. A small amount of weighting is expected after assembly of the whole system to fine tune the balance.

For M of I, the flywheel is the conventional disc type like you find in most car engines so this will be 0.5((m.r)^2), as near as is required for this stage of the project.

Gepman, you refer to your drill and this is how I was hoping to skip the next stage of calcs (and any errors I manage to build in!), by someone having a feel for this sort of application already. The mass and rpm of the flywheel can both be reduced if that's required to fit my 10 x D type battery source (unless another power source can be suggested of course !).

I can see that I may not avoid all those extra calcs but hopefully someone can estimate what sort of acceleration and rpm I can get from 10 D types for a given mass and flywheel geometry.

Thanks again for your comments.
Best regards,
74Spider.

 

[MetalworkerMike]

You mention that the flywheel will be similar to an automotive flywheel.. is there any reason why you can't drive it from the edge? It would be a heckuvalot easier on the motor, and you wouldn't need to use batteries.

Mike

 

[gepman]

74spider

I don't like to skip calculations on things that I haven't done before. Calculate the energy in the flywheel and the energy in the batteries if you want to see if it even feasible. You can't get something for nothing. See following formulas for speed and torque requirements.

seconds (to reach operating speed)=[Wk2(lb-ft2)*speed change (rpm)]/[308 * avg. accel. torque (lb-ft2)

Avg. accel. torque = {[(FLT+BDT)/2]+BDT+LRT}/3

BDT = Breakdown torque
FLT = Full Load torque
LRT = Locked Rotor torque

I am not sure I agree with MetalworkerMike on it being easier on the motor since the same energy needs to be imparted to the flywheel (I think that the torque will be less but the rpm will need to be higher) but otherwise I think it would be a good idea just like a car flywheel ring gear.

 

[sreid]

1) We really need to know the inertia of the disk. Can you guess a radius and thickness?

2)High torque, low voltage, low RPM motors are hard to find. Driving through, say, a belt reduction to the OD of the flywheel would result in a more efficient drive (with an available motor).

 

[74spider]

Thanks again for your comments guys.

Hi Mike, yes the motor can be geared to the flywheel to give us a higher revving motor but I think this would still put the same power draw on the motor(?). Not sure what you mean about not having to use batteries though (neither of the other guys mentioned this so I expect I'm missing something !)

Hi Gepman, thanks for the formulae. No probs with a ring gear type arrangement, in fact this way I could use 2 or 3 motors equipspaced around the flywheel axis for balance (smaller motors but same power source).

Hi sreid, estimating the flywheel dims, plain disc type flywheel, 20cm radius, 1cm thick. If steel is 8000KG per metre^3, this equates to about 10KG (22lb?). I make the M of I for this size of flywheel to be 0.2

Gotta shoot guys as I'm travelling for work all day tomorrow and need to get sorted but I'll spend some time progressing the power calcs to accelerate this flywheel from rest to 500rpm in say 5 seconds. That should give me a good pointer for the power source and motor(s) required.

Thanks again guys.

 

[sreid]

For I= 0.2 kg-m^2 and 500 RPM, I get an energy of 274 N-m = 274 Joule = 274 Watt-Second.

12 Volts x 2.6 Amp-Hr = 31.2 Watt Hour = 112,320 Watt Second of power available.

Accelerating to 500 RPM (52.3 Rad,sec) in five seconds requires an acceleration of 10.46 rad/sec^2

The torque required = Ta = .2 x 10.46 = 2.1 Newton-meter.

 

[MetalworkerMike]

Using a motor that drives from the edge of the flywheel will require far less torque to start which makes it easier to get it up to speed. I was assuming that by driving the flywheel from the edge you can have the motor mounted 'off' the rotating arrangement so that you can have mains power running to the motor. This assumption may be false.
If the motor can be off of the rotating frame then not only can you power it from mains but it reduces the inertia of the system.

Mike