# Powering A Flywheel

#### Cardinal799

Joined Feb 9, 2021
7
Hi! I am working on a project for school. I am trying to find a way to power a metal flywheel (about 2.5" in diameter) off of a 9V battery using an electric motor. The speed should be variable with the use of either a potentiometer or a pair of buttons, between 100 and 300 RPM. The goal is to maintain a consistent speed even when there is some outside resistance against the flywheel edge (like a finger pressed against it), and the torque from the motor should be reasonably similar/consistent at both low and high speeds. This should be able to fit in the palm of your hand.

I have some knowledge of basic circuit components, not enough to know what my requirements would look like in order to design a functioning circuit though. I do want to learn for the sake of this project and I have a mentor to help me, but I do need some help first with figuring out what this circuit might involve. I am trying to stick to analog only to keep it simple, unless that doesn't actually make the circuit less complicated. I'm also wondering how many of these components I can find easily online. I also am not sure how to come up with the formulas I need in order to see what is going on physically, like how much energy will be taken out of the spinning flywheel when it encounters resistance and how that relates to the power coming from the motor. I can try to figure that out on my own though.

Any help would be greatly appreciated!

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#### wayneh

Joined Sep 9, 2010
17,498
The goal is to maintain a consistent speed even when there is some outside resistance against the flywheel edge (like a finger pressed against it), and the torque from the motor should be reasonably similar/consistent at both low and high speeds.
Those are arguments for a gearbox. Run the motor at 1,000RPM (or more, whatever) and gear it down to your target. This would give enough torque to resist a bit of drag on the flywheel. But there's really no way around the fact that your hands can overcome the power delivered by a 9V battery and a small motor. Grab it tightly and it's going to stall.

#### Cardinal799

Joined Feb 9, 2021
7
Those are arguments for a gearbox. Run the motor at 1,000RPM (or more, whatever) and gear it down to your target. This would give enough torque to resist a bit of drag on the flywheel. But there's really no way around the fact that your hands can overcome the power delivered by a 9V battery and a small motor. Grab it tightly and it's going to stall.
I can see that. Under normal operation, it should only see friction comparable to a finger on the outside edge. It's okay if it stalls out from grabbing onto it.

#### Externet

Joined Nov 29, 2005
2,236
I would not know how to bring the speed to that low RPM as I would use a small 5V discarded hard drive instead (you can remove the cover to enjoy)
Some may work just applying power. Explore if the alternative pleases you. A toy/hobby store should be a good place to find something to repurpose.

----> https://www.micromark.com/Flywheels...MIp4C27ILe7gIVZ1XVCh0RPAwvEAkYByABEgKGcfD_BwE

Or any toy motor can have a flywheel installed in its shaft.

#### wayneh

Joined Sep 9, 2010
17,498
I have some knowledge of basic circuit components, not enough to know what my requirements would look like in order to design a functioning circuit though.
Don't sweat the electronics. Assume it's easy, because it will be once the mechanical issues are worked out.

If you want to jump ahead and get started, I believe you'll want a PWM motor speed controller. (They're also used for driving LEDs.) You can buy these cheap on eBay or Amazon, it's just a matter of finding the specs you need. In your case, anything rated for 9V and 1A should be fine. Something like this (just an example, not an endorsement):

Yes you could build your own, but not cheaper than you can buy it.

#### Cardinal799

Joined Feb 9, 2021
7
Don't sweat the electronics. Assume it's easy, because it will be once the mechanical issues are worked out.

If you want to jump ahead and get started, I believe you'll want a PWM motor speed controller. (They're also used for driving LEDs.) You can buy these cheap on eBay or Amazon, it's just a matter of finding the specs you need. In your case, anything rated for 9V and 1A should be fine. Something like this (just an example, not an endorsement):

Yes you could build your own, but not cheaper than you can buy it.
I'll give those a try. Is there a cheap way to get 9V from a plug in device that I can use with those controllers so I'm not running through batteries while I do research on this circuit?

This is a motor I found that would probably work for now. It looks like the usual operating speed range for this is much higher than I need though. If I add a resistor to knock it down to my range, with the motor controller doing the fine tuning, would the torque of the motor be affected?

#### SamR

Joined Mar 19, 2019
5,060
Is there a cheap way to get 9V from a plug in device

The trick is going to be for the motor to overcome the 0 RPM starting torque to get it to spin. That motor parameter will limit the mass of the flywheel.

#### wayneh

Joined Sep 9, 2010
17,498
If I add a resistor to knock it down to my range, with the motor controller doing the fine tuning, would the torque of the motor be affected?
Yes. That’s why PWM is used instead, it allows torque to be maintained while rpm drops. More or less.

#### Cardinal799

Joined Feb 9, 2021
7
The trick is going to be for the motor to overcome the 0 RPM starting torque to get it to spin. That motor parameter will limit the mass of the flywheel.
Can't I just give it a push to start it?
Yes. That’s why PWM is used, it allows torque to be maintained while rpm drops. More or less.
So if I am ordering parts right now, I shouldn't bother buying assorted values of pots to mess around with?

#### wayneh

Joined Sep 9, 2010
17,498
Can't I just give it a push to start it?

So if I am ordering parts right now, I shouldn't bother buying assorted values of pots to mess around with?
Pots are relatively expensive as electronics go. So no, I wouldn’t bother with that approach. You’ll be much happier with a PWM controller. A variable voltage regulator such as LM317 would be useable with a pot but controlling a motor by voltage is not ideal.

#### SamR

Joined Mar 19, 2019
5,060
Yes, you can push start it. However, the greater the mass of the flywheel the greater the power required to maintain its rotational velocity in addition to the starting torque. Which in return reduces the power to recover from the loss of rotational velocity by braking. Research the field of dynamics although much of it uses calculus. So go for a low mass flywheel. Diameter also comes into play.

EDIT: Flywheel balance also comes into play. Better balance equals less energy to maintain rotational velocity.

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#### DickCappels

Joined Aug 21, 2008
10,232
My solution to a similar problem is to power a drill motor with a power supply that has negative output resistance. As the load increases, the voltage out of the power supply increases to compensate for IR drop in the wiring and the motor.

Chart 1. Above is the plot of the current-voltage response curve of
the power supply. As the load current increases from zero
milliamps to 200 milliamps, the output voltage increases from
approximately 20 volts to approximately 24 volts.

Schematic 2. The circuit is based on the TL431 voltage regulator.
A second feedback loop increases voltage with current while a
third loop limits and regulates the maximum current.

#### Cardinal799

Joined Feb 9, 2021
7
Yes, you can push start it. However, the greater the mass of the flywheel the greater the power required to maintain its rotational velocity in addition to the starting torque. Which in return reduces the power to recover from the loss of rotational velocity by braking. Research the field of dynamics although much of it uses calculus. So go for a low mass flywheel. Diameter also comes into play.

EDIT: Flywheel balance also comes into play. Better balance equals less energy to maintain rotational velocity.
So if I used aluminum for this, it would be about between 1.9 and 3.8 ounces. I need to do some testing first to see just how much mass this thing needs to overcome the braking. Once the parts come in, I should be able to figure that out quickly.

Since I do need to factor cost into this project, I'm willing to not add more complexity if all it takes for it to start is a simple push. I will be machining the flywheel with my own custom design, and I need to figure out where the mass needs to be for this all to work. Can I make it thinner in the middle and just have a thicker outer band in order to save weight, while also ensuring that it maintains more rotational velocity? Or is that not how this works?

Don't I need special devices to balance a flywheel? Or can I do this myself by watching it spin?

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#### shortbus

Joined Sep 30, 2009
10,049
I will be machining the flywheel with my own custom design, and I need to figure out where the mass needs to be for this all to work. Can I make it thinner in the middle and just have a thicker outer band in order to save weight, while also ensuring that it maintains more rotational velocity? Or is that not how this works?
You seem to have done your homework on flywheels. It is the rim that makes the difference with them, that, the way you explain it is how a flywheel for a machine is made, a thick rim with a thin center.

Is 9 volts a requirement? You said 9V battery in the OP. The regular 9V transistor radio battery, like the old PP3 type are a bad choice to run a motor,they are low amp hour. If 9V is a requirement you would be better off with using 6 flashlight batteries.

#### SamR

Joined Mar 19, 2019
5,060
Balancing is a challenging task, especially with a small radius and low mass. Best I can suggest is to use as frictionless bearings as possible on a horizontal shaft going through the flywheel. Spin it and mark the low spot when it comes to rest. Repeat. IF the marked spot continues to be the point at which it comes to rest remove mass from that spot and repeat until random. Better to remove too little mass than too much. Easier said than done.

#### wayneh

Joined Sep 9, 2010
17,498
The regular 9V transistor radio battery, like the old PP3 type are a bad choice to run a motor,they are low amp hour. If 9V is a requirement you would be better off with using 6 flashlight batteries.
Even a 5V USB supply would likely give better performance than the PP3.

#### wayneh

Joined Sep 9, 2010
17,498
Balancing is a challenging task, especially with a small radius and low mass. Best I can suggest is to use as frictionless bearings as possible on a horizontal shaft going through the flywheel. Spin it and mark the low spot when it comes to rest. Repeat. IF the marked spot continues to be the point at which it comes to rest remove mass from that spot and repeat until random. Better to remove too little mass than too much. Easier said than done.
That's how I (try to) balance my mower blades during sharpening. Balance makes a HUGE difference though and it's worth the effort.

#### MrChips

Joined Oct 2, 2009
30,982
There are two ways that I can think of to maintain a constant speed. One way is to detect the speed and compensate the applied voltage to keep the speed constant. I suspect that this will make your setup too complicated.

The second method which Dick as already suggested is a power source with negative resistance. This would require putting together an electronic circuit.

#### wayneh

Joined Sep 9, 2010
17,498
There are two ways that I can think of to maintain a constant speed.
Don't forget a gearbox. That's how every home appliance does it. It doesn't really hold speed constant, just minimizes the effect of torque changes at the final shaft.

Electric pepper mills are a great example. A tiny and cheap 6V motor can generate enough torque to grind peppercorns with only a slight dip in motor rpm. Of course the final shaft rpm is too slow (by 100X!) for this application. But gearing works.

#### Bernard

Joined Aug 7, 2008
5,784
To maintain a constant speed with variable load seems to call for some feed back like a slotted disc, optical sensor, & assorted electronics. Reminds me of a desk display of a Jaguar speedometer which used a 20 slot disc with a slot detector with a variable input voltage to generate a PWM signal. 1 to 120 MPH.