Dynamic brake with capacitor storage for rotation of flywheel - Page 1
Forum section: electronics / projects

author: OskerMike
author_rank: Newbie
timestamp: February 01, 2020, 06:04:05 pm
content: Hello, ok so i will try to give as much info as i can but i am not very educated in this field so please be patient.I am doing an experiment at home and need to spin a 250g cylindrical disk (flywheel) up to 3 different speeds (a low, mid, full sort of set up, with a 3 way switch) as quickly as possible (capacitors??). I will be using a standard 3v-6v toy motor that i purchased from amazon. No load max rpm is 10,000 and has a 27mm diameter if that helps. After it has spun for an amount of time i would like to be able to flip a 2 way toggle switch, engaging a dynamic braking system which i have read is done with resistors? The flywheel needs to immediately stop if possible. Once the toggle switch is flipped again the flywheel should quickly achieve the chosen speed as quick as possible again (Due to quick release of energy from capacitors?). I have 2 LiPo 3.7v battery packs that i would like to use with this system.I would like to repeat this process repeatedly and possibly in quick succession, would a fan with heat sink be required for this or would the ambient cool air of a basement suffice?If anyone has the time to maybe make up one of those awesome schematics or wiring diagrams i would be so appreciative.. i am trying to learn what i can but reading and retaining information has become difficult and i would really like to do this experiment.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2020, 07:43:52 pm
content: Just how immediate is "immediate"? And how much budget is there to achieve it?I'm not sure you realize just how much is "possible" to attain here, given any price whatsoever...Put another way, what's theslowestacceleration that is still satisfactory for the application?Using a "toy motor" sounds like it doesn't need to be all that fast at all, perhaps seconds? Which the batteries will more than suffice to supply, no additional storage needed.Any interest in regenerative braking, to extend battery charge? (Added complexity of course, but might come for free with the right motor driver.)Is the wheel unloaded? How about bearings, and their losses? Is it spinning in air, what about windage loss? Anything touching it? (And if nothing is touching it, does it really need to spin at all? Might there be another approach entirely that you haven't considered?)Tim

author: max_torque
author_rank: Super Contributor
timestamp: February 01, 2020, 07:48:05 pm
content: Define "immediately"To stop the flywheel instantly will require an infinitely powerful motor!As you overall power flow is negative (ie using power) then a simple diode rectifier, followed by a capacitor bank, and then an H bridge will work. You need to calculate the energy in the flywheel, measure or find the motor's maximum torque, and then you can calculate the H bridge current maximums, and the power flow during the starting and stopping. You also may need some way of measuring the flywheels speed to determine when it has reached peak speed and therefore when to start braking it to a halt etc. A brushed motor has no intrinsic speed measurement (unlike a brushless unit when you can calculate the motor speed from the controllers output frequency.

author: Ian.M
author_rank: Super Contributor
timestamp: February 01, 2020, 08:13:51 pm
content: Quote from: max_torque on February 01, 2020, 07:48:05 pmSensorless speed control of a brushed PM motor is possible as the averaged back EMF is proportional to the speed, and equal to the averaged generated voltage when coasting.

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 02, 2020, 03:35:08 am
content: A 10Krpm toy motor probably doesn’t have enough torque to barely move a 250g flywheel, given they have really crappy bearings, if you can even call them bearings and the topic of using capacitors is just a red herring, torque requires current and capacitors have internal resistance which will limit the of instantaneous current.If you calculate the moment of inertia of the flywheel (which is easy), you can calculate how much torque is required to accelerate and decelerate it in a given time. What you don’t know is the friction but whatever it is, it hurts the acceleration time and and helps the deceleration.Whatever the point of this experiment is, it’s quite pointless as it’s just physics. The problem is you have no data for the motor. With no data, you cannot even begin to guess how fast you could possibly accelerate and decelerate the flywheel. What is known and is guaranteed is it will not and cannot be instantaneous. Not even close. Tens of seconds best case, maybe a minute to 10k rpm if it can even be reached, given the fiction with that high of a load. Anything faster and you just let the smoke out of the motor. Every system has its limits.

author: OskerMike
author_rank: Newbie
timestamp: February 02, 2020, 07:54:17 pm
content: Ok so i don't need any specific speed attained. I only wish to spin the flywheel at 3 different speeds. Preferably doubling in rotations each time. I only wish to see the effects of the wheel in different situations. Its nothing fancy, just a personal hobbie in learning.Yes 100% to power regeneration if possible. As for the speed of deceleration, i was hoping for 1/10th of a second. Anything lose to this would be more than satisfactory.If need be, i can lower the mass of the flywheel to 150g but would prefer to use the 250g flywheel as mass has a big factor in what i am looking at.I would also like to be able to switch direction of the spin with another 2 way switch.I truly appreciate all the responses and will try to learn as much as possible. You all are awesome!

author: OskerMike
author_rank: Newbie
timestamp: February 02, 2020, 07:56:32 pm
content: Oh yeah and my budget is very small.. i would like to stat within the $25-$50 range if possible. I have a really nice hobby shop around the corner from me and can get many small electrical parts for quite cheap. Just need to put it all together myself..

author: OskerMike
author_rank: Newbie
timestamp: February 02, 2020, 08:00:55 pm
content: The flywheel is directly connected to the motor shaft.