Generator project for my 8 year old (and me apparently)

[e1p3dr0]

Hi All,

My daughter is super keen to build a generator. So that I don't kill that enthusiasm straight away:

* I plan to start simple and then make it progressively more interesting
* I want the circuits to work on the first attempt <- that's where you can help!

To start, I'm planning to use a DC motor connected to an old school tungsten filament bulb. We'll make a bigger wheel, connected by a rubber band to a pulley wheel on the motor shaft. So she'll crank the big wheel around by hand to drive the motor and see the bulb flicker into life. Makes sense?

What specs should I look for? Would these do:
Motor: https://uk.rs-online.com/web/p/products/2389737/
Bulb: https://uk.rs-online.com/web/p/filament-indicator-lamps/1716958/

I'm guessing that we might be able to get towards 1500 RPM with a suitably sized drive wheel. That's way below the design RPM of the motor, does that mean it will generate a much lower voltage than the input voltage it's been designed for?

I haven't bought any parts yet, so open to suggestions. Strong preference is to keep things cheap and simple so that we don't have to be too precious about the kit.

Thanks!

[Ian.M]

For PM DC motors and generators speed is proportional to voltage over a wide range.  Therefore running it as a generator at 1500 RPM. you can expect less than 15% of its max. rated voltage, which is a fairly good match to your chosen bulb.  However reaching 1500 RPM may be difficult, so I'd suggest adding a 1.2V bulb to your order, e.g.https://uk.rs-online.com/web/p/filament-indicator-lamps/6559075/

[e1p3dr0]

 

Great. Thank you @Ian.M

I was wondering whether we could build this into some sort of fun experiment to gauge spin speed.  E.g. Have a row of lamps and the faster you turn the wheel the greater the number of lamps that light up.

Circuit design aside for a sec, is that going to be a bit of a stretch due to the low voltage I'm going to produce?

[DrG]

8 year old (and me apparently)...the and me part makes me wonder if that is not the weak link (and yes, I am joking). Having been a judge at science fairs, I think I am a bit jaded by the parent's projects.

Throwing out an alternative approach...one of the many varieties of these around....

https://www.banggood.com/DC-Micro-Motor-Small-LED-Lights-Vertical-Axis-Wind-Generator-Wind-Turbines-Blades-p-1277416.html?rmmds=search&cur_warehouse=CN

...modified by the adult.

BTW: I really think it is cool to feed a kid knowledge when they show interest in anything like that.

[Jeroen3]

You know, you can buy these things for your bike...
https://hollandbikeshop.com/en-gb/bicycle-lights/hbs-bicycle-lights/hbs-bicycle-dynamo/aufa-bicycle-dynamo-6v-3watt-black-576509/
https://hollandbikeshop.com/en-gb/bicycle-wheels/front-hub/dynamo-hub/sturmey-archer-hds12-front-hub-36-hole-2-4w-silver-487484/

[e1p3dr0]

@DrG
Yeah - the me part definitely is the weak link. She's got all the enthusaism and time, and all I can bring is patchy recollection of electromagntic principles - learnt at degree level, but with almost zero practical experience and a couple of decades to forget all the theory too!

That wind turbine is exactly the kind of thing that I'd like to do for stage 2.  Though we might make the tubine ourselves -e.g. from some card or plastic cups mounted on some sticks. I wonder if balance will be an issue, or whether we'll be able to connect our construction to the shaft firmly enough to transmit all that wind power.

In that kit, the turbine is connected directly to the shaft. I wonder what RPM you could achieve like that. The motor specs from the kit seem to be:
Output voltage : DC 0.01v - 5.5v
Output current : 0.01 - 100mA
Rated speed : 100 - 6000 rev/min

That doesn't seem a million miles off the motor I linked to above:
Supply Voltage    4.5 - 15 V dc
Power Rating    7.98 W
Output Speed    9869 rpm
Current Rating    990 mA

Do you think I could reuse my motor to create a similar wind turbine?


@Jeroen3 - Yes - I did think about a dynamo. But I felt it was perhaps a bit too pre-built. I'm trying to get as close to first principles as possible. I started by thinking about wrapping wire around cylinder. But then I found out I'd be looking at 1,500 or so turns. She swears that she'd be up for that, but I reckon I'd probably end up having to do the final 1,450 of those turns 

At least with a motor, it's a component that we can make into other things too - so it feels a bit closer to fundamental building blocks.

[DrG]

@DrG
Yeah - the me part definitely is the weak link. She's got all the enthusaism and time, and all I can bring is patchy recollection of electromagntic principles - learnt at degree level, but with almost zero practical experience and a couple of decades to forget all the theory too!

That wind turbine is exactly the kind of thing that I'd like to do for stage 2.  Though we might make the tubine ourselves -e.g. from some card or plastic cups mounted on some sticks. I wonder if balance will be an issue, or whether we'll be able to connect our construction to the shaft firmly enough to transmit all that wind power.

In that kit, the turbine is connected directly to the shaft. I wonder what RPM you could achieve like that. The motor specs from the kit seem to be:
Output voltage : DC 0.01v - 5.5v
Output current : 0.01 - 100mA
Rated speed : 100 - 6000 rev/min

That doesn't seem a million miles off the motor I linked to above:
Supply Voltage    4.5 - 15 V dc
Power Rating    7.98 W
Output Speed    9869 rpm
Current Rating    990 mA

Do you think I could reuse my motor to create a similar wind turbine?
/--/

There are others here that can answer that much better than I can. I have the "product" I linked to as well as another cheapo wind turbine demo. I wanted to see if I could power a BLE beacon with one (which I believe can be done with a few diodes, regulator and cap, but I didn't completely test it out). For the beacon, I planned on using one of those luggage tags. The simplest version (app) would simply infer the presence of wind (at some intensity) when the beacon broadcast was present.

What impressed me was that even a quick flick of the propellers was enough to see a brief flash on the LED. When I played around with sticking a propeller on some other cheap toy motors, I could get the same output - This is a long way of saying that I am not smart enough to know what the characteristics of a motor/generator to look for in this context.

[Doctorandus_P]

(Hybrid) Stepper motors make pretty good generators at low rpm.
BLDC motors too. Pretty much any motor that has permanent magnets will work.

For a DC motor, use one with built in gearbox to get the motor to a high enough RPM.
Maybe build both and compare the results.

Some mechanical stability is probably also advisable. A way to clamp the motor itself to a table, and a lever on the axle to crank it.

Are those flashlights with built in generator and hand crank still a thing?
Personally I never managed to crank those longer than a minute or so before getting bored of them.

There is also a big difference of hand cranking, and what you can get out of a home-trainer bycicle with bolted on dynamo.

Some time ago I demolished an old washing machine, and to my surprise it has a BLDC motor in it, and with the Multi-V belt this can be relatively easy to bolt onto a hometrainer.

[e1p3dr0]

"There is also a big difference of hand cranking, and what you can get out of a home-trainer bycicle with bolted on dynamo.

Some time ago I demolished an old washing machine, and to my surprise it has a BLDC motor in it, and with the Multi-V belt this can be relatively easy to bolt onto a hometrainer."

Ha! Gives me some ideas for Iteration #14 of our little generator project:

Her: "Dad, can I watch TV?"
Me: "Sure, just as long as you can keep cycling fast enough to power the TV"

Stepper? BLDC? No idea what they are - I've got to do some more research. Thanks for the pointers.

[wizard69]

There is a big mechanical consideration here, you have a limit to how much reduction you can get with one belt drive.    In commercial systems 10:1 is often consider a limit.   A rubber band drive will have a limit that is less than that but hard to guess at.   

So what does this mean?   If you need a specific voltage you need to know how fast the motor will have to rotate to give you that voltage.   The parameter to look for is Ke which is "V/(rad per sec)" sometimes expressed as "V/RPM".

[e1p3dr0]

Ah. yes, I guess that's what Ian.M was getting at too. My back of an envelope calcs were indeed based on 1:10 - which seems overly optimistic. Maybe 1:5 might be achievable?

The motor specs are 4.5V-15V with 9869 rpm. No V/rpm specs unfortunately. So for ballpark calcs, I'll assume linear V/rpm and that 9869 corresponds to max 15V input. If I get a 1.2V bulb that Ian suggested, then I'd need about 800rpm to get max brightness. Hopefully the light will illuminate from around 500rpm. If I get 1:5 out of my rubber band, she need to turn it a couple of times a second. Hopefully it'll be ok.

This is great - I'll be able to get some mechanics & material sci in there too. Hopefully we'll get to try different sized wheels made out of diff materials to try and optimise this thing before the excitement fizzles out.

[Ian.M]

You'll need more speed than the simple calculation suggests, as the voltage drop due to the winding resistance is effectively in series with the EMF induced in the windings by their motion in the magnetic field.  When its motoring that reduces the effective voltage that determines the speed, and if its generating at the same speed, with no load, you get the reduced voltage not the motoring terminal voltage.  Then you loose voltage proportional to load current due to the winding resistance.

[Doctorandus_P]

For a wind turbine you can get some usable results by cutting the right shape out of a single piece of PVC pipe

https://duckduckgo.com/?q=wind+turbine+blades+pvc+pipe&t=h_&iax=images&ia=images

[e1p3dr0]

It worked!

She's taken it in to school today so we'll see how much abuse it can take from 30 8-year olds.

Ian.M was right - we struggled to get the required rpms to start with. I took wizard69 observations on board, so kept the step up ratio well in single figs. So we ended up having to do two step ups in the mechanical transmission - first: circa 8cm diameter to 1.5cm diameter, then 4.5cm to 1.5cm. Two thin rubber bands on each stage.

No lubrication or bearings, so the axles are just dowel rotating in a hole drilled in pine. So there's a load of friction.

When we had only one step up, we only managed to get the light to flicker when cranking pretty hard. But with the two stage step up, she can crank the handle and get quite a consistent illumination. We used the 1.2V bulb and the 4.5 - 15 V dc, 78.4 gcm, 9869 rpm motor.

There's quite a few other opportunities to build on this, mech and elec.
* Mechanical power smoothing - possibly via a flywheel?
* Electronic power smooting - capacitor?
* Power indicator - it could be some sort of voltmeter but that seems a little too pre-fabricated. Even better would be an arrangement of lights so that the more power you generate, the more lights that illuminate. But I'm not sure we'd be able to generate sufficient power. Perhaps there are some ultra low power leds?
* Wind/Water turbine

Thanks for all your inputs. She's delighted.