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Mechanical Inverter Again
By RogerAS, Section Homebrewed Electricity Posted on Wed Nov 19, 2003 at 03:53:04 PM MST
all

Greetings,

Here is what I was talking about with a mechanical inverter.

This is an end view of the rotor and brush setup I mentioned in an earlier thread. The colored lines are connections between the commutator positions. As the commutated rotor spins the direct current is redirected in an alternating current fashion. A really small motor could be used to turn this rotor as only the brush resistance is a factor (in my tiny mind anyway :-) ). The brushes are on the sides and top, no feed or supply wires shown. I think for 60 hrtz the RPM would have to be 3600, but it could be half that(it's late in the day). Or with more commutation slots far less RPM.

I think that 12V DC could be fed to this with the addition of the stepup transformer on the AC side. I feel, but don't know, that if the right size motor run capacitors were used on the AC side this could make some pretty good quality AC.

What do you guys think?

RogerAS

Mechanical Inverter Again | 14 comments (14 topical)

Re: Mechanical Inverter Again (none / 0) (#1)
by Old F on Wed Nov 19, 2003 at 04:58:26 PM MST

Roger

In stead of a transformer you could use a DC to DC converter 12 volts in 120 out.
I don't see why your commutator would not work. Its like a two pole alternator you would
find on an engin driven gen set 3600 rpm gives you 60 hz and a good wave form.

Have fun

Old F

Re: Mechanical Inverter Again (none / 0) (#2)
by drdongle on Wed Nov 19, 2003 at 05:04:00 PM MST

No mater the speed/frequency the output will be a square wave. Just soze you know:)

Dr.D
Carpe Vigor, Dr.D

Re: Mechanical Inverter Again (none / 0) (#3)
by Norm on Wed Nov 19, 2003 at 06:14:53 PM MST

If you put some capacitors in the circuit would that 'round the corners' a little? Know how I could make a home-brewed Electronic ignition for the LawnBoy? (:>) Norm.
( :>) Norm
[ Parent ]

Re: Mechanical Inverter Again (none / 0) (#6)
by zubbly on Wed Nov 19, 2003 at 07:32:39 PM MST

hello Dr. D. first i will admit that i am not much of an electronics person. i do understand that the ac output will be in the square wave form. if Roger uses a step up transformer ( 12vac-120vac) and two seperate coils in the transformer, would the 120vac coil round off the sine wave somewhat because of the slight lag behind the primary coil? just a hunch. also, could you reply with what items can be succesfully run from a square wave form and what cannot.

thanks--zubbly

[ Parent ]

Re: Mechanical Inverter Again (none / 0) (#4)
by drdongle on Wed Nov 19, 2003 at 07:10:48 PM MST

While caps can "round off the corners" you need to use them in conjunction with a transformer or inductor. I have built low frequency inverters 20-30 Hz for ringing telephones that I have done this with. Basically I had a 555 timer driving two out put transistors ( 1 NPN and 1 PNP)which in turn drove a transformer connected backwards ( 12 volt secondary became the primary) so that I had a 120 volt square wave output, I would then hook up my scope and try various caps bridged across the secondary to get the best approximation of a sine wave. It was never a perfect sine wave and it had some transient artifacts but it was better than with out the cap. What I had was a "resonant tank circuit" that was tuned to 30 HZ. So if you did build one yes you can approximate a sine wave but you will need to experiment with the caps and you will definitely will need a scope. Note that if the speed and frequency change the "resonance" will be lost and you will no longer have a sine wave output.

Dr.D
Carpe Vigor, Dr.D

Re: Mechanical Inverter Again (none / 0) (#5)
by zubbly on Wed Nov 19, 2003 at 07:22:32 PM MST

hellow Roger. i have sort of been dreaming along the same lines. your idea does seem much more simple than what i was thinking. i was thinking of using a commutator from an old large dc motor, but then couple the connections to a set of slip rings on a common shaft. basically would come out to the same thing i think. the current if from a 12vdc source will be of significant high amps. may i offer the idea of using brushes from an industrial lift truck motor as these are designed for very high current and are often arranged in a row in the brush holder. perhaps you may be able to salvage some parts from a lift truck motor. another source for high current brushes are from the old motor and gas driven Lincoln and Hobart welders. hope this helps and please post your results. i am very interested to here how it goes.

keep having fun--zubbly

Re: Mechanical Inverter Again (none / 0) (#7)
by Norm on Wed Nov 19, 2003 at 08:12:07 PM MST

As for brushes and brush holders wouldn't just plain old parts from car starters be a good source they carry about 300 amps or so....well maybe not probably weren't meant to carry a lot of amps for more than a minute at a time? Norm.
( :>) Norm
[ Parent ]

Re: Mechanical Inverter Again (none / 0) (#8)
by Simon on Wed Nov 19, 2003 at 11:12:44 PM MST

Hallo Roger,

I have been thinking about the same thing! A mechanical inverter. The only thing obvious which will result in a pure square wave is the ON-OFF effect of the commutator contacts. Maybe the solution is to use wider contacts on the stationary part of the inverter which have variable resistance. This will sort of create a curve just like on a "potentiometer". You might need a bigger wheell to do this and many contact points to enable 60Hz at lower RPM than 3600! You could try with lower voltages first for this setup. Have fun and tell me when it works.

Simo'

Re: Mechanical Inverter Again (none / 0) (#9)
by RogerAS on Thu Nov 20, 2003 at 07:11:49 AM MST

Thanks guys,

For the advice and encouragement.

I really don't know when I can find time to build my little idea, so anyone with a little motivation go ahead and try.

I'm building a house, not like on HGTV "build" but rather actually doing the work, all the work! So, little time for anything else.

It seems this could work for a cheapie square wave inverter. Maybe only good for lights or heating, Sure the brushes will wear out and the commutator will go south, but oh well. Thanks for showing me how tuff it would be to aproximate a sine none-the-less.

If I used a step up transformer with the square wave output, wouldn't that create another set of problems? I think I read somewhere that trying to setup the output of a square wave inverter was bad, or would the output from my spinner work differently?

DrD? Others?

P.S. I only have net access at work so excuse any delays in responses.

RogerAS
Roger AS
9 Years off-grid & counting

Re: Mechanical Inverter Again (none / 0) (#10)
by drdongle on Thu Nov 20, 2003 at 08:26:14 PM MST

Square wave forms tend to saturate a transformer more so than sine waves and induction motors don't realy like them either. They can also produce more RF noise, which is bad for radios and such.

Dr.D
Carpe Vigor, Dr.D

Why not use two motors? (none / 0) (#11)
by dconn on Fri Nov 21, 2003 at 06:13:29 AM MST

Why not make an old-fashioned mechanical Inverter and use a universal motor on the DC side and couple it to a induction motor on the other side - you'd get a perfect sine wave output and the only brushes to wear out would be in the universal motor - and at least they are engineered to carry the current.

I've often thought about a mechanical inverter but really I'm [and everyone probably] better off using an off-the-shelf inverter.

Derek

Re: Why not use two motors? (none / 0) (#12)
by RogerAS on Fri Nov 21, 2003 at 07:12:46 AM MST

Hi Derek,

I'm thinking this might be useful in places where electrnoic inversion is impractical or outside the budget. In the developing world this little gizmo might come in handy, or in Stone County Arkansas, where I live.

If one had a hydro setup OR maybe a distant VAWT it seems it would be a simple matter to run a take off to drive the spinner. This would make long distance runs to the primary battery bank more effiecent, maybe, if run as AC. If one could build a real good rectifier on the storage end.

I dunno, I think I think too much.

RogerAS
Roger AS
9 Years off-grid & counting
[ Parent ]

Re: Why not use two motors? (none / 0) (#13)
by drdongle on Fri Nov 21, 2003 at 07:50:54 PM MST

It would work, in the past these were called MG sets ( Motor/Generaters) and Dynomiters.

Dr.D
Carpe Vigor, Dr.D
[ Parent ]

Re: Why not use two motors? (none / 0) (#14)
by Jerry on Wed Nov 26, 2003 at 11:30:29 PM MST

I have this set up. I coudn't aford the inverter to power my well pump. I wanted power for the pump if we had a grid power failier. I couldn't aford the big inveter that could start and run my pump.

I did however have some surplus 2 HP dc motors that are rated 180 volts. I had 24 12 volt batteries from a hospital power backup system.

The motors were rated 1750 rpm. I purchased a stand alone belt drive ac generator rated 5000 watts 120/240 v. Its requiered power was listed at 10 hp 3600 rpm.

I used larger pullys on the motors and smaller on the genny. I put a motor on each side of the genny with a double belt pully on the genny.

I used 20 of the batteries (240 volts) and used a 400 amp aircraft relay to swich power to the motors. No load the genny puts out 260 volts and has no truble starting the pump. And while the pump is pumping the voltage is 242. Close enough. These two 2 hp dc motors have no problem meating the 10 hp power requierment for this genny.

I figure the pump wont run continuas so the system is good for back up. I'll post pics.

My small batteries went bad last winter and I've not replaced them yet but the setup is still in the pump house wich is inside my 40X60 shop. Its a small room in the back.

JK TAS Jerry

Airheads Page

[ Parent ]

Mechanical Inverter Again | 14 comments (14 topical)

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