Single phase electric motor control

[stillvodka]

Hello,
I have been trying to find out how I can control the speed of a 230-240volt Single Phase 3/4 hp electric motor, I know there are inverters/variable speed controllers out there, but not sure really what sort i would need, they seem to only control 3 phase motors, they seem quite expensiveas well, my Budget will only manage say £100 not sure the $ rate
I know there are very clever people on this forum, Who seem to be able to come up with cheaper alternitives for the DIYers, Anybody can help would be most appreciated

Thanks

[Tater]

They are venerable speed motors out there that's single phase but are quite expensive .Gear reduction one way of slowing one down.What speed ya wanting to get one down to ?

[rad14701]

Out of curiosity, what will this 3/4 HP motor be driving...???

[cob]

i did a simple search for "motor controller" the second listing had a 25a. unit for $125 us. i don't know the conversion to euros. the question would be how cheap do you need it to be ? cob

[Dnderhead]

not all AC motors can be controlled for speed. and the ones that can need there own specific controller for that type.

[cob]

we used rampers for soft start, you could control speed to any rpm, our operation was up to 25 hp. but they will go higher, there was nothing inexpensive in that operation. and yes ac.,dc., induction, or brushed all makes a difference. inexpensive seemed to be the point of the question. cob

[stillvodka]

Here you go , this is what I am going to build, It's a Telescope Mirror Grinding machine, I want to grind a mirror for a 24'' telescope, I had a 12'' which is pretty good but 24'' is in the realms of the fantastic night time sky viewing. I need to build this machine on a smallish budget if possible, I am using two cement mixer gearboxes to keep the torque running properly, they turn a cement bucket at 24rpm , I will need to get the Table turning at around 1-3 rpm, I can do this with pulley arrangements, My problem is with the Overarm mechanism which moves the tool on top, across the Mirror Blank, I need to control the speed the arm will move across the mirror for purpose of grinding sequences.
My electric motors are single phase, I will most probably will need to get a 3 phase and a controller/inverter, That is unless someone can come up with another way of doing it?

[HookLine]

Cool. Gonna show us some pics when it gets up and running?

[stillvodka]

Cool. Gonna show us some pics when it gets up and running?

Yes , I'll put pics up when it's done,

Got to get past the motor speed control problem first, these are the motors I have http://cgi.ebay.co.uk/Electric-Motor-Si ... 3eeee78bb4" onclick="window.open(this.href);return false;" rel="nofollow
Harry suggested this Kit, I'll have no problem building it, but I am not sure it will work with the motors I have, what I gather is the kit is only suitiable for brush motors, do mine Have brushes? I dont think they do, but I may be wrong.
http://www.electronickits.com/kit/compl ... ek2636.htm" onclick="window.open(this.href);return false;" rel="nofollow

[HookLine]

I meant pics from the telescope, not of it, though they are welcome too.

Helped make an 8" Newtonian when I was a kid, worked pretty good too. One of our teachers was a keen astronomer, showed a bunch of us how to do build them and use them. Still love astronomy pics.

[stillvodka]

OK

[rad14701]

Just my opinion but a 3/4 HP motor sounds like overkill... I'd think something small, like 1/4 HP or smaller, running the lowest RPM's available, reduce to a worm/gear transmission, would be a better choice... I was initially thinking hydrostatic drive, but that might get expensive, especially if this is for limited run production... How about a salvage rotisserie assembly...???

[heynonny]

I dont know if it would work for you, but how about a 'Craftsman' 3/8 or 1/2 in. drill motor, or some other "cheap" brand (Harbor Freight)? Use the 'trigger mechanism' out of an old drill. I had a 1/2 in drill motor go bad, one of the 3 jaws in the chuck fell out, and it was as cheap to replace as to fix. I ripped it apart (I rip EVERYTHING apart!) to see if I could salvage anything, it was an ac-dc motor wired to the speed/fwd-rev switch unit. Is this worth a try? -hey-

[goose eye]

electric sewin machine moter rigin the foot pedal an sackin pulleys

[Dnderhead]

your motor is most likely a induction , can be controlled but expensive. so why cant you use a "split" pulley or a "plate and wheel"?
another idea just use "stepped" pulleys

[cob]

stillvodka if you already have these motors open the little tin box on top it should have a capacitor (small aluminum can/w 2 wires) if it does it is a cacacitor start induction motor and the kit you linked will not work. the controller you need is a pulsed controller similar to the controllers used by some of the electric stillers here. (search "induction motor controller") if your standard house voltage is 230v. i have no more options. if you don't already have those motors and have 120v. available brushed motors and router controllers could be an option. i am assuming 50hz. but then we have to return to inexpensive. i wonder if neuton ground his own by hand or hired someone? cob

[Hack]

Just to elaborate on what dnderhead said about stepped pulleys. If you need different speeds, but don't need to change them while you are running, why not get a used drill press and replace the arbor with a pulley. There are a variety of speeds you can get by rearranging the belts, and it's already setup to do that. I got my big drill press for $60 at a garage sale. You can buy the little ones new from harbor freight for about the same amount. Also if you plan it right you can reassemble the drill press after you are done and sell it for what you paid for it.

[stillvodka]

Just to elaborate on what dnderhead said about stepped pulleys. If you need different speeds, but don't need to change them while you are running, why not get a used drill press and replace the arbor with a pulley. There are a variety of speeds you can get by rearranging the belts, and it's already setup to do that. I got my big drill press for $60 at a garage sale. You can buy the little ones new from harbor freight for about the same amount. Also if you plan it right you can reassemble the drill press after you are done and sell it for what you paid for it.

I have one of those little drill presses , it will only go down to 540rpm, it will still be too fast, Looks like I will be doing a lot of pulley changing, I do need the aid of variable speed really , if push comes to shove, I'll exchange the Single Phase for the 3 phase, get a variable speed controller. plenty of them out there for the three phase, i'll just have to run over budget a bit.

[Dnderhead]

Is this what he neads?-----
http://cgi.ebay.com/CUTLER-HAMMER-ADJUS ... %26ps%3D54

[cob]

dnderhead probably like that. that particular one is only rated for 1/2hp. he needs 3/4hp. rating. the bigger issue is every thing on this continent is 60hz. stillvodka's power is 50hz. (correct me) right idea wrong toy. cob

[cob]

I meant pics from the telescope, not of it, though they are welcome too.

Helped make an 8" Newtonian when I was a kid, worked pretty good too. One of our teachers was a keen astronomer, showed a bunch of us how to do build them and use them. Still love astronomy pics.

hookline you ever seen the hubble site? they have pictures a newtonian can only dream of. i hooked the laptop to the bigscreen, you can see the pillars of creation three feet tall. cob

stillvodka any of this help? cob

[HookLine]

Yeah, I got LOTS of astronomy pics (thank you Internet). Visit the APOD site most days.

Beats any human art, hands down.