Electrical engineer? I'm not giving up here!

H

hallboy

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I have a Gast Roc-R piston type compressor. model ROA-P101-JK mounted to a speedair 3 gal tank. It's been in our shop collecting dust for years.... This thing is very quiet when running..... the problem is it's 24 volt DC motor....... Now after doing some reserch, I finally purchased a step down transformer to get me to 24 volts AC, I've made a diode bridge to convert the AC to DC but this has caused about a 2 volt loss.......... Now for the tech part.... I reserched and found out it's a 1/8 hp motor and that converts to approx 100 watts.....and to calculate the amps needed 100 watts divided by 24 volts converts to approx 4 amps......... The only transformer I found was rated at 100 watts,(barely enough) but when the compressor gets to around 35psi, it starts straining for power like it needs more amps....... I was really hoping I could make use of this compressor being that it was FREE!!! except the diodes..... one burned up transformer..... a second transformer now that almost fixed the problem....... OK so how many electrical engineers are airbrush artists on here.......... Some of you know what got me into this new hobby so you know I'm determined to get this going..... I know there is a way to get 120 voltAC to power this 4 amp, 1/8hp 24 volt DC motor that wispers when it runs.......
 
Sorry hallboy, not sure what language that post was in, but I didn't understand any of it, the tumbleweed is blowing through my empty little head. Hope someone else can help you, a compressor is a big outlay so getting a free one working would be fab!
 
Yeah, that looked like you were speaking French/German/and a little Cambodian to me, lol!! Hope you get it working, but I can't help ya there. Mitch is an electrician maybe he can help when he gets back from vacation.


Josh
 
Yep I pulled my hair out reading and googling all this only to still be sitting here with a great 24volt compressor that I can't feed...... unless I go buy two car batteries and wire them up to push 24volts.......... hmmmmm maybe a battery charger and I'm in business............ Iwata might not be so bad after all.... lol
 
Well you are on the right track , Except rule of thumb , if a unit calls for 24 volts DC you have to factor a + or - 10% flux , so instead of 4 amp draw you are really looking at a 6 to 8 amp draw at peak power, 4 amp is minimum draw on the system.
When you are calculating a load you always want to look at the max load of a unit, this helps gauge wire thickness , Type of wire to use and energy source.
But it may be cheaper to just grab a pancake compressor at wally ville aka walmart...The make a little noise but they are not really that bad.
Or find a old RV generator most of them have a 24 volt DC cycle built into them. at 8 amp circuit.
 
Any more specs on the motor? can it run at a slightly higher voltage?

With a 24VAC supply I get 20.7VDC to the motor, you need at least a 27.65VAC supply, closest you'll probably get is a 30VAC which will give you 26.11VDC to the motor.... that's using a full bridge.
Did you put a capacitor on the output of the bridge?? bigger the better ;)

You'll need at least a 240 to 300VA transformer as Mr. M said 6 to 8 amps.

Disclaimer - Um, this was all theoretical so I haven't tested it, might have forgotten to carry the 1 ;)
 
Capacitor?? Well there's another piece of this mystery contraption I didn't google....... And the 240 to 300VA transformers get expensive. The only one I found was a 24volt 100VA ..... Yep seems to be too small to handle full load.... Oh well, it will make a great addition to my offroad truck for on-board air on the trail...... I'll just grab a little loud one at Lowes and turn my music up. Thanks again for the feedback guys.
 
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Capacitor?? Well there's another piece of this mystery contraption I didn't google....... And the 240 to 300VA transformers get expensive. The only one I found was a 24volt 100VA ..... Yep seems to be too small to handle full load.... Oh well, it will make a great addition to my offroad truck for on-board air on the trail...... I'll just grab a little loud one at Lowes and turn my music up. Thanks again for the feedback guys.

LOL, a Capacitor helps smooth out the voltage ripple coming from the bridge rectifier.... Yeah, power stuff can get quite expensive, you can get ready made gadgets and bits so cheap now a days it's hardly worth pulling out the calculator :(
 
Well I'm pretty sure you've already found a solution to your problem, but I'm unsure if you bought a power supply or a new compressor. I spent some time modeling some circuits with NI multisim using an LM338 linear voltage regulator. This regulator can be configured to output ~24V at 5A. The circuit would be configured like this:
AC Mains>Transformer>Rectifier>[LM338 configured to 24V with proper configuration(input/output caps)> Load

And then I remembered what most of my college professors said: Just go buy one.

As stated before at time=0+ (moment you flip on the compressor) you have a theoretical infinite current draw, after that the current draw will continually fall until it reaches steady-state. And then we get to the issue with high power linear Vregs - they give you stable lower power by dissipating a higher power through heat.

The solution is a switching power supply, and as aficionado said, it's much cheaper to buy.

For example : gah, can't link yet. google "parts express" Search for part number: PS-SP11154

Other things I'd always keep a look out for are old computers. The hard drives are great at the 300 yard gun range, and the power supplies are easily configured for other uses. I've converted a few into bench power supplies for a few friends and I so we didn't have to go to the lab to use theirs. You can also wire two together to produce a 24V supply, but I'd rather purchase the parts express one. Probably the best use I found for old electronics was a combination of low quality 2 channel car amplifier, a bluetooth adapter, an Antec Earthwatts 350W PSU, the stock speakers from my Fiesta, and some scrap MDF. It's a fantastically ghetto garage stereo that honestly sounds pretty decent with ear protection on. Best of all I can keep my phone inside the house so I don't paint it, get it filled with saw dust, or drown it with the power washer when I convert from wood shop to paint booth.

Be Creative
~Michael
 
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