[chrispy]

This is a little off topic, but I figured there is a ton of knowledge on motors here. Probably more experts than most places.

My dilemma:

I have a generator on a Arc welder. Was using a circular saw on it, no problem. Only when I hooked up the compressor did the smoke tell me something wrong. Took me some head scratching, but what I learned is it is putting out 120vDC. Great.

Well that should explain why the cap start compressor would not work. Thought about taking the DC and oscillating it. Problem was that it would of cost more than a generator. It is 15amps of DC,

Than it dawned on me, why not put a DC motor on the compressor. Any thoughts?
The only thing I can imagine is that the DC motor will not have enough torque to start the compressor, due to the way a compressor starts.

It is only a 3/4 hp compressor, so this might be a cheap way out?

Comments appreciated. If you want to tell me I am crazy, please do.

Chris

[gar]

050903-1919 EST USA

You need to tell us why you want to run a compressor off the output of a dc welding generator. Probably not a good idea.

I have to assume this is a portable application.

If so there is probably an internal combustion engine that runs the welding generator. If this is the case directly drive the compressor from that engine.

Using continuousl duty as a comparison a dc motor of a horsepower rating equal to an ac induction motor the dc motor starting torque can be many times that of an ac induction motor. Universal motors are a different story.

The problem with a dc motor across a dc arc welder is the possiblity of large voltage transients. Whether these would damage the dc motor is hard to say.

.

[Geof]

I suspect a 3/4 horse 120V DC motor is going to cost a few dollars. If this is a portable application and you need the compressor put a gasoline engine on it; probably cost less than the DC motor.

[chrispy]

Ok I should of been more clear.

The welder has a separate regular AC outlet. Old 1953 unit that runs great. Welding DC normally has a lower voltage, thus the DC supplied to the outlet has a separate generation unit inside the welder.

This might make more clear my original surprise when I found that the outlet was actually spitting out DC.

Was thinking about getting a regular generator with real AC, than the DC motor dawned on me. I have seen 3/4 DC motors going for pretty cheap. Just curious about the torque as compared to a compressor duty cap start AC motor.

Thanks again,
Chris

[Geof]

Oh; DC out of an "AC" outlet would surprise me also. As has been mentioned you should have no problem with the starting torque on a DC motor. Check that the motors you have seen can take 120v, a common voltage for DC motors I have encountered is 90 v. Also be careful about using your AC switches on DC; most switch gear is greatly derated when it comes to switching DC.

[ViperTX]

Well generators put out DC and alternators put out AC.....so, you gotta have something in between to convert it...I wonder why power companies call there facilities 'generation facilities'......hmmmmmm...

[tommylight]

Generator=whatever generates something, it does not have anything to do with AC or DC.
Back on subject, find a screamer (brushed motors) or just bolt a powerfull hand drill to the compresor and off you go.
Some major tips regarding this idea: be avare that brushed motors can run up to but not limited to 30000 rpm ( yes thirty thousend ) so you need some kind of reduction with it. Most brushed motors run on AC and DC happily and with no complaints, been there done that several times. Brushed motors will run at very low voltages but at the cost of torque and RPM.
If using a drill you will still most likely have to use a small pulley on the drill side so you can get 600 to 1200 RPM required by the compresor.
etc. etc......
Tom

[Al_The_Man]

Resurrecting a 9 year old thread??
Al.