AC, DC, or what?

Now that my welder is running, I have a question about running grinders and such. The manual says 115v. DC power receptacle, which looks like the three pronged standard receptacles, like in a house. They are rated at 15a. in the manual. I put my little AC tester on there, and it came lit up the

110v. light, so I used my AC Makita stringer brush to buff up some 7018 passes. About 30 seconds, a minute tops. IIRC, I seem to remember discussions about using DC tools on these, or not to use AC, as they burned out, or whatever it was. What's the deal? Do I need to go buy special DC tools to use on this?

Just thought I'd ask before I fry something. If I haven't already .....................

Steve

All small grinders, drill motors, etc. will work just fine on DC, Steve. They are actually a kind of DC motor which will work on AC, called a "universal motor". That's why welders sometimes have DC outlets on them.

Grant

Even those with variable speed, or "slow start"? I would think not.

"Tim Wescott" wrote

Sez so in the manual.

I hate that word. And "should."

This is going to be for grinders, drills, sawzalls, die grinders, and the like.

I try not to play Martian Laser Gonzo Blondes on my laptop at remote muddy sites. Even in the cab with the windows rolled up and steamy. ;-)

My electrician buddy is visiting this weekend. I'll have him check it out. Although last time he fried my multimeter testing something.

Think that's what I have now you describe it. I'll look at it a litle closer. It was a buck at a yard sale. Probably $.99 originally at the Dollar Store.

Steve

"Universal motors". These are a classic - work on DC and AC. The traction motors for railway locos were these (not now that use power-electronic devices and AC traction motors) - current received was fast enough AC that you could use a transformer to set the power through voltage tap, yet slow enough AC that a "universal motor" could run on it. The tap on the transformer is selected using a big multi-position switch turned by a big wheel on the driver's console which looks a bit absurdly like a steering wheel(!).

How the motor works - the current flows through the static "stator" coils to create the magnetic field - that react against the self-same currnet continuing via the carbon-brushes through the rotor coils / windings causing the mechanical force and roatyional movement.

If you are continuity testing a power tool like an angle-grinder looking for a fault - you need to indpendently continuity test the rotor coils and the stator coils. Their resistance to DC current for intact windings is about the same, isn't it?

On AC - when the current reverses, so does the magnetic field it produces in the stator - so the motor keeps running in the same direction - the reason why uniquely this motor keeps happily spinning in one direction producing power whether the current is AC or DC.

I think some claim that angle-grinders, etc run smoother on DC than AC

- maybe that is a claim when using a "Lincoln pipeliner"

Here it is:

"DC Auxiliary Performance 1,750 total watts of DC auxiliary power from a single 120V outlet for grinders and lights. DC power is less likely to bog down a grinder compared to AC power. (Not for use with AC voltage equipment.)"

Rich Smith

Many years ago I used to hang around a gun smith's shop and chat with him while he worked. He had commented that both his big milling machine, and his big lather were actually DC motors, and claimed that all the big low tolerance machining tools like that used DC motors because it ran smoother. I never dug into the connections, nor I think would he have let me. Those were some expensive pieces of equipment.

Bob

Just recounting what I've been told...

3-phase induction motors have a constant turning torque. It's a geometric perfection of having 3 phases rising and falling in the three sets of windings, all balancing eachother out with their peaks and troughs to give an even turning moment.

So I'd be surprised if common industrial machines had other than

3-phase motors to power them.

Single-phase induction motors have an uneven turning torque and that can sometimes be seen in a "thrum" in the drive system, can't it? And single-phase motors are not self-starting, unlike 3-phase motors which are self-starting, giving a simple and very ideal set-up.

Rich Smith

I'm with Rich - Industrial machines use 3, 5, 7, 9 and 11 phase in exotic machines. These are wyes and stars and twisted stars. More phases the smoother the movement.

Large can machines use the poly phase power and custom AC control.

Years ago when I taught power in a class, a young electrician under his dads master sought help in determining which phase came first. Order counts on some complex machines - and there was an electronic device that measured just that. Neither he nor his dad had seen the lab instrument at that time.

Now there is a small hand unit that defines phase order.

My surface grinders are 3 phase.

Mart> "Bob La L>

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