AC motor on DC?

[Hillhater]

Guys, are we suggesting that i could convert many of my AC power tools ..(drills, saws, grinder, sander, polisher, , hedge trimmer , etc etc) to run from a suitably sized Lipo pack ????
That could be very useful if so.
however, i guess the fact that they are all 240v would mean a juicy lipo pack would be needed

On a similar thought train, what is the typical voltage for striking a welding arc ?
Has anyone built a lipo based portable welder ( light duty) ??

 

[liveforphysics]

Guys, are we suggesting that i could convert many of my AC power tools ..(drills, saws, grinder, sander, polisher, , hedge trimmer , etc etc) to run from a suitably sized Lipo pack ????
That could be very useful if so.
however, i guess the fact that they are all 240v would mean a juicy lipo pack would be needed

On a similar thought train, what is the typical voltage for striking a welding arc ?
Has anyone built a lipo based portable welder ( light duty) ??

Welders seldom need to exceed 48v, the heat of the arc is more current dependent and the welder is a current control device. But yes, a lipo pack and a ~300amp brushed motor controller could make a portable welder.


Most of your power tools will run equally well, or slightly better on DC (less hysteresis loss for some parts). If they are syncronous, brushless, or single phase induction motors, they will just smoke rapidly when connected to DC. If they are universal motors, which includes most vacuums, drills, grinders, blenders, hand saws (but NOT table saws or band saws etc), yard tools, will all work perfectly on DC, though some of them will only have full speed rather than speed control.

 

[strantor]

My welder goes up to 24V; I think up to 48V is common. lots of amps. if a power tool has a universal motor, sometimes it will have a symbol on the rating plate with a single bar and a sine wave under it indicating it is "officially" AC or DC. That's not very common though; I haven't tried to run power tools on DC myself, but I've seen people do it all over the internet. Angle grinders mostly. Here's a thread I'm following right now on the topic (http://forum.allaboutcircuits.com/showthread.php?t=60972)

 

[dynamo dave]

thought I would jump in here.We plug drill motors into the 74VDC receptacles on the locomotives at work .They have a little less torque but we can apply a hole where we need it. Remember a universal motor is basically a series DC motor.So if you hit with 120vdc with no load on the shaft it will turn really fast. Comutator segments coming at you fast.That is why when you put your hand over a vacuum cleaner hose you hear the motor speed up. It spinning in a vacuum no water in the pump.

 

[SamTexas]

... Remember a universal motor is basically a series DC motor.So if you hit with 120vdc with no load on the shaft it will turn really fast. Comutator segments coming at you fast.That is why when you put your hand over a vacuum cleaner hose you hear the motor speed up. It spinning in a vacuum no water in the pump.

Huh? Can you elaborate?

 

[strantor]

he's just saying that the unloaded DC motor's speed is only limited to (nothing?) and might spin itself to pieces (flying towards your face). a vacuum cleaner impeller spinning in an environment with no air is basically an unloaded dc motor. so don't block the inlet of the vacuum or it will vacuum itself into a vacuum and spin too fast.

 

[SamTexas]

Ah, I see. But isn't that true with all types of motor? AC or DC, brushed or brusless, universal or not. Correct?

 

[amberwolf]

Not really--it's a "feature" of series-wound motors.

Reduce the load on teh motor, and the current thru the armature drops, which also drops the current thru the field windings, since they're in series. That lowers the magnetic field in the windings, which reduces the "drag" on the armature, which speeds it up, which reduces the current, which reduces the field, which speeds it up....

You see where this is going.

(my specific description might not be totally accurate, but it is the series-wound motor that does this, for essentially those reasons. If you have permanent magnets it won't happen unless you just increase the voltage so high that it reaches fly-apart speeds, and that could be such a high voltage that the insulation on the windings breaks down first. If you have parallel field/armature, I am not sure that it would happen at all.)

 

[liveforphysics]

Ah, I see. But isn't that true with all types of motor? AC or DC, brushed or brusless, universal or not. Correct?

The universal motor and series wound DC are the only types that behave this way.

If you have some load on it, including just the little cooling fan you see in a handheld drill, then it will never be an issue. If you try to run it just bare, coupled to nothing, it may reach an RPM where mechanically it can't stay together and fails catastrophically. In practice though, most all of these power tool motors have thick enough laminations that eddy loss counter torque reaches equilibrium with high RPM torque and they stop accelerating before they reach a critical RPM.

It is a valid concern to be aware of though if you're testing a motor unloaded.

 

[dynamo dave]

Hi Sam. strantor ,amberwolf, and liveforphysics explained it great . I just wanted you to be aware of that overspeed thing.I just put that in about the vacuum cleaner cause i thought it interresting. Like a bicycle is not held up by the bottom spokes but hung from the top spokes.Amberwolf I don't think there is any shunt universal motors also because of the number of turns involved with the shunt field. The inductive reactance of such a field would make it so different from when it is changed from AC to DC that you would know what you had. Have fun Sam enjoy the journey .

 

[SamTexas]

Thanks all for the explanations.