Originally Posted by Mike Henderson
Nonsense!
Everyone knows that running a tool on 220v really impresses your neighbor. (That guy who only owns Black & Decker tools from Wallmart.)
- and running at 240v impresses him even more.
Member
Nonsense!
Everyone knows that running a tool on 220v really impresses your neighbor. (That guy who only owns Black & Decker tools from Wallmart.)
- and running at 240v impresses him even more.
Member
I am also a lifetime electrical engineer and I agree with Mike's opinion and examples 100%. No practicing electrical engineer or master electrician I have ever worked with (several hundreds) would refer to 240VAC house wiring as two phase. Mike's example, if anyone takes the time to read it, is a crystal clear explanation why.
Contributor
It also explains why you can't just use a transformer to convert single phase to 3 phase. If the residential distribution transformer could "make" "2 phase" from single-phase, then logically, it would be possible to buy a single-to-3-phase transformer (which obviously does not exist, because phase shift cannot be realized with a transformer)
Member
It's because each winding only "sees" 120V, regardless of whether the motor is wired for 120V or 240V. If it's wired for 240V, that 240V is put across (as I explained before) all the windings in line (series wiring): LEG1-winding1-winding2-LEG2. The voltage between LEG1 and LEG2 is 240V, and since there are two equal windings between those legs, each winding has a 120V drop across it.
For 120V, the wiring is changed so that you get LEG1-winding1-neutral-winding2-LEG1 (only one hot leg in 120V). LEG1 to neutral is 120V, so each winding is hooked in parallel and has 120V drop across it.
Moderator
As a Moderator who lives in Silly-con Valley withOUT a PE but with a MSEE and who is married to a PhD in EE, I'm going to have to say..........................
...........that electromagnetics is very cool and that coupled with gravity created and sustains life.
Member
Indeed. Maxwell's Equations are a thing of beauty.
(Yes, yet another MSEE)
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
Member
Now let me throw a real hook in there. How bout 220 3phase on a variable frequency inverter. I've been running any machines I have on them since the mid 90s. You can run the saw at anything from 0 to full rpms. You're thinking, what's the point? Try running your planer at 1750 instead of 3300, it's whisper quiet and you don't loose any performance. I've run everything on them: drill press, tablesaw, planer, lathe (of course), bandsaw... Inverters are dirt cheap and usually the better ones run right out of the box without any configuring.
But not joiner, not sure about safety with respect to grabbing and possible kickbacks as lower rpms. Not that you have to but the temptation is there...
Sent from the bathtub on my Samsung Galaxy(C)S5 with waterproof Lifeproof Case(C), and spell check turned off!
Contributor
Brian, I'm no engineer, but I don't understand how there would be no loss of performance in a saw or planer at half the rpm. We know from Newton's second law of motion that Force = Mass x Acceleration. The equivalent for rotation around a fixed axis is Rotational Force (Torque) = Rotational Mass (Moment of Inertia) x Angular Acceleration. How can the cutting edges of a planer, or the teeth of a table saw, cut as well traveling at half the speed?Perhaps you mean something else by performance. I have a bench grinder that runs at 1725 rpm for sharpening woodturning tools, and several that run at 3450 rpm for everything else. The slow speed grinder removes material much slower, with less heat build-up, than the others, and for that reason performs better for sharpening lathe tools.
Contributor
Horsepower is the ability to do work (in a period of time). So you can remove more wood in a certain period of time with a higher HP tool.
When you slow a 3 phase induction motor down, the motor generally maintains its torque (at least within what I'd call the "working band of RPMs). But HP is torque times RPM times a constant factor. So when you slow down an induction motor with a VFD, you reduce the HP of the motor - and that reduces its ability to remove wood as quickly. If you're not using the tool at it's limits you may not notice any difference, but if you push the tool, you'll stall it quicker at a lower RPM.
Mike
[Just an added comment: You may notice that a 2HP induction motor that runs at 1725 RPM will be physically larger than a 2HP induction motor that runs at 3450 RPM. That's because the 1725 RPM motor has to generate twice the torque of the 3450 RPM motor. A Universal motor that runs at 12,000 RPM will be quite a bit smaller to produce a real 2 HP than either of the previously mentioned motors.]
Last edited by Mike Henderson; 09-10-2015 at 2:26 PM.
Go into the world and do well. But more importantly, go into the world and do good.
Member
All good theory but in reality not much value unless you're running a production line. As far as I know we're all hobbyists were such productivity needs don't exist. So have maximum hp isn't an issue. Yes at 200 rpm I can bog the bandsaw, so I dial it up a bit and no more bogging... After a while you find the right Htz setting to run each machine... My grinder runs at 10Htz, bandsaw at about 30htz, tablesaw (when I had one) was 40Htz, planer (when I had one) at about 30Htz. I'm sure everyone knows how loud a planer is, especially when you have a dust collector hooked up to it... I could plane wood without ear protection it was that quiet when run at half speed. The scream of a tablesaw is also really reduced, but there was no extra effort to push wood through it at reduced rpms.
When I first found this I had to pull the motor on a planer and put in what I had laying around, and all I had was a 1750 motor. So I don't know about size difference (I never noticed one) but I do know throwing a 1750 motor on a planer had no detrimental effects on the work, the motor, planer or the work. And there were no problems with it bolting in, obviously don't try to fit a motor with a different mounting plate. I can still remember when I first turned it back on and started up the dust collector. I just stood there thinking the motor must be shot or the planer head has somehow not been hooked up and not spinning because it was so quiet - the wail was completely gone. And then running the pine it was only marginally louder... All I know is I won't be going back to a stock configuration. If I didn't have a vfd or 3ph motors but had the money for 1750 motor I'd swap all the motors in my machines except the jointer - not sure if there could be a problem there and I wouldn't want to find out the hard way there was.
Last edited by Brian Ashton; 09-10-2015 at 5:44 PM.
Sent from the bathtub on my Samsung Galaxy(C)S5 with waterproof Lifeproof Case(C), and spell check turned off!
Member
How are you measuring what is "best"? Are you going by sound? Cut quality? Your observations do not correspond to any facts I have ever read or any personal observations. There is no difference between "theory" and "reality". If you think there is then you are not taking all the facts into account.
Contributor
Art, my reading of Brian's post is that he agrees with what I posted - that slowing down an induction motor reduces the HP. I think his position is that - for the motors/tools he has - the motor has sufficient HP left at the slower speed to do the work he demands of the tool.
There's really no arguing against the concept that HP goes down as you slow the motor down.
Mike
[And certainly, if he prefers running his tools are a slower RPM for whatever reason, that's okay.]
Last edited by Mike Henderson; 09-15-2015 at 6:57 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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