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Translucent-Thin Bowls (Read 1,392 times)
 
Don Stephan
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Translucent-Thin Bowls
Feb 19th, 2016 at 1:18pm
 
Book be Michael O'Donnell explains the process very well, but cautions about wet sanding near electricity.  I've been led to believe that variable speed VFD won't work on GFCI protection.  Do people simply take their chances?
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Larry Matchett
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Re: Translucent-Thin Bowls
Reply #1 - Feb 19th, 2016 at 7:39pm
 
My lathe is on a GFCI circuit.  It has a VFD.  I don't know of anyone having a problem.  I wet sand by dipping the sandpaper in whatever liquid I am using.  Have never experienced a problem.  Not saying it couldn't happen just that I have never heard of it happening.
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Carl Jester
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Re: Translucent-Thin Bowls
Reply #2 - Feb 20th, 2016 at 9:59am
 
My variable speed Jet pops the GFI, I have to have it plugged into a non-GFI outlet.

I put some paper towels under what I'm sanding, and dip the sandpaper in whatever liquid I'm sanding with. I doesn't have to be super sloppy drippy wet. Sandpaper isn't spongy, so it's not going to soak up a lot of extra anyway.
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Louie Powell
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Re: Translucent-Thin Bowls
Reply #3 - Feb 20th, 2016 at 11:53am
 
Don Stephan wrote on Feb 19th, 2016 at 1:18pm:
I've been led to believe that variable speed VFD won't work on GFCI protection. Do people simply take their chances?



My variable speed Turncrafter Commander is on a circuit with a GFCI, and it works just fine.  But there is a potential interaction between variable speed drives and GFCIs that can cause a problem.  This is not something that will happen in every case, but is a possibility whenever a variable speed device is powered from a circuit with a GFCI.

Electronic speed controllers operate by creating either a variable magnitude DC voltage to power a DC motor, or by producing a variable frequency AC to supply an AD motor.  In either case, the drive first converts the incoming line voltage into DC using a rectifier circuit. 

A characteristic of rectifier circuits is they inject harmonic currents back into the supply system.  Harmonic currents are AC currents that have a frequency that is an integral multiple of the supply frequency.  The most typical injected harmonic current to be the fifth harmonic (ie, 300 Hz in North America, 250Hz in Europe and Australia) and for that current to have a magnitude of about 1/5 of the supply current into the rectifier.  This is a perfectly normal situation and does not indicate that anything is wrong - is is purely a consequence of the physics of how a rectifier converts an AC voltage into a DC voltage.

The purpose of a GFCI is to detect that there is a very low-magnitude abnormal current flowing between the 'hot line' in the circuit and ground.  At the same time, the GFCI cannot misinterpret the ordinary load current in the circuit as abnormal.  Since they are intended to protect people against fatal electric shocks, they have to detect abnormal current levels as low as 5 miliamperes. 

The problem is that in some instances, the very sensitive detection circuit in the GFCI can be confused by the harmonic current injected by the variable speed drive on a lathe or other tool connected to that circuit.  As a result, the GFCI will trip when the variable speed device is powered from the GFCI circuit.  Tripping will be erratic since the injected harmonic current is always 1/5 of the load current - so everything will be fine when the lathe is spinning at no load, but the circuit will trip when you apply load, for example  by cutting on the perimeter of a bowl.
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Bert Delisle
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Re: Translucent-Thin Bowls
Reply #4 - Feb 20th, 2016 at 5:07pm
 
It is interesting to note that when something doesn't run then it is blamed on the device that was engineered to protect us. A GFCI is designed to trip when a device is leaking current to ground instead of going back to the distribution panel through the neutral. At the point of distribution a three wire edison installation like in Canada and the US has the neutral bar and the ground bar tied together. Prior to GFCI technology devices could function even if there was a faulty or open neutral as long as the ground path was available. (Lots of electrocution / shock events). Now the technology GFCI is installed and the hot and the neutral are connected to the breaker, the ground is not. Therefore the GFCI electronics knows how much hot is going out and expects that much neutral coming back. If the wiring or end device is leaking to ground then there is a problem with "that device " NOT the GFCI breaker, from a personal protection point of view. Just because it will run doesn't mean it isn't compromised, the tripping may seem inconvenient but it is actually warning the problem exists.
Now new Electrical codes in Canada and US are coming out that encourage Arc Fault Circuit Interuppters that are different again from GFCI. This technology is even more sofisticated and will warn when even a switch is arcing in use. When everything is in good working order no tripping, but when starting to fail it pre-warns of the condition BEFORE a total failure. But note these are two different types of circuit breakers. Huh If in doubt discuss with a Journeyman Electrician or Electrical Inspector.
Turning safe isn't limited to just flying pieces of wood. Smiley
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« Last Edit: Feb 20th, 2016 at 5:10pm by Bert Delisle »  
 
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Louie Powell
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Re: Translucent-Thin Bowls
Reply #5 - Feb 20th, 2016 at 7:48pm
 
To Bert's point - arc fault interrupters are very different devices, and based on my understanding of how they work, I would think it even more likely that they would behave badly on circuits feeding variable speed devices.

Arc fault detectors look for a characteristic signature in the current flowing to an arc.  When there is a sputtering arc, the current is discontinuous and includes random higher frequency components.  A protective device that is specifically intended to detect random higher-frequency current components is even more likely to respond to the harmonics injected by variable speed devices than is a GFCI.

The original objective for arc fault detectors was to detect and interrupt low level arcing faults in bedrooms before they can start smokey fires that suffocate sleeping occupants of those rooms.  They should never be used in shops.
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Glenn Roberts
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Re: Translucent-Thin Bowls
Reply #6 - Feb 20th, 2016 at 8:39pm
 
Louie & Bert, Why no arc faults in the shop? Is it because of the high current draw during the motor start up? Does the arc breaker distinguish between a good switch arcing vs a defective switch arcing? Is the voltage & current difference in arcing, the signature Louie refered to? I am installing a new 150amp panel box (soon) and was all set to purchase arc fault breakers. Squirrels and mice seem to me a bigger problem (chewing the insulation), yet code doesn't require bx (steel shielded) in residences.
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« Last Edit: Feb 20th, 2016 at 10:12pm by Glenn Roberts »  

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Bert Delisle
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Re: Translucent-Thin Bowls
Reply #7 - Feb 21st, 2016 at 12:24am
 
Glenn Roberts wrote on Feb 20th, 2016 at 8:39pm:
Louie & Bert, Why no arc faults in the shop?.

Check your local Electrical regulator's interpretation of the current Code. One city jurisdiction has adopted AFCI for all 120v circuits, in effect Jan 2016.
Again check with your local regulator / inspector prior to installation to reduce "do over" work. DAMHIKT
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Louie Powell
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Re: Translucent-Thin Bowls
Reply #8 - Feb 21st, 2016 at 8:35am
 
Glen

At the time arc fault interrupters ere introduced, the justification was to address situations where a lamp or appliance cord becomes abraded and starts a sputtering arc. The concern was not that it would start a fire, but rather that it would smolder for a while (possibly hours) generating smoke, and if people were sleeping in the room where this happened they could succumb to the smoke long before it evolved into an open fire.

And Bert is absolutely correct - you need to check the codes that apply in your area. The requirement to have arc fault interrupters on bedroom receptacle circuits first appeared in the 1999 revision of the National Electrical Code. My understanding is that New York did not adopt the 1999 Code until 2003 (our house was built in 2002 and does NOT include arc fault interrupters).

I also understand that the 2014 revision of the National Electrical Code requires arc fault interrupters on receptacle circuits in all residential living space. I don't know if New York has adopted that revision of the Cod ye, but it would certainly not be a bad thing to do.

However, I'm not convinced it would be a smart idea to use arc fault detection on a shop receptacle circuit, and I don't believe the NEC would require that.

Years ago, I replaced the electrical service in our former home (and installed a GFCI on the circuit that fed the bathroom as was required at the time under New York codes). I had the work inspected, and the inspector made me redo portions of the work. the issues were matters of interpretation of the code - but he was the 'authority having jurisdiction' and that automatically made his interpretation right. The point is that it pays to ask.

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« Last Edit: Feb 21st, 2016 at 2:25pm by Louie Powell »  

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Don Stephan
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Re: Translucent-Thin Bowls
Reply #9 - Feb 21st, 2016 at 2:02pm
 
Thanks for all the great explanations.  Bottom line seems to be if I want to wet sand using my Powermatic 4224 I will not be able to use a GFCI breaker on that circuit.  And of course the lathe has to be turning to wet sand.  If I'm not feeling lucky I'll have to postpone wet sanding projects until I have a non-variable speed 120V lathe (240V GFCI breakers are so much more expensive than 120V GFCI receptacles).
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Re: Translucent-Thin Bowls
Reply #10 - Feb 21st, 2016 at 7:46pm
 
My electrician told me to be sure have both the ground and neutral  going to separate buses in the panel which I did.  I have not had any problems except when my shop got fried with lightning, all the gfci switches were fried.  I don't have a vs 220 machine so maybe that is where the problem is as 220 does not need a neutral.  Just wondering if your ground wire goes to ground bus or neutral bus.  I know they end up in the same place somewhere.  I don't think you will have a problem wet sanding anyway.  I can't imagine using that much WET since you are just washing off your sand paper with whatever you are using not pouring it on the piece.  I guess I would have to ask are there any turners with 220v machines on a GFCI circuit who do not have a problem?  Interesting to know.  I would like to present the results to my turning club.
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Bert Delisle
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Re: Translucent-Thin Bowls
Reply #11 - Feb 21st, 2016 at 8:51pm
 
I am a bit confused, the thread title and the discussion seems to have gone down a couple of rabbit holes. All good discussions however, if the concern is wet sanding on a lathe and electrical equipment.
Correct me if I am mistaken wet sanding involves using water and or oil on the sand paper. No where will this liquid get any where near the motor or the controls. When I wet sand there is a definite streak of sanding sludge on both side of the lathe across the ceiling and at times up my chest and across my face shield.  Cheesy
Wet sanding is great to find the line of fire and where pieces of wood will likely go on those less than ideal timbers that sometimes fly apart. Even then the electrics are well out of the line of fire for these too. JMNSHO.
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« Last Edit: Feb 21st, 2016 at 8:53pm by Bert Delisle »  
 
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Larry Matchett
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Re: Translucent-Thin Bowls
Reply #12 - Feb 21st, 2016 at 10:25pm
 
May be a rabbit hole but I am learning a lot about electricity anyway thanks to Bert and Louie.
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Re: Translucent-Thin Bowls
Reply #13 - Feb 22nd, 2016 at 3:49pm
 
given the relatively small volumes of water involved (you can "do" wet sanding with a spray bottle, or as someone said, dunking the sandpaper)   the threat of grounding seems remote, PROVIDED your motor is out of the line with the work space.....most full sized lathes are like this, the motor and electronics are all off to one side.   But midi lathes seem to mount the motor directly below the ways....   On mine, take the thin plastic "chip shield" out of there, and you can see directly to the motor housing between the ways.   so I would say it depends on your specific lathe and setup.
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Re: Translucent-Thin Bowls
Reply #14 - Feb 22nd, 2016 at 4:06pm
 
I'm with Bert on this one...Two topics

1) Wet Sanding  and   2) issues with VFDs on GFCI circuits.

I fail to see how they may be interrelated. 

The only issue I have with my VFD is that it causes interference with the AM radio that I have nearby (even though I purchased the lathe with the interference reduction module.).
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