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LAN_403

Physics Junkie

Fri May 03 2013, 10:39PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Yes! I know exactly what you're talking about! I'll have to take apart some bulky old stereos receivers I have and really investigate this some more since I've never dabbled in using relays and similar current protection schemes and don't really know how they work, maybe come up with a scheme for a higher power application (Tesla coils over 2500W). Thanks

klugesmith

Sat May 04 2013, 01:16AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

PJ,
What kind of receptacle did you choose for your new 220V circuit?
Not a rhetorical question.
I am thinking of adding a 4-prong receptacle to the same circuit as my old
electric clothes dryer circuit, which has a 3-prong NEMA 10-30 receptacle IIRC.
Need to see if the steel conduit run includes a white neutral wire.
Even if it does, I expect the code does not permit a second receptacle.

The 240-V variac I have never yet powered up is a dual 120 sharing one shaft and control knob.

With a 4-wire outlet and some care,
I think you could use your 120/140 variac on one side of the new circuit,
safely getting continuous adjustment from 120 to 260 volts with respect to the other hot line.

There must be ground fault circuit interrupters for split 240-V circuits.
Anyone know if they run all 3 conductors through the same sense-coil core?
Is there another way to do the job?

Physics Junkie

Sat May 04 2013, 04:13AM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

The receptacle is a 3 prong.. a ground with the two angled prongs for hot and neutral. I may be wrong but I think it is very similar to the European style outlets. I could have had many different types of outlets but I chose the large angled one so that nobody would mistakingly plug a 120V device in that outlet. When I discovered it, it was a 4 prong for an electric dryer that was never used, probably because it was installed not in compliance with code (not in a box) and the breaker was mislabeled. So I had an electrician check it and put it in a box. I'm not sure if it's against code to extend another receptacle, I don't think it is as long as you put it in a box because I was going to do that with my washer and dryer until I found this receptacle all by itself. So yeah like I said I had an electrician fix it up properly and as far as I know it was ground faulted.

I dont know about using my 140variac on this 220 line, certainly don't want to ruin my variac. Anyhow, this one is 3 prong and not 4

Edit: plug I bought is a NEMA TT30 type plug. So the receptacle is the same type as would fit that plug

teravolt

Sat May 04 2013, 05:26PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Steve Conner recomended this in a cuple of threads earlyier

Physics Junkie

Wed May 08 2013, 10:59PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

With setup I'm using right now (variac->step up transformer) I'm blowing fuses because my variac is 10A 1.5kW so its really limiting the capabilities of my Tesla coil. This might be a silly question, but... whatever route I go in making a ballast, should I make the power rating equal to the max power output of the breaker? (This would be about 6.6kW) even though the Tesla coil might not pull that much...

HV Enthusiast

Thu May 09 2013, 12:49AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Circuit breakers are designed to protect the wire only, not the circuit or load.

The ballast should be designed to the limit the power to the device you are using (i.e. transformer, variac, etc...) It should be sized independently of the breaker.

Obviously, the ballast should limit the current to a value less than the breaker current.

Physics Junkie

Tue May 14 2013, 03:52PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Other than about $1000 difference, why is this

so much cheaper than this

Moreover, the mastech is cheaper than all autotransformers with similar specs.

Steve Conner

Tue May 14 2013, 05:43PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The Mastech is a cheap and cheerful Chinese-made unit. Powerstat is made in the USA.

That one could be B stock. Maybe it's already been beaten up in shipping. Still looks like a bit of a bargain.

Physics Junkie

Tue May 14 2013, 07:21PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Yeah I wouldn't doubt if it's not brand new like advertised. And other then obvious quality factor, the mastech is not fused and I read a review somewhere that you have to chip the paint off to ground it. If I bought the mastech I would probably scrap the red case and make my own with receptacles and proper fusing.
In other news, Tesla Motors opened at about $97 a share on nasdaq this morning. Maybe sell a few shares and get some quality product

Thanks Steve.

Physics Junkie

Mon May 20 2013, 11:52PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Steve Conner wrote ...

If you live in the US, a 240V line is actually two 120V lines 180 degrees out of phase,

Forgive my elementary EE knowledge, but here in the US, a 3 wire 240V line is going to be two 120's and ground. So this means that it is two HOT wires and a ground. The only reason for a neutral would be for devices/components that also require 120V operation i.e. clocks, timers, etc... which is typical for a 4 wire electric dryer or stove or whatever the device is you would normally have operating from two 120V legs.

So here's why I am asking: I have my eye on the 28F-1 variac

So when I look at common single phase variacs for 240V operation, the schem shows a hot and a neutral, the neutral being the wire at the bottom of the schem (based on just about any variac schem you look at) and the hot wire is the "variable" part, for lack of proper term . So would this pose any problem if I were to use a variac with two hot wires instead of one hot one neutral? Instead of being operable between 0-280V, would it be like 120-280 since the 'neutral' is actually 120V hot?

Edit: but if I understand correctly, two 120V legs being from the same pole will not trip the breaker, instead it will just equal zero volts. And if from opposite poles, they will short and trip the breaker. So this puts me out of luck in with using "split-phase" like this? Because I would have to have a 240Hot and neutral single phase to use an auto transformer.

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