120v and 240v variacs / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

120v and 240v variacs

1 2 next

Move Thread

LAN_403

IamSmooth

Sat Apr 10 2010, 10:38PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

What is the issue with taking a 120v variac and connecting it to 240v? If this were to be done, the original neutral wire would be one of the hot lines, and it would not be connected to the ground. The ground would simply be connected to the chassis.

Is it that the core would saturate above 120vac?

Proud Mary

Sat Apr 10 2010, 11:00PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It would conduct too heavily, overheat, and fail.

radiotech

Sun Apr 11 2010, 03:26AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The variac woud burn up.

There is a way to use a 120 volt variac on 240 volts. Find a choke
that has the same DC rersiatance at the variac full winding AND
when connected in series will drop 120 Volts AC across each. A junkpile motor stopped from rotating could do.

Now you would have a variable AC supply from 120 VAC to 240 VAC
OR from 0 to 120 VAC. but not both.

Also 240 volt variacs work fine on 120 VAC at derated outputs,,

The magic of a variac happens when the brush lies on top of several turns, shorting them out, yet not burning those windings out whhile delivering rated rated current.Science of commutaion is interesting
to those with hugh motors using many expensive brushes.

IamSmooth

Tue Apr 13 2010, 12:07AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

If I wanted to make my own, does the toroidal core need to be made from a particular iron? Any suggestions where to get a core?

GhostNull

Tue Apr 13 2010, 12:22AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

I think as long as it won't saturate and you can get the proper DC resistance most materials should be okay, with mains frequency most other factors are not a problem, I think.

Well I know that www.electronicsurplus.com as some cheap toroids here:

and BGmicro is selling some cheap toriods here:

and then there are all the other surplus sites and ebay

radiotech

Tue Apr 13 2010, 07:23AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Before you start the variac building project design the single layer winding to magnetize the cross section of the core up to its required number of lines. Remember it is a laminated iron core like a transformer, BTW variacs dont have to be round. Also variacs dont have be autotransformers.

Round stacks of laminations can be salvaged from induction motors. Just remember the motor shaft was pushed though in a hydraulic press.

Good luck on making a brush.

Somewhere I have a homemade variac about 1 foot high and a foot in diameter. Someone made it for about 1000 watts.

It sparks, growls, and smells. It tries to convince you it isnt a rheostat.

Steve Conner

Tue Apr 13 2010, 09:08AM

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

radiotech wrote ...

The magic of a variac happens when the brush lies on top of several turns, shorting them out, yet not burning those windings out whhile delivering rated rated current.Science of commutaion is interesting
to those with hugh motors using many expensive brushes.

The brushes in big motors are processed to have anisotropic resistivity. Low in the plane of the wanted currents, an order of magnitude higher in the plane of the unwanted short-circuit currents.

I imagine large variac brushes will be the same.

If you can't get a 240 volt variac, surely you can get a stack of two or three 120 volt ones.

If you wanted to make your own, the no-brainer choice would be a core from a toroidal transformer: it's exactly the same kind used for variacs. Unwinding one is tedious, though.

radiotech

Tue Apr 13 2010, 05:14PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"The brushes in big motors are processed to have anisotropic resistivity. Low in the plane of the wanted currents, an order of magnitude higher in the plane of the unwanted short-circuit currents."

These brushres are homogenous in compostition. They have no planes of currents. The brush operates by creating an electrographitic interface layer on the surface of the communtator which transfers the current and lubricates the surface. This layer can be poisened by a myriad of polutants and destroyed by a load and or mechanical conditions.I have changed thousands of brushes
on hugh motors and have me crew chief on the 'brush maintenance team' You literaly have to read the patterns on the commutator surface to know what is happening with the machine, then talk to the motor shop commutator machinist and brush manufacturer's salesman for for informed data. Brush composition is modified within the standard grades to solve issues. The sheen on a com that is working well is a thing of beauty.

Just an aside: The coal strike in Britian years ago wreaked havoc with trolley busses because the special graphitic sliding current collectors were in short supply and the replacement brushes just didn't have the performance.

Every coiler's dream- A DC source with a inertia af a small locomotive!

Hazmatt_(The Underdog)

Wed Apr 14 2010, 04:31AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Go to Ebay and buy a variac, or the local electronics store and buy a Chinese import variac. You can get these things for about $100 and its worth it.

I thought I would never in a million years have a 1256D and now I have 2, so be patient and all things will come!

radiotech

Fri Apr 16 2010, 06:14AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Here is a 6.3 KW variac for 120 Volts. Each section is wound with # 14 wire approx 2 turns/volt. the cores are 1.25 inch x 6 inch.

If used on single phase with the 3 sections parallel, it could carry 3600 watts or 1200 watts per section. What am I not revealing about this bad boy?

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob