# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: panzerfaust on October 19, 2021, 08:56:41 pm

Title: variac wiring
Post by: panzerfaust on October 19, 2021, 08:56:41 pm
Has anyone connected their single phase variac input to 2 phase i.e between L1-L2 ? ::)
Title: Re: variac wiring
Post by: Benta on October 19, 2021, 09:13:11 pm
As you're in Sweden, I presume you have 3-phase 400 V mains. This means 3 phases wtih 230 V each to neutral.
Unless your variac is rated at 400 V, connecting it to L1/L2 is a really bad idea.
And not just because of the voltage, but also the complete lack of of a neutral voltage. Variacs are not isolated from the mains.

Don't do it and stay alive.

Title: Re: variac wiring
Post by: Doctorandus_P on October 20, 2021, 03:31:27 am
Transformers have a pretty strict voltage limit.
If you put too much voltage over it, the core goes into saturation, with the result that inductance drops a lot and it will start drawing too much current.

Transformers are also frequency dependent.
Simplistic view is that the amount of energy that can be transferred between 2 zero-crossings is constant, which means that at lower frequencies (50Hz) a transformer can handle less voltage then at higher frquencies (60Hz).
Title: Re: variac wiring
Post by: Vovk_Z on October 20, 2021, 05:15:52 pm
Has anyone connected their single phase variac input to 2 phase i.e between L1-L2 ? ::)
Until it is not rated for that voltage it will burn.
Title: Re: variac wiring
Post by: TimFox on October 20, 2021, 06:02:37 pm
Transformers have a pretty strict voltage limited.
If you put too much voltage over it, the core goes into saturation, with the result that inductance drops a lot and it will start drawing too much current.

Transformers are also frequency dependent.
Simplistic view is that the amount of energy that can be transferred between 2 zero-crossings is constant, which means that at lower frequencies (50Hz) a transformer can handle less voltage then at higher frquencies (60Hz).

These points are very important.  Since the core has a finite area and a saturation flux density (B-field), there is a maximum flux in the core before saturation.  The voltage across the windings is the rate of change (time derivative) of the flux linking the windings (turns times core flux).  The time rate of change depends directly on that flux and the frequency.  Any voltage across the windings above that saturation limit essentially goes into air, and the current through the copper becomes excessive.