Core Gaps (fwd)

From:  Barton B. Anderson [SMTP:mopar-at-mn.uswest-dot-net]
Sent:  Wednesday, April 01, 1998 9:19 PM
To:  Tesla List
Subject:  Re: Core Gaps (fwd)


Tesla List wrote:

> ---------- Forwarded message ----------
> Date: Tue, 31 Mar 1998 20:04:48 +0000
> From: "Gregory R. Hunter" <ghunter-at-mail.enterprise-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Core Gaps
> Coiler Types,
> I've noticed that transformer cores with an air gap tend to have
> current limiting properties, whereas transformers with continuous,
> gapless cores have no ability to limit current.  (For example, cutting
> a gap in a variac core to make a current limiter of it)  How does that
> work?  Just curious.

Without the airgap, the core would act as a dead short without some sore of
external current limiting. With an air gap, the core is prevented from
saturating and prevents a short circuit. With the variac slot in a current
limiting configuration, the core is prevented from saturating and the
inductive reactance (resistance to AC) is derived by constant frequency of
50/60Hz, the voltage applied, and the inductance as based on the tap point of
the winding. You have a variable current limiter because you can change the
resistance to the AC signal by tapping the winding at different points
(turning the dial for brush contact position).

> Also, I've gotten ahold of a 120V/12 amp variac.  I took it out of its
> enclosure and studied it.  It's so simple, even a novice like me can grasp
> how it works.  Seems like it would be real easy to build a big,
> homebrew variac for 30-40 amp service.
> Greg
> UK

I don't know, it looks difficult. First you need the core material. Then
you've need to insulate and wind the core. Then there's the epoxy over the
winding for the contact portion. Then there's the grinding of the
opoxy/winding creating the contact points. Then there's the polishing of the
contact points to prevent brush arcing. Seems like quite a bit of work. I
guess it could be done, but it seems a heck of a lot easy buy a used one.