Re: 60A Variac on Ebay: inrush current problem

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Sparktron01@xxxxxxxxxxx 

Justin

Yes, you comment is basically correct.

We scoped a 1.0kVA variac for giggles at work when initially powered.
(used a high speed digital storage oscilloscope, Tek TDS754 for the
curious).  First 20-30mS was a huge L/R  saturation offset (can be
positive or negative depending on turn on point a AC sine wave).  Then
a self excited resonant oscillation (200Hz to 1.0Khz, kind a
fuzzy with memory) lasted about 8 HF cycles.  AC forcing function
took over about 50-100ms (3-10 AC cycles) after initial powering.

It is very fast, ugly, and can easily damage sensitive electronics
(along with tripping breakers).  The "step-start" method mentioned
in previous postings has been around for many years (decades?),
is robust and reliable, and "cushions" AC line from impulse starting.
This method is exactly equivalent to "reduced voltage" starting
methods that have been around also for decades using first resistors,
now solid-state electronics to "soft start" motors/inductive loads.

Start up inrush on large transformers, AC motors and variacs can be
as much as 3X to 10X rated line current, so this is a big deal, especially
when you consider the thermal (and magnetic [mechanical] ) forces
on windings are generally a function of inrush current squared.
Early in my career, I watched the power company disconnect and reconnect
34.5kV 3 phase power to a 20MVA transformer(s) in 34.5kV switchyard at
a chemical plant where I started my career.  The connection was
_significantly_  more exciting then the disconnection and this was with
no load on secondary, and using motorized HV switches on the
primary...   :^)

Regards

Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA


> Original poster: Just Justin <rocketfuel@xxxxxxxxxxxxx>
>
>
>
>  > Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx>
>  >
>  > This is a common problem when powering up big inductive loads (like
>  > transformers or variacs) that have high magnetizing currents. Several
>  > relatively easy "fixes" are available.
>  >
>
> I hope you'll pardon the newbie question:
>
> Are you saying that a lot of current will flow through the coil before the
> core has had a chance to magnetize and thereby oppose the current flowing
> in the coil?  Do you have a ballpark figure on how long this takes for, say
> a 60A variac?
>
> Thanks!
>
> Justin
>
>