Re: saturable reactor vs choke

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Steve,

I just had another thought and perhaps a different way to look at 
it.  If one looks at the flux thru the core with no saturation (and 
lets also assume no hysteresis and an ideal inductor), it will look 
like a sine wave.  The back emf will be its derivative (dphi/dt).  As 
the peak of the flux sine wave is reached, the voltage across the 
inductor will be crossing thru zero volts (90 degree lag of current 
wrt voltage) because dphi/dt = 0.  Now if one has an ideally sharp 
knee for the core material (still no hysteresis) and the flux 
sinewave is larger than the core's saturation point, the peak of the 
sinewave will be clipped at the saturation point.  Hence its 
derivative will be zero before and after the point where the peak 
flux would have occured.  If this thinking is correct, then it seems 
that dphi/dt will be zero while there is still voltage on the 
inductor and current will scream.   I'm thinking that a sharp knee is 
not desirable and a soft knee will keep dphi/dt non zero until zero 
volt crossing occured.  Would this be correct??

Gerry R


> Original poster: Steve Conner <steve@xxxxxxxxxxxx>
>
> Hi Gerry, David,
>
> I believe a saturable reactor works much the same no matter if the 
> "knee" of the magnetic material is "hard" or "soft", but hard is 
> preferable. As far as I know, a saturable reactor works as a phase 
> angle controlled switch just like a lamp dimmer, and the control 
> coil bias determines the point in each half-cycle where it flips 
> from unsaturated to saturated.
>
> I've seen ferrites for HF saturable reactors in a Fair-Rite catalog 
> and they seem to be designed with a vey hard knee. And a warning not 
> to operate too near their acoustic resonance frequency in case they shatter :-/
>
> Steve Conner
> http://www.scopeboy.com
>
> At 20:56 14/04/06 -0600, you wrote:
> > Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
> >
> > Gerry,
> >
> > I'm not too sure of exactly what your asking but I'm assuming that 
> > you are making referance to the "bell
> > curve" or "J curve", actually, that would represent the magnetic 
> > saturation range of the reactor's core on an x-y graphic? From what 
> > little personal experience that I've had with SRs, by gradually 
> > increasing the input DC voltage (usually something like 0 - 90 VDC 
> > at a few amps), you will proportionately increase the allowed 
> > current flow to the source load through the main coil. I BELIEVE 
> > once full saturation is achieved (90 VDC input to the control 
> > winding) then the main coil behaves almost like a short circuit. 
> > Conversely, with a zero volt DC input to the control coil, the main 
> > work coil will have maxi-
> > mum inductance and L reactance to the main AC volt-
> > age and will allow a relatively small current flow, irreagardless
> > of the load, even if it's a short circuit.
> >
> > David
> >
> >
> > ----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
> > To: <tesla@xxxxxxxxxx>
> > Sent: Friday, April 14, 2006 1:44 PM
> > Subject: Re: saturable reactor vs choke
> >
> >
> > > Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
> > > Hi David,
> > > A few questions:
> > > If one has a saturable reactor and the BH curve is sorta ideal 
> > > with respect to the knee ( ie: it has constant slope until it 
> > > saturates and then the curve precisely flattens out), I'm 
> > > wondering what happens when this knee is reached.
> > > Does the current scream very high or is there a smooth transition 
> > > to larger current as the saturation point is changed??
> > > Does a saturable reactor require a soft knee??
> > > Gerry R.
> > >
> > > > Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
> > > >
> > > > With a "saturable reactor" (SR), in addtion to the "working"
> > > > inductive coil, there is also a "control coil" of usually many
> > > > more turns of smaller guage wire than the main "working"
> > > > coil. A relatively low, variable DC voltage is introduced into 
> > > > the control coil to control the magnetic saturation of the iron
> > > > core so as to control the current flow through the main
> > > > working coil. This allows infinite range of the current that's
> > > > allowed to flow through the main working coil, irregardless
> > > > of the source load.
> > > >
> > > > David Rieben