Re: [TCML] Freau Phase Controller - Circuit Theory

[Mike Gray <graymp89@xxxxxxxxx>]

Thanks John. That's improved my understanding somewhat.

Okay, so you're effectively mapping the adjustable phase range, 45 degrees,
onto the mechanical rotation range of the variac. And when you increase the
capacitance, you stretch the adjustable phase range across more of the
variacs rotation, thereby increasing the sensitivity of the adjustments you
can make, but at the cost of increasing resonant stresses. So it's a
tradeoff between sensitivity and resonant stress.

I'm still wondering about the actual circuit theory though. By increasing
capacitance you approach resonance at 50Hz. So the variations in phase
caused by turning the variac must become greater (hence a greater total
range across the variac) as you approach resonance? Is that correct? If so,
why?




On 21 July 2013 06:22, Futuret <futuret@xxxxxxx> wrote:

> Mike,
>
> My reasoning for using only enough rise (3 to 5 volts max) is to reduce
> the resonant currents to a minimum to not overstress the components.
> A smaller cap value can be selected which gives no resonant rise, but
> this usually causes the motor to lose sync-lock at some point along
> the variac's range.  Many people use the system this way, and either
> install a mechanical stop so they they don't rotate the variac knob too
> far to the right,
> or they simply remember not to go past a certain point.  A 5 volt or
> so rise should allow for the variac knob to be turned all the way to
> the right without losing sync-lock.  Many folks run with a 10 volt
> resonant rise.  It's a matter of taking a sort of middle of the road
> approach.
>
> The oriental motors don't need a very large phase cap, only about 4uF
> I think.
>
> Cheers,
> John
>
>
>
>
>
>
>
> -----Original Message-----
> From: Mike Gray <graymp89@xxxxxxxxx>
> To: Tesla Coil Mailing List <Tesla@xxxxxxxxxx>
> Sent: Sat, Jul 20, 2013 6:12 pm
> Subject: [TCML] Freau Phase Controller - Circuit Theory
>
>
> Hi all. As I've mentioned in a separate post, I'm going to build a Freau
> phase controller. So I've started looking into it, searching the archives
> and reading up etc. The first thing I always do before starting something
> new is make sure I understand the theory of how it works. Now I've tried
> and tried but I can't seem to find any info on the actual theory behind the
> phase controller. Maybe it's so simple that it doesn't need explaining! I
> understand the general concept (I think), but would appreciate a fairly
> detailed explanation if someone could provide it.
>
> Here's my broken understanding. A motor run directly off mains will look
> almost purely inductive. A variable inductor (variac) in series with this
> will not affect the phase since the circuit is still completely inductive.
> So by introducing capacitance into the circuit, you can now play with the
> ratio of the reactance of the inductor and capacitor, by turning the
> variac. By varying this ratio you are effectively varying the phase angle
> between the voltage and current.
>
> That's about as far I go. I've read that you should use a cap size that
> gives roughly 3 to 5 volts maximum resonant rise when you're turning the
> variac through its range. Larger caps will cause greater voltage rise,
> presumably because they become more and more resonant with the inductance
> of the variac & motor. So you're intentionally picking a cap that won't
> resonate perfectly with the variac at 50Hz, but will resonate enough to
> produce a small voltage rise?
>
> What is happening it terms of voltage/current phase here? Why is this
> slight resonance a good condition for controlling the phase of the circuit?
> Does the max voltage rise represent the point at which the phase between
> current and voltage is zero, and so ahead/behind on either side of this
> point?
>
> Lastly, I'm using a US motor (one of the small oriental ones) in the UK,
> and so I've already got a variac in place to run the motor. This must
> influence the design of my phase controller. The variac introduces even
> more inductance into the circuit, and so I imagine I will probably need a
> larger cap to compensate? Has anyone else come across this?
>
> Cheers
>
> Mike
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