RE: PFC on MOT Pair

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pkomen-at-zianet-dot-com>

Hello Matthew,

The formula I use (mostly untried but matches others' values.  My NSTs are
high power factor.) is a manipulated form of the resonance equation.

PFC - pfc cap in microfarads
VA - VA of the transformer
60 - frequency (or 50 Hz)
120 - line voltage (or 240V)
120^2 - 120 squared.
1000000 - gives the answer in microfarads

PFC = VA * 1000000 / (2 * PI * 60 * 120^2)

Testing may yield different values, but this should put you in the ballpark.

AC ammeters show up on eBay often.  Measuring the primary current of a
shorted MOT would give you maximum VA (assuming that these are current
limited).  Using a somewhat smaller than calculated Cap would be OK.

Regards,

Pete Komen

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Saturday, February 02, 2002 7:56 PM
To: tesla-at-pupman-dot-com
Subject: PFC on MOT Pair

Original poster: "Matthew Smith by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<matt-at-kbc-dot-net.au>

Hi All

College was a long time ago and I can't remember how to work this out -
anyone help?

Two MOTs - primaries in parallel, secondaries in series; they are rated
for 240V input and both have approximate turns ratio of 1:7.95, each
giving approximately 1.9kV.

Mains frequency is 50Hz.

I have determined the primary restistances to be 2.7 and 3.0 Ohms and
the secondary resistances 85 and 109 Ohms respectively.  These figures
were measured using a Wavetek 27XT and confirmed using decade resistance
+ Wheatstone Bridge of a "traditional" Megger tester.  (The inductance
didn't half make the needle vibrate at low cranking speeds - like trying
to measure AC with a DC voltmeter.)  I attempted to measure inductance
using the Wavetek, but it was either two low or too high to register.  I
have yet to check primary current since I currently lack a CT and don't
like using DMMs directly for current measurement! (I've had one fry before.)

I guess that is not enough data to calculate the value of a PFC
capacitor but I would like to know

a) is there a rule of thumb value for capacitor sizes for an array like
this?
b) what is the actual formula that I should be using, if I had the
inductance figures (which is what I guess that I'm missing)?

Cheers

Matthew Smith