Battery Charger PFC

["Mitchell Oates" <mitchoates@prodigy.net>]

     I ended up getting two T650-26 toroids from Micrometals ($26 
each) to try and do some PFC on my big variac charger. I plan on 
wrapping with 2 or 3 separate leads of THHN 10 AWG wire (toroids 
are 6 1/2" od, about 4" ID). The AL number for them is 434 (ie, 100 
turns will give 4.34 millihenries). Below are the readings and info I 
sent to Micrometals for advice on where to start. I tried using their 
download software, but the way they have the categories listed is 
confusing as to which section to use for calculating how much 
inductance and how many turns to wrap with. Anyone on the list 
have any advice they'd like to add in. especially with reference to 
the questions at the end?

------- Forwarded message follows -------

      I recently downloaded your software and tried using it to 
analyze for winding two T650-26 toroids I recently purchased for 
doing passive PFC in a 60 hz application. However, I found the 
categories listed to be confusing. If you could, after I describe the 
application below, I would appreciate it if you could tell me which 
section I should be using.
      The application is for a variac-based battery charger. The variac 
is a Power Designs Model W50 120V AC 50 amp output, wiring 
connected for 0 to 140 V AC output. The output is being rectified 
by two 35 amp 600 PIV potted bridge rectifiers in parallel, the 
output of the rectifiers being fed to the battery pack. The battery 
pack consists of 30 US Battery 8V golf cart batteries, 165 ahr 
nominal, the batteries paralleled in pairs to form 15 8V 330 ahr 
pairs, and the 15 pairs connected in series to form a 120V DC 
nominal 330 ahr battery pack. The AC outlet is 50 amps fed from a 
50 amp breaker. All wiring from the breaker panel to the battery 
pack is 6 AWG.
      Checking with a Greenlee digital clamp-on Ac ammeter, I've 
taken the following readings when charging. 
      Battery pack voltage 128V, DC current 25 amps by Cruising 
Equipment E-Meter. Variac set at 105V on dial, 35 amps AC in.  
Between variac and bridge rectifiers, 39 amps AC current.
      Battery pack voltage 140V, DC current 18 amps. Variac set at 
110V on dial. 28 amps AC into variac, 28 amps between variac and 
rectifiers.
      On the last set of readings, I checked the positive and negative 
leads going to the battery pack.  With 18 amps DC current 
showing on the E-Meter, the clamp-on ammeter showed 30 amps 
AC current in the negative lead and 20 amps AC current in the 
positive lead.
      The goal is to be able to charge initially at 35 amps DC current 
with battery pack voltage at 128V. This setting would enable full 
charging of the pack without any further adjustment of the variac 
and the charge terminated by a shut-off timer. The toroids are to be 
used for passive PFC to avoid exceeding any current limits on the 
AC outlet breaker, variac, or other components. From what I've 
seen, a variac dial setting of 110V is sufficient for a full charge.
      Would reconnecting the variac for 0 to 120V full scale instead 
of 0 to 140V help any with PF?
      Is there a specific place in the circuit that would be more 
effective for placing the toroids, ie between the variac and rectifiers, 
or would placing them between the AC outlet and variac be just as 
effective?
      Would there be any advantage to adding a bank of large 
capacitors in with the toroids as far as PFC?
      Given the seemingly large AC ripple current in the leads to the 
batteries, would another set of toroids and/or a capacitor bank be 
worth looking at on the DC ouput?

                                                 Mitchell Oates


------- End of forwarded message -------