Re: To Inductor...or Not to Inductor...

[Lee Hart <leeahart@earthlink.net>]

RichSJ@aol.com wrote:
> I am building the "Boat Anchor" battery charger [list of parts deleted]
> Rich Rudman thinks I'll need a big inductor if I want to get 40 amps or
> so of DC out of this charger. I thought the variac might be enough
> inductance. I was going to add some AC capacitance across the line
> input to the variacs to try to improve power factor. If an inductor
> would help where in the circuit would it be most effective?

What is a) your AC input voltage, b) the desired DC output voltage, and
c) desired output current?

Comments so far: 

1. You don't want to just parallel the variacs, because their wipers
will never match exactly. Even a single turn difference will lead to a
large circulating current.

2. Variacs make great adjustable inductors. Just use one end and the
wiper as the two connections.

3. Inductors go in the AC side, not the DC side. You can't put them on
the DC output side unless they are specifically built to operate with DC
flowing in them (i.e. they have a deliberate air gap in the core).

4. The amount of inductance controls the charging current, the same as
if you had a resistor in series. However, the inductor has the advantage
of not burning up the excess as heat; it instead shifts the phase angle
between voltage and current (lowers the power factor).

5. Capacitors can be added to improve the power factor. Inductance
causes the current to lag behind the voltage, and capacitance causes it
to lead. The right value of capacitance can bring the power factor
closer to 1.0. But it will take substantially more than 15uF; you need
50-100uF just for a 10-15 amp charger.

6a. If you need to reduce the voltage (peak of the AC line is greater
than desired battery voltage), then the capacitors go across the AC
input, and the inductor in series with the bridge rectifier's AC input.

6b. If you need to increase the voltage (peak of the AC line is less
than desired battery voltage), then the capacitors go across the AC
input of the bridge rectifier, and the inductor goes in series with the
AC line.

That will get you started.
--
Lee A. Hart                 Ring the bells that still can ring
814 8th Ave. N.             Forget your perfect offering
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<leeahart@earthlink.net>            Leonard Cohen