[Prev][Next][Index][Thread]

Resitive vs Inductive Ballast

Hi Mike, and all,

I am no pig expert, however I do use power transformers which
deliver up to 4kW with inductive ballasting.  Here is my summary
of the functions of each type of ballast:

Inductive ballast:-

1. Limits short circuit current drawn.
    Series Inductance limits the s/c current without dissipating any
    real power. Only reactive power is drawn.  (Power is limited by
    shifting the phase of the current by up to 90deg relative to the
    voltage.) No power is dissipated, therefore NO HEAT is generated.

2. Controls the capacitor charging current.
    Series inductance limits the current which can flow whilst
    recharging the tank capacitor.  This depends on many factors such
    as capacitor size, break rate, etc,  The supply almost looks like
    a constant current source giving rise to a linear charging ramp.

3. Increases firing voltage due to inductive kick (or inductive
    flyback.)  Current flowing through the ballast as the capacitor
    charges,  continues to flow and drives the capacitor voltage up
    to two times higher than the source voltage at each firing.  Since
    power goes up with V^2 this effect gives a real increase in
    performance. (I beleive this is the biggest benefit of inductive
    ballasting if setup carefully.)

4. Increases firing voltage due to resonant rise.
    The inductive ballast L and the tank C form a resonant cicuit which
    can give an increase in voltage over several supply cycles.  Can be
    more of a hazard, if allowed to occur over many cycles,  but gives
    a some increase in performance at low sync break rates.


Resistive ballast:-

1. Limits short circuit current drawn.
    Series resistance limits the s/c current by dissipating real
    power. It gets real hot !  (If your resistive elements are glowing
    red hot it is obvious where most of the power is going.)

2. Controls the capacitor charging current.
    Series resistance limits the current which can flow whilst charging
    the tank capacitor.  Again real power is dissipated as real heat.

3. No increase in firing voltage can occur with resistive ballast alone.
    I beleive this is why performance is often poor when using R alone.

4. No resonant charing effects are seen with resistive ballast alone.
    Less chance of overvolting components.

The only disadvantage I see with Inductive ballasting alone is that of
time lag.  If there is VERY LITTLE stray resistance in the charging
circuit the ballast gives a long time constant, t=L/R.  The system is
then slow to respond to changes in current and can experience
oscillations and "bucking".  If only a few ohms of resistance are added
in series with the inductor this shortens the time constant
considerably and operation is far smoother.

A while ago I did a test using a 10kv 2kva power transformer which was
ballasted to give 200mA into a short circuit using first inductive
ballast, then resistive ballast.  My TC tank circuit was run from
each supply and the results for each case are shown below.  Notice the
lower firing voltage and power throughput with the resistive ballast.

> Tests at 200BPS sync with 47nF tank capacitor,
> Rotary phase was altered for 2 equal bangs in each case.
> 
> 10kv/200mA Inductively ballasted supply
> 
> Rotary firing delay:  1.762 ms (after supply voltage zero cross)
> Peak firing voltage:  18.85 kv
> Real power input:     1670  W
> Real power output:    1670  W  (assumes ideal components)
> Ballast dissipation:  0     W  (assumes zero stray R)
> Supply VA draw:       1964  VA
> Power factor:         0.85
> 
> 
> 10kv/200mA Resistively ballasted supply
> 
> Rotary firing delay:  4.130 ms (after supply voltage zero cross)
> Peak firing voltage:  8.075 kv
> Real power input:     982   W
> Real power output:    306   W  (assumes ideal components)
> Ballast dissipation:  676   W  (assumes zero stray L)
> Supply VA draw:       1162  VA
> Power factor:         0.85

These are just my experiences.  I hope they help,

Comments welcome as always,     Cheers,

					- Richie,
					- In Sunny Newcastle,

PS.  I plan to put up a Web page shortly on the University server with
some details about Resonant Charging and how Break Rate, Ballast, and
Capacitor value affect power throughput etc.  Any interest ?