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Re: ballasting

In a message dated 5/9/00 4:14:33 PM Pacific Daylight Time, tesla-at-pupman-dot-com 
writes:

<< 
 Maybe my questions are a bit funny for the pros... but I would like to ask 
the
 same.
 I was wondering about power limiting issues, in effect a friend told that a
 primary ballast, 
 is it resistive or inductive, will reduce the voltage input to the 
transformer
 ... is this right ? 
 So, what do you think is the good combination ?  Variac and ballast ? Variac
 only ? Variable ballast only ???
 Imagine I want to build a "Triac based" (I have a 400 Volt 20 Ampere one) 
watt
 absosption limiter rated 
 for something like 1.5KVA maximum, am I going the right way to limit current
 absorption from the transformer ?
 Is this triac suitable for coil use ???
 This cause I would like to have a "precise" limiting value... with a nice
 tweedling knob there to play with power ;) 
 They also sell kits for power limiting use nice for brush motors and the
 like... What about these ?
  
 I imagine all of these will cause a voltage drop or any weird sinusoid split
 too...   :-\
 
 
 Tnx to all for any help
 ciao ciao
 
 Daniele
  >>
Daniele,

What type of high voltage transformer are you planning to use?  Some 
transformers like oil burner ignition transformers (obits) and neon sign 
transformers (nsts) are internally current limited and need no ballasting to 
limit the primary current.  Pole transformers, plate transformers, potential 
transformers and most microwave oven transformers are not current limited and 
will require the addition of some external ballast in series with the input.  
Inductive ballast is best because it is most efficient as the ballasting is 
accomplished with inductive reactance and most of the loss is due to the 
resistance of the wire.  Resistive ballast, using something like high wattage 
heating elements also works but you will have a voltage drop according to the 
amount of resistance used and current in the primary, thus the ultimate high 
voltage output will be reduced.  Many folks, myself included use inductive 
ballast with a small amount of resistance (like .5 to 1.5 ohms) in series.

I also use a variac to control the voltage applied to the transformer 
primary.  It sounds like you are describing a triac phase control circuit 
like for controlling a light or variable speed drill motor.  These will not 
work to control the input to a transformer.

Ed Sonderman