220906-2140 EDT
Jpflex:
To continue from my last post.
Now I will create a theoretical circuit that will approximate an MOV as a transient limiter.
A typical real world MOV in an AC circuit will have clamping characteristics in both the + and - directions that are about equal. For my analysis I am going to just look at what happens for one polarity.
An electrical circuit that I am going to describe has an adjustable DC voltage source of v, and a linear internal impedance of X resistance.
If we consider this source to be adjustable from 0 to 500 V, and have a linear internal impedance of 0.1 ohms, then if my normal load is a 100 ohm linear resistance the load current is:
0 A when v = 0 V, and load voltage is 0 V,
0.5 A when v = 50 V, and load voltage is 50 - ( 0.5 * 0.1 ) = 49.95 V,
1.0 A when v = 100 V,
2.0 A when v = 200 V, and
5.0 A when v = 500 V, and load voltages 500 - ( 5.0 * 0.1 ) = 499.5 V
Next I place an MOV in parallel with the 100 ohm load resistor. For an approximation for the MOV I will a use a series equivalent circuit of a 200 V DC voltage source, a 1 ohm resistor, and an ideal diode. This equivalent shunt circuit draws no current when the applied voltage is less than +200 V.
When this series circuit has a voltage less than 200 V it draws 0 current. Above 200 V the current thru this combination is ( Vin -200 ) / 1. At 300 V across this combination the current is 300 A. Thus, the voltage drop across the internal impedance of the source is 300 * 0.1 = 30 V. This is an approximation, but sufficiently close for our purpose here. The clamped voltage across the load at 500 V input is about 470 V. Not much clamping for the values I chose. The current to the actual load is not much different from no transient limiter.
Does this help you understand a little about operation of a shunt transient MOV limiter? Different values will give you more likable values.
There are other considerations that are of major importance, such as series inductance.