Hi,
The equation for voltage drop is I*L/1000[ R*cos(theta) + X*sin(theta)]. But I am confused on when to use the plus or minus sign in between the resistance and reactance? Different problems in CI show different approaches. Someone please help!
Hi Reema
The key to remember is that voltage drop is just the voltage across the line impedance connecting the source to the load (unless you are calculating the percent voltage drop instead of the voltage).
For example, the equation you wrote:
I*L/1000[ R*cos(theta) + X*sin(theta)]
Is really just Ohm's law solving for the voltage across the conductor impedance depending on the total line current:
V = (I)•(Z)
Where:
Z = (L)•(Conductor Impedance in ohms)
To answer the specific question you asked:
"I am confused on when to use the plus or minus sign in between the resistance and reactance?"
This depends on the type of load. Sine the load in question in the conductor connecting the source to the load, the sign will almost always be positive since conductors tend to be more inductive than they are capacitive.
This means that the sign between the resistive and reactive term will be a plus sign in the form of:
Z per 1,000 ft = (R per 1,000ft + jX per 1,000ft)
Where R is the AC resistance of the conductor
And X is the reactance of the conductor.
