Newbie Needs Help

Thread Starter

Taffy100

Joined May 29, 2007
2
Hi All,

My name is Martin Cole and i'm new to all this...

I've decided to start studying Electrical Engineering, and my topic touches on Series AC Circuits. I have been set some homework questions, and was needing a little help...

Any help will be appreciated... Its pretty hectic i think...

Martin

QUESTION 1
A coil with an unknown resistance and inductance is connected in series with a resistor of 15 Ω and a capacitor of 150,5 μF. The circuit is connected across a 240-V, 50-Hz supply. The power factor of the load is 0.7771 lagging. A voltage drop of 142,5 V is measured across the
resistor.
Calculate the voltage drops across the capacitor and the coil.
Calculate the resistance and inductance of the coil.
Calculate the power dissipated in the circuit.
Calculate the value of a capacitor to be connected in parallel with the
circuit to raise the overall power factor to 0,9563 lagging.

QUESTION 2
A voltage, represented by ν(t) = 339,411 sin (315t + (pi)/6 ) volt, supplies three parallel connected loads. The loads draw current from the supply as follow:
Load a : 17,2 A at a power factor of 0,5878 lagging
Load b : 9,5 A at unity power factor
Load c : 18,8 A at a power factor of 0,7071 lagging
Calculate the following:
The total current
The active and reactive components of the current
The value of the components in each load
The total impedance
The total active power dissipated in the circuit
The overall power factor of the circuit
The value of a capacitor to be connected in parallel to the circuit to cause
resonance to the parallel circuit
The total current and the overall power factor, should these loads be
connected in series
The value of a capacitor to be connected in series to the circuit to cause
resonance to the series circuit

QUESTION 3
A circuit consists of a coil in series with a 9,6–Ω resistor. When a direct current of 6,5 A is passed through the coil, the voltage measured across the coil is 117V. With an alternating current of 6,5 A at 60 Hz, the voltage across the coil is 273 V. When the coil is connected to a 240 V/50 Hz supply, calculate the following and also draw the power triangle:
The voltage across the coil
The value of a capacitor to be connected in parallel with the coil to improve the power factor to 0,866 lagging
 

bloguetronica

Joined Apr 27, 2007
1,359
Some hints:
Zc = 1 / (2 * Pi * f * C)
Zl= 2 * Pi * f * L

Being:
Zc and Zl, the impedance, or resistance at a frequency f of respectively, a capacitor and an inductor, C the capacitance, L the inductance and f the frequency.

I won't solve the exercise. I'll be more helpful by doing this.
 

recca02

Joined Apr 2, 2007
1,211
u must really try this one on your own as these are quite elementary problems
after solving them u'll learn a lot;
like mr cumesoftware i'll try to help u by providing some clue;
to avoid confusion of symbols;
z---is used for impedance;
Xc-capacitive reactane ;XL--inductive reactance;
i recommend u to have a look at volume 2 of the site.
r--resistance
the formula to calculate Xc,XL is in the above reply;
in general;
Z= sqrt(R^2 + (XL-Xc)^2)

i would like to give the most detailed answer to above question;
plz try and post your difficulties.
 

Thread Starter

Taffy100

Joined May 29, 2007
2
Thank you both for the assistance. I will attempt today and post my solutions for you to see if i'm doing anything wrong...

Cheers,

Martin
 

recca02

Joined Apr 2, 2007
1,211
additionally
u will find this helpful
for z impedance voltage across it is given by V= i*z
power factor of load z is cos(phi)= r/z ; where Z= sqrt(R^2 + (XL-Xc)^2)

for v= Vsin(wt + epoch)
frequency of above voltage is w/2pi; while epoch is initial lead/lag angle of the quantity with respect to some reference.
 
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