# AC-Power factor of an RL circuit

Discussion in 'Homework Help' started by Rubberfrog, Nov 12, 2008.

1. ### Rubberfrog Thread Starter New Member

Nov 12, 2008
7
0
Hello, im new to this forum and have used it in the past for studying, im wondering if anyone could give me help on a question Ive been trying to figure out involving calculating power factor of an RL Circuit when the frequency has changed.

A 240v 50hz appliance is rated at 2Kw and has a lag PF of 0.7
Calculate the new PF at 60hz.

Heres what Ive done so far:

Sketched a Power triangle True power (P) = 2000w
Apparent (S)= 2860VA (S= P/PF)

Calculating I
Vs=240v
Apparent (S)=Vs x I

I=S/Vs = 11.92A

Finding Q (Reactive Comp.) by Pythagoras.
b= root (c2-a2)
Q= 2044va
Finding Inductive Reactance
Xl=Vs2/Q = 240squared/2044
Xl=28.18

Total Cct Impedance
Z= Vs/I = 240/11.92= 20.13Ω

Using XL = 2piFL
Transpose to find L
L= 632mH

Cos θ = PF=0.7
Cos-1PF= θ
Θ=45.57degrees

I have tried changing the Freq in the equation 2piFL =Xl giving two new values of Xl but I cant seem to be able to construct a new power triangle with just this info. Having got to this point I am not sure what my next step is any pointers would be of a great help.

Cheers
Rubberfrog

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Now, you have the inductance so you can calculate the new Xl for 60Hz. After you do it calculate the total impedance of the circuit. The arccos of the angle of this impedance is the new power factor. Another way to do it is to calculate again the total impedance at 60Hz, then find the current, then the real power dissipated by the real part of the impedance and finally divide the real power by the new apparent power to find the power factor.

Mar 20, 2007
1,068
4
Rubberfrog,