# stumped on obtaining angle phaseof 0 degrees,and power factor close to unity?

Discussion in 'Homework Help' started by baldhead2, Sep 28, 2009.

1. ### baldhead2 Thread Starter New Member

Sep 28, 2009
1
0
Hi, how do I choose a capacitor to get a phase angle equal to 0 degrees and a power facter that is close to unity? This is for power factor correction for an R-L motor with the motors voltage being 120Vrms,the current is .282Amps rms amps,the frequency is 60Hz,the true power(P) = 79.5mWatts,the reactive power(Q)=unknown,the apparent power(S)= 33.84 VA. What should the capacitor value be to obtain this task?

Thanks for taking time to help me with this head scratcher!

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Are you intending to put this capacitor across the "L" of the motor? If that is the case then you start with a capacitor that has the same reactance as the motor at the frequency of interest: 60 Hz. in this case. Whatever you do make sure you pick one with a voltage rating 2*Vpk= 2*120*SQRT(2) ~= 340V, or for whatever your mains voltage is.

I don't know if there is more that can or should be done, but you might want to ask someone who has tried it, to find out what results they achieved.

I would not be surprised if the results were a disappointment to you. Running the motor at its rated speed is one thing, but starting it from a dead stop is quite another thing altogether. If a simple capacitor is right for running it may be wrong for starting, or vice versa.

Last edited: Sep 29, 2009
3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
If the true power is only 79.5 milliwatts then you are a very long way from unity power factor! Where did these values come from - some measurements you made?

BTW - what exactly do you mean by a R-L motor? Is that a single phase induction motor?

4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
I am also having trouble believing the true power is 80 milli Watts. If the apparrent power is (as stated) almost 400X the true power, the reactive power and the apparrent power are nearly the same and real (true) power is negligible.

5. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
That's my understanding as well. That's the "click" you hear when the refrig motor kicks on: the start cap is disconnected by a relay a second after it runs up.

6. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Since he did not appear to get what he wanted, I'll give odds against his return.