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Power factor correction

polarmystery

polarmystery

Diamond Member
Is there an EE around here that can help me with a problem? In my power systems class, one of my hw problems gives me the hp of a motor, the voltage, and the original power factor of the circuit, asking on what capacitor value I would need to correct the power factor to another value, but I can't for the life of me how to do it. I've been killing myself for about 2 hours now and I know it's simple, but I can't seem to piece it together. Anyone offer any help? Here is the question in all details, I even know the answer because it is in the back of the book, but I need to know how to "solve" it. Here it goes:
🙁
Problem:
A single-phase 3hp, 240V pool pump(motor) runs at full load with 92% efficientcy and a power factor of 0.73 lagging. Calculate the capacitor needed to improve the power factor to 0.98 lagging.

Thanks for the help!
 
Here's a start for you...

You know that the motor delivers 3 hp, so you should be able to calculate the total power input from the efficiency. From there you should be able to figure out what the effective resistance (balanced Y-connected across 240 l-l) must be in order to deliver the power.

Knowing the power factor, you can also figure out an effective (parallel) inductance that will bring the power factor (from 1.0 for just the resistance) to 0.73 for the resistance/inductance parallel combination.

Now it's easy to calculate what the size of the capacitors need to be (balanced Y-connected) to raise the total resistance/inductance/capacitance parallel combination back to 0.98.

Good luck!
 
Yes, and I figured it out. It turns out that I was racking my brain for nothing being as the answer in the book was wrong :-\. The circuit is a voltage source, with a load Z, in parallel with a Capacitor C. Solving yields 70microfarads, not 82 like in the book.
 
Hmm. My attempt at this problem gives an answer of 82 µF.

Would you care to explain why the answer is 70 µF?
 
The answer is 70 because I can't figure out how to get 82 and it's due at 9am today and since no one wanted to explain it to me, that's what I came up with.
 
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