Discussion in 'Homework Help' started by DYLH, Nov 25, 2013.

1. ### DYLH Thread Starter New Member

Aug 13, 2013
28
1
Appologies, this isn't actually a homework problem, but I thought that this may be the best place to get academic input.

Which is the correct way to solve this?

Find the total VA and PF for a circuit with the two loads:
Apparent Load 1 = 250 VA w/ 0.75 PF
Apparent Load 2 = 150 VA w/ 0.90 PF

Method 1
Real Power 1 = 0.75 * 250 = 187.5 W
Real Power 2 = 0.90 * 150 = 135 W

Total Apparent Load = 250 VA + 150 VA = 400 VA
Total Real Power = 187.5 + 135 = 322.5 W

Resulting Power Factor = 322.5 / 400 = 0.80625

Method 2
Real Power 1 = 0.75 * 250 = 187.5 W
Real Power 2 = 0.90 * 150 = 135 W
Total Real Power = 187.5 + 135 = 322.5 W

Reactive Power 1 = sqrt(250^2  187.5^2) = 165.36 VAR
Reactive Power 2 = sqrt(150^2  135^2) = 65.38 VAR
Total Reactive Power = 230.7 VAR

Total Apparent Power = sqrt(322.5^2 + 230.74^2) = 396.55 VA
Resulting Power Factor = 322.5 / 396.55 = 0.813

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Method 2 is correct.

Aug 13, 2013
28
1
4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
How about just doing the math from the definitions.

Do you know about complex power? If so, things are very easy. If not, then things are a bit harder, but not too much.

5. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You are neglecting to take into account whether the power factor is leading or lagging. If both are leading or both are lagging, then you will get the results in Method 2. But if one is leading and the other is lagging, which is precisely the case when you are doing power factor correction, then you have to take that into account.

Last edited: Nov 25, 2013
DYLH likes this.
6. ### DYLH Thread Starter New Member

Aug 13, 2013
28
1
Good Point.. Thanks!