Solution 30 - Eng Pro Guide Test

[rmsg]

Hi All,

To my understanding of Power Systems 3 Phase Circuits, I found something which does not make sense to me.

Please see Solution 30 of Eng Pro Guide Test.In my opinion, Justin Kauwale should make some corrections here. See enclosed PDF,

 In the question above it is assumed 0.85 lagging power factor, yet in the solution below it is stated 0.85 leading (current leads the voltage)

Then later below it is said :

cos (theta) = 0.85 -> theta = -31.8 degree (negative for lagging)

Actually in my opinion, cos (theta) is power factor and therefore cos inverse (theta) here is power factor angle which should be positive instead of calling it negative for assumed lagging system.

Yet I see that in the expression of Vp after 2 lines, I think he got it right there by saying current = 1.0<-31.8 degree.

In actuality current angle is negative (opposite sign of power factor angle).

so I believe in the end by taking  current = 1.0<-31.8 degree he got the final answer right, but there are mistakes/discrepancies in the explanation above

I believe some corrections needs to be made here.

Please comment if anyone thinks other wise.

View attachment Solution 30 - Transformers - Correction.pdf

 

[Drewism]

Lagging power factor means a positive angle. The power factor angle is cos -1 (0.85) = 31.78 degrees. We're assuming the reference angle is 0 degrees therefore (0 degrees - 31.78 degrees) which is -31.78 degrees. I guess it should have mentioned the reference angle is 0 degrees.

If you have the Eng Pro Electrical Study Guide refer to section 2 (circuits) - page 32.

Leading power factor = (Voltage Angle - Current Angle) so, 0 - (+ theta) = -theta.

The solution seems correct. There's just not enough explanation of getting into why the angle is negative.