Three-Phase Power Calculations

Question 1

What is the simplest way to view power in a three phase system?

Answer 1

By analyzing it as 3 - single phase circuits.

Question 2

Why does S_T = V_L•I_L•3^1/2 hold true for a wye connected load?

Answer 2

Because I_Φ = I_L &

V_L = V_Φ • 3^1/2.

Question 3

Why does ST = V_L•I_L•3^1/2​ hold true for a delta connected load?

Answer 3

Because V_Φ = V_L &

I_L = I_Φ • 3^1/2.

Question 4

What is the unit of measurement for reactive power?

Answer 4

The Voltampere reactive;

VAr.

Question 5

What is the unit of measurement for active power?

Answer 5

Watts;

W.

Question 6

What is the definition of power factor?

Answer 6

The ratio of true power to apparent power.

Question 7

Is a 3Φ motor normally balanced?

Answer 7

Yes, it does not require a wye neutral.

Question 8

Typically, are there many ways to calculate circuit power in a balanced circuit?

Answer 8

Yes, You can calculate for one phase and multiply by 3, use the line values formula or add horizontal and vertical components for the whole circuit.

Question 9

Can you use the line value formula for power in an unbalanced 3 Φ delta load?

Answer 9

No, the horizontal and vertical values must be summed separately.

Question 10

What is an example of a normally unbalanced load?

Answer 10

A 3 Φ 120/208V panelboard.

Question 11

Which load connection uses more power? wye or delta.

Answer 11

Delta uses three times the power that wye does;

However the components connected in wye need only be rated for phase voltage.

Question 12

Can a delta connected load be shown both as a rectangle and a triangle?

Answer 12

Yes, generally you can use your preference.