2.1 Determine the value of the phase voltage in a delta-connected system if the line voltage is 380 V - NSC Electrical Technology Electronics - Question 2 - 2016 - Paper 1

The voltage phasor diagram for a three-phase delta-connected system consists of three phasors, each separated by 120 degrees, illustrated as follows:

1. Label each phase as $V_{L1}$, $V_{L2}$, and $V_{L3}$.
2. The angles between the phasors should be 120 degrees.

Make sure to indicate the rotation direction and label each phasor appropriately.

To determine the current delivered by the alternator at full load, we use the formula:

I_L = rac{S}{ ext{sqrt}(3) imes V_L}

Substituting the given values:

• $S = 20 imes 10^3 ext{ VA}$
• $V_L = 380 ext{ V}$

We find:

I_L = rac{20 imes 10^3}{ ext{sqrt}(3) imes 380} $I_L ext{ approximately equals } 30,39 ext{ A}$

The power rating can be calculated using the power factor:

$P = ext{sqrt}(3) imes V_L imes I_L imes ext{cos}( heta)$

Where:

• $V_L = 380 ext{ V}$
• $I_L = 30,39 ext{ A}$
• $ext{cos}( heta) = 0,87$ (lagging)

Substituting the values:

$P = ext{sqrt}(3) imes 380 imes 30,39 imes 0,87$ $P ext{ approximately equals } 17,4 ext{ kW}$

A kilowatt-hour (kWh) meter measures the amount of energy consumed over a specified period. It quantifies the total electrical energy used by a load.

1. Adding power factor correcting capacitors in parallel with the load.
2. Using synchronous motors to adjust the power factor to unity.

The power consumed by the load can be calculated as:

$P_n = P_1 + P_2$

Where:

• $P_1 = 120 ext{ W}$
• $P_2 = 50 ext{ W}$

Thus:

$P_n = 120 + 50 = 170 ext{ W}$

1. The power of a balanced load can be measured accurately using only two wattmeters instead of three, reducing both costs and complexity.
2. It allows for determination of power factor in both balanced and unbalanced conditions.