Introduction:
In this scenario, we have a 3-phase 450V, 10KVA star-connected alternator with an impedance of 0.4+j5 ohm per phase. We need to determine the power factor at which the voltage is zero. To solve this problem, we will first calculate the current flowing through the alternator using the given values and then determine the power factor.

Calculating Current:
To calculate the current, we will use Ohm's law. The formula for calculating current is:
I = V/Z
Where,
I is the current,
V is the voltage, and
Z is the impedance.

In this case, the voltage is 450V and the impedance is 0.4+j5 ohm per phase. We will calculate the magnitude of the impedance using the Pythagorean theorem:
|Z| = sqrt(0.4^2 + 5^2)
|Z| = sqrt(0.16 + 25)
|Z| = sqrt(25.16)
|Z| ≈ 5.016 ohm

Now, we can calculate the current:
I = 450V / 5.016 ohm
I ≈ 89.7A

Calculating Power Factor:
To calculate the power factor, we need to determine the phase angle between the voltage and current. Since the voltage is given as zero, the angle between the voltage and current will also be zero, indicating a purely resistive load.

The power factor (PF) can be expressed as the cosine of the phase angle (θ):
PF = cos(θ)

In this case, since the voltage is zero, the power factor will also be zero.

Conclusion:
In conclusion, the power factor at which the voltage is zero is zero. This indicates a purely resistive load. The current flowing through the alternator can be calculated using Ohm's law, and the power factor can be determined by finding the phase angle between the voltage and current.