Elevator Mechanic Practice Exam

If a 480VAC step-down transformer to 240VAC is connected to a 3-phase rectifier, what is the output voltage?

1. 320VAC

2. 360VAC

3. 400VAC

4. 480VAC

The correct answer is: 320VAC

To determine the output voltage from a 480VAC step-down transformer to 240VAC when connected to a 3-phase rectifier, it is important to understand how the rectification process impacts the voltage. The transformer reduces the voltage from 480VAC to 240VAC, which is the root mean square (RMS) value. In a three-phase system, the DC output voltage after rectification can be calculated using the formula for the peak voltage from the RMS value and then applying the rectification factor. First, we find the peak voltage from the 240VAC RMS value. The peak voltage (V_peak) is given by: \[ V_{peak} = V_{RMS} \times \sqrt{2} \] So, for 240VAC: \[ V_{peak} = 240VAC \times \sqrt{2} \approx 339.41V \] In a three-phase rectifier configuration using a full-wave rectification method, the output DC voltage would be approximately: \[ V_{DC} \approx V_{peak} \times 0.866 \] This accounts for the nature of the full-wave rectification in a three-phase system: \[ V_{DC} \approx 339.41