3phase full wave rectifier question

[Jimbz]

Hello, doing schoolwork, nearly done, but I dont know what to make of one thing.
We were doing works on diode rectifier, we reached 3phase full wave rectifiers with having a source voltage of 24 VA on a single phase. We had hooked up a multimeter to read the DC readings across a load and it gave us ~32 V. According to the formula (3*SQRT(3)*Vpeak)/PI() it should read roughly 56 V. However there is a massive difference, why ? I do not know if the phases were 120 degress apart or not and im fairly certain it was fullwave not half wave rectified. Could anybody enlighten me on what is going on in here ? Also could anyone enlighten me why the average voltage of 56 V is above the amplitude of 34 V ?

[IanB]

You need to consider various details of the system to be able to answer your question. Some details to consider might be:

Was the source voltage the line voltage or the phase voltage?

Was the source voltage the RMS average voltage or the peak voltage?

How was the three phase rectifier wired in relation to the three phase supply?

Were you using a true RMS multimeter to measure the voltage, or a simple sine averaging meter?

About the voltage magnitudes, consider the following:

If the supply was 24 V AC RMS measured phase to neutral, then you would have 42 V RMS phase to phase. If you considered the peak voltage of this 42 V sine wave, you would have about 59 V. So the available voltage in the system is certainly higher than the 56 V that you measured.

Note: you may want to sketch out the system you were examining, and then sketch the wave forms you expect at various parts of the system with magnitude and phase angle.

[Zero999]

If you connect 24V phase to phase across the rectifier input, you should expect 32V on the output.

[IanB]

If you connect 24V phase to phase across the rectifier input, you should expect 32V on the output.

If you have 24 V between phases (i.e. the line voltage is 24 V) this is true. But if you have 24 V phase to neutral, you will have a line voltage of 41.5 V phase to phase. And if you connect this to the bridge rectifier you will have 56 V DC on the output.

When working with three phase supplies it is important to know what is your phase voltage and what is your line voltage.