Alternator Rectifier

[Strongbow]

Hello,

Does anyone know what the effect would be of replacing a delta wired diode pack/rectifier with a star wired one?

Any advise would be awesome.

Many Thanks,
Alan.

 

[marconi]

I have never worked on auto alternators so I am drawing on my knowledge of electrical engineering to provide an answer. So I don't know if auto alternator windings are usually wired 'star' or 'delta'. I suspect they are normally wired 'delta'. For my response I assume the auto load is mainly resistive.

The 3 phase rectifier with 3 blue diodes is the star configuration which provides 3 phase half wave rectification. It is a so-called 'single way' configuration because the current in each line always flow in the same direction through the windings. To connect this to an auto alternator it would have to have its windings also connected in star and the node at which all three windings are connected together brought out of the machine for connection to the star rectifier block - see my diagram 'alternator.png'.

The 3 phase rectifier with 6 blue diodes is the delta configuration which provides 3 phase full wave rectification. It is a so-called two-way configuration because the currents in the alternator windings now flow in both directions. This rectifier only requires 3 lines as input and so could be connected to the three lines of either a star or a delta connected alternator.

For reasons I will not explain now - it involves some calculus - the average direct current output voltage from the rectifier configurations are not the same for the same line-to-line input voltage. If you have a dekko at the second attachment you will see the three phase input voltage waveforms and the rectifier output voltage waveform for the half (star) and the full (delta) wave rectifiers.

The average value of the dc voltage for star is 67% of the rms line to line voltage.
For delta the average dc voltage is 134% of the rms line to line voltage. Or simpler - the output voltage would be 50% (2 x 67 = 134) less if you used a star rectifier instead of a delta one.

The other thing you note is that the full wave delta rectifier output waveform looks smoother than that for the half wave star rectifier. This is a feature of two way current flow for the full wave delta. The undulations of the dc output are called 'ripple' and one can calculate a 'ripple factor' to compare different waveforms. The figures are 0.18 and 0.04 respectively for star and delta. The rate of change of the undulations - their ripple frequency - is 3 x alternator frequency for star and 6 x alternator frequency for delta.

I will begin to draw to a close but before I do the full wave/delta rectifier 'uses' the windings of the alternator more effectively since the current flows in both directions. What this means is that a star rectifier would require an alternator with thicker winding conductors than a delta one to produce the same dc power output.

So the more direct reply to your questions is stick with a delta rectifier bridge if that is what was originally connected.

(Or something like this - others can confirm, correct or amplify).