boosting car alternator voltage

[tcmtech]

If I understand you correctly, you change a 3 phase delta alternator to become 3 phases in series, then use the 3 coils in series to drive an AC load?

I've never tried it, so this is likely a dumb question, but wouldn't the 3 coils and their 3 voltages have significant cancellation?

No they don't cancel out exactly. If you look at the layout of a normal 3 phase delta connection all three phases are in a closed loop series where all three balance each other out.

With a Z connection the bottom phase is shifted off one way or another putting it 180 degrees out of phase from what the other two are doing. Thats why a 120 volt delta connection changed to a single phase Z connection ends up with double the voltage but keeps the current equal between all three supply phases.

I have an old military gen set that has multiple output configurations for 120 VAC delta 3 phase, 120/208 VAC wye three phase and 120/240 VAC single phase.

The 120/240 VAC single phase output just moves the one leg of the 120 VAC three phase delta configuration over and uses the middle of the Z as the common center tap point for the 120/240 VAC single phase output.

 

[Mr RB]

Ahh I think I understand you now!

So you don't just open the connection between two coils on a delta, you need to open the connection AND reverse one end coil (of the 3 series coils).

 

[tcmtech]

Yep you got it!

 

[MikeMl]

...With a Z connection the bottom phase is shifted off one way or another putting it 180 degrees out of phase from what the other two are doing. Thats why a 120 volt delta connection changed to a single phase Z connection ends up with double the voltage but keeps the current equal between all three supply phases...

However, with the winding currents the same, the power delivered to a single phase load with the Z-connection is only 2/3 of what the generator can deliver to a three phase load.

 

[tcmtech]

I have never tried it with an automotive alternator.

What I do know is that on normal three phase AC power gensets they are not derated on output capacity, wattage or KVA wise, when configured for single phase output.

For whatever reason when setup for a single phase Z connection the amp ratings are always 50% higher than the three phase phase to phase ratings.