Join two power sources

[MAB]

As a newbie, this might be a off question.
I am trying to join two PMG generators into a single source of power.
the generators are 3 phase 220 volt AC, I am using two rectifiers to convert to DC
can I then join the two power cables together to output into a single inverter, even
thought they might be generating un-equal power at the same time.
Would the out put be A + B = C or do I lose efficiency to the lowest output.

Thanks
MAB

 

[davenn]

Hi MAB,
welcome to EP

Not sure if there is anyone here qualified to answer that one ??
if so some one will chime in

else I would suggest you consult a qualified electrician

cheers
Dave

 

[Gryd3]

What are you trying to accomplish?
I've done some light reading into it, and although it sounds doable to run multiple generators in parallel the application is usually done with synched phases and some form of protection (like a small value shunt resistor) to prevent current loops between the generators.
This does not sound like a diy project to me unless you are working with a much smaller scale for educational purposes.

 

[Harald Kapp]

If the generators were to be paralleled at the AC side you'd have to sync them to avoid power swings between the generators.

But: As you state you are first going to rectify the AC into some DC, then you want to parallel the DC outputs to provide one single DC input to an inverter.
This should be possible without much trouble. The diodes in the rectifiers will block any backwards current flow between the generators or between the common output and the generators. You may need to add smoothing capacitor(s) to obtain a stable DC input to the inverter. Note that these capacitors need to be rated for the peak operating voltage (DC) plus a good safety margin and need to be fairly large to obtain good smoothing.

As Gryd3 observed this seems to be a high voltage / high power project. This is potentially life threatening. I advise you to seek the help of an experienced electrician when setting up your circuit and have him double check your setup. Otherwise keep your hands off.
Especially note: Even with the generators off, there may be dangerous residual voltages on the capacitors. Additional bleeder resistors may be required to safely discharge the capacitors in th eoff state.

 

[Colin Mitchell]

There is no problem in joining the two generators and you don't need any capacitors.
In fact if they are out of sync it is simply equal to 100Hz input and that is exactly what you want as each generator will be delivering to the load.

 

[Harald Kapp]

Colin,
with all due respect: I disagree.

You cannot connect the genertaors on the AC side without proper synchronisation and protection circuitry. Fortunately this is not what the op wants to do.

As these are 3-phase generators, the DC from each generator (without filtering) will be a DC signal with an overlaid 150 Hz ripple. If the generators are out of phase, the ripple will be max. up to 300 Hz.

Smoothing capacitors may be necessary, depending on the requirements of the inverter for a clean input signal. If the inverter tolerates the ripple, the better and no capacitors are required.

To the op:

Would the out put be A + B = C or do I lose efficiency to the lowest output

The genertor's nominal output powers should not bee too different, otherwise the smaller one will not contribute much to the compound output power. If the power ratings for the generators are comparable, you will get indeed the sum of the output powers of the two generators - minus the losses in the bridge rectifiers.

 

[Colin Mitchell]

You cannot connect the generators on the AC side without proper synchronisation and protection circuitry. Fortunately this is not what the op wants to do.

READ THE ORIGINAL POST

 

[Harald Kapp]

READ THE ORIGINAL POST

Colin, you don't have to shout at me. Normally I'm quite reasonable and even willing to admit an error. However, this is a quote from the original post:

I am using two rectifiers to convert to DC
can I then join the two power cables together

Forgive me if I do misunderstand this, but this definitely sounds like
generator1 - rectifier1 - connection - rectifier2 - generator2
to me. Why else would the op use two rectifiers?

 

[davenn]

sorry Colin, but I'm with Harald on this one
I think you mis-understood the original post

I am using two rectifiers to convert to DC
can I then join the two power cables together to output into a single inverter, even
thought they might be generating un-equal power at the same time.

emphasis on the sequence of events ... Can I then

Dave

 

[Y2KEDDIE]

The confussion may be in the terminology. I tend to think of generators as DC producing devices. An Alternator produces AC, (Single or polyphase). My co-workers refer to our standby power sources as Gen-Sets, but they really are Alternators (produce 3 phase power when comercial power is not available.)
I guess one could say, a generator is an alternator with a commutator.