Current and Power Ratings

With the recent hurricanes I decided to drag out my generator, a 30kw Onan 3 phase unit I bought at an auction a while back. It has a 6 cylinder Ford motor, runs great. I'm not real familiar with 3 phase. When using for single phase, I get 120 across the neautral and any leg. I get 240 across any of the three legs. If I just leave one leg disconnected can I still get the maximum 104 amps output, or do I have to reduce the amount it puts out? Thanks, J.B. Castagnos

[Harry]

To my knowledge, it comes down to current. That is the current through any part of the circuit. If a winding is made of #18 wire for example, the current flow multiplies with the internal voltage drop within the winding to create heat. If the heat is only coming from one winding rather than all three, that helps, but there are many factors to consider.

If your rating is 104 amps, surely they made a statement about total kilovolt amps (KVA) output. That would be the rating to adhere to. Power factor also comes into consideration if you are running motors.

Hope I don't get bombed. I'm sure as hell no expert.

J.B. Your 3 phase generator will do fine on 1 phase output provided you only pull 66% of the 3 phase rating since you are only using two of the 3 phases. You cannot pull the rated 3 phase current output. Also, if the voltage regulator sensing circuit is only across two of the phases, you want to use those two phases for your output. If the genertor is producing 120/240v, I think you might be connected in a 3 phase delta arrangement. If so, one of those three phases to neutral will read 200v and not 120v. This is the wild leg. Be carefull. Since you have at least 20kw of capacity you will have more than enough power for a very large home. I get by nicely with 8kw.

The rated nameplate output capacity is assumed to be based on the capacity of all the power producing windings in the alternator contributing to the overall rating. When some of the windings are not doing their job, the resulatant rating obviously has to be less.

Another thing to consider might be the waveform of the output with an unbalanced loading of the windings. It probaly won't have much effect with light loadings, but might with heavier unequal loadings of the windings. I am thinking of an analogy of an engine where two cylinders fire one right after the other and the engine coast for a couple revolutions before it fires in the same sequence again. If your engine/alternator was built by John Deere, then maybe this would be normal

Many of these large generators can be re jumped (change the hookup in the junction box) to produce single phase. I don't know how, but I think the manual should tell, or you might try the good people at http://www.dynagen.com/dynagen/