Three phase power questions

I think I came across a great deal, IF I can make it work properly.

Picked up an Onan model DKD Genset, 7.5 KW, 3 Phase 120/208 60 hz. 13 hours since new and was told to basically get it out of the company's warehouse. It's been there 10 years and the bean counter sez "get the damn thing outta here if you have no purpose for it".

So it's now mine and charged off the books.

I intend to use it to provide backup power in my off grid home and guitar shop running a CNC router and dust collection system. The data plate sez...

110 vac, single phase output .... 26 amps
120/208 vac, 3 phase output ..... A phase - 23 amps, B phase - 25 amps, C phase - 23 amps.

Looks like I can run any single phase at 26 amps max load right? Or...

I can run each phase up to the rated load (or therabouts!) Right?

As an example of what I am thinking of doing:

Connect A Phase to the Trace inverter for battery charging the bank.

Connect B Phase to the CNC.

Connect C Phase to a dust collection system.

(alternatively B and C to run a 220 volt well pump. Both C and C options are mutually exclusive as both options NEVER occur at the same time. (does 220 allow for a 120 degree phase shift??)

My rationale for all this is, a three phase generator runs well under peak efficiency when the loads on each phase are wildly imbalanced, right.

Thoughts, suggestions, criticisms of my understanding or "power" to say that "power" and "electronics" that I used to do.. are VERY different animals.

Am I missing something or, otherwise, dangerously misinformed?

Thanx in advance!!

Twombo

Comments

  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Three phase power questions

    Twombo,

    The generator sounds interesting, but I doubt that it will function as you’ve suggested. Assuming a continuous rating of 7.5 kW, it should be able to supply 7,500 W / 120 VAC = ~62.5 A total across all three legs, and the maximum per leg would be as rated. But, if you were to pull 23 A from phase A, and 25 A from phase B, I would think that you’d be limited to 14.5 A from Phase C.

    23 A + 25 A + 14.5 A = 62.5 A.

    And, don’t forget power factor. Non-resistive AC loads typically draw higher VA than they do W, so loads may require more current than it would appear.

    120 VAC three-phase supplies 208 VAC between each leg pair. You’ll need to check with your well pump’s manufacturer to see if it can run on 208 instead of 220.

    HTH,
    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: Three phase power questions

    Crewzer

    That is a good start for me! So, in theory, I can split things out this way. Each phase to neutral can feed a load (or collection of loads) within certain limitations.

    I am most interested in coming up with a loading that ensures the genny has a roughly balanced (recommended is within 10%) loading phase to phase and about 50 percent or better total load. I'll have to survey the target loads with my Kill-a-Watt. I think I can compensate for power factor with to make sure all my measurements are "apples vs apples".

    As it stands right now, the rough numbers look like about 15 amps a phase tops!

    The Onan genny really is only a dedicated source for my shop, as the rest of my home is covered by solar panels and a bit of makeup as needed from a little homebrew single cylinder kubota diesel turning a 160 amp large frame truck alternator.

    As to the well, I hope to have a solar pumping system in soon so that becomes a moot point.

    thanks

    twombo