Running 3 phase 780s on single phase power (was RE: hams on classiccmp)

[Roger Holmes]

>
>> Ian King (IanK at vulcan.com) wrote:
>> We *are* running the machine off three separate 110V single-phase  
>> circuits
> -
>> apparently you missed that part.
>
>  People need to understand the danger in this - in any multiphase  
> power
> system, the neutral current is the DIFFERENCE of the current in the
> individual phases.  If the load is perfectly balanced, then the  
> neutral
> current will be zero.
>
>  But, if you run the same load from three separate single phase  
> circuits
> and (worst case) the three circuits are all the same phase, then the  
> neutral
> current will be the SUM of all three phase currents.  That's 3 times  
> what it
> would be otherwise and runs a serious danger of melting the neutral  
> and
> starting a fire.

I agree with all but the last bit, because if he's running it from  
three separate supplies, surely he will have three neutral conductors,  
so no problem.

Roger Holmes
(Who has a 440v three phase computer drawing 13kVA which has three  
phase bridge rectifiers to generate fairly ripple free DC)