RE: Three phase conversion In Rush current Concern.

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 03:22 PM 9/29/2006, you wrote:
> Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>
>
> Hi Jim,
>
>
> Depending on your budget, a 3 phase variable speed drive (there is a 50
> HP unit on e-bay right now for $330 for example) would not only allow
> you to control start up, but it would create true 3 phase from single
> phase. Most of the drives out there use a static rectifier (diode, not
> SCR or transistor) and will run on single phase provided that you
> de-rate them accordingly (50% is typical).

They'll run a motor (and relatively smooth running too), but the 
filter capacitance usually isn't enough to hold the dc bus voltage up 
between half cycles.


> Otherwise, a pair of TRIACs slowly ramped up would be effective as a
> soft start.




> Simpler, but more expensive would be a tapped autotransformer or
> reactor, or even a resistor bank, stepped out with contactors on a time
> delay or current basis. All of this was the norm for starting large
> motors 30+ years ago, and is still in use today in many cases.

How many steps do you need.. One resistor (or light bulb) in series 
with a big relay might be enough.


> You may find starting a bit troublesome (although you won't require much
> starting torque) with reduced voltage techniques however, given that you
> are faking your 3 phase with capacitors.