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RE: Three phase conversion In Rush current Concern.



Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>

I can't say that I have ever noticed any appreciable ripple, even on a
low inertia load, however I haven't gone to measure it per se either.
The premise is that by de-rating, the filter inductance and capacitance
are sufficiently large to keep ripple down on the DC link, and its
inherent modulation effect on the output.

Bear in mind that these drives are designed for 200% over current for 3
seconds, and 150% for 30 seconds. So if we are only drawing 1/2 the
current that the filter is designed for, there is a lot of time for the
rectifier to re-charge that filter capacitance. That being said, our
ripple fundamental frequency is 1/3 the design frequency.

Nonetheless you've gone and done it now Jim! I'm going to have to scope
Vdc, and Vout on my drives this weekend.

Cheers,

Leigh

-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: September 30, 2006 8:10 PM
To: tesla@xxxxxxxxxx
Subject: RE: Three phase conversion In Rush current Concern.

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 11:26 AM 9/30/2006, Tesla list wrote:
>Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>
>
>
>         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.
>
>
>
>         <LC>Very true, but as long as you de-rate the drive by/to
> 50% the DC link filter components are large enough. I have 8 of
> these things from 2 HP to 50 HP running in my own shop; most of
> them being over 5 HP. I live several kM away from 3 phase, so I
> step up the 240V single phase to 575 VAC single phase, and power
> the drives from that. My lathe is 25 HP spindle + roughly 5 HP of
> hydraulics, feed motors and coolant pump, and it runs from a single
> 50 HP Allen Bradley Powerflex 700VC. Allen Bradley actually stands
> behind the product as long as it is de-rated by 50%, and you follow
> their connection requirements (internal cooling fans are power off
> of a particular pair of line input terminals, so it matters which
> phase you leave unconnected).


Fine for running a motor, which can tolerate the 120Hz ripple voltage
on the three phase coming out of the inverter, especially if it's
running a high inertia load.

However, if you run that three phase into, say, a 6 pulse rectifier,
and expect to get the 8% ripple unfiltered that you'd expect, it
won't work.  Likewise if you're running a low inertia load (like a
fan) where the torque ripple manifests itself as a noticable hum.