Re: 3-phase power at home

awright wrote:

> Lots of ideas here, but I think ken o's comment, "I also omitted that
> I need 550 volts to tun this baby," slipped under everybody's radar.
>
>
> That's a pretty big hooker unless he has 440 VAC service or a step-up
> transformer capable of transforming the full power demanded by the
> motor plus reserve. In fact, while I strongly disagree with the
> comment by another poster that 3-phase conversion at home is not
> practical, doing it for a 550 volt motor may well be. ken o will
> either have to provide a single-phase transformer to transform his
> input power to the converter up so the DC bus in his VFD is high
> enough to provide 550 volts AC out, or he will have to provide a
> 3-phase transformer between the converter and the motor.
>
> The former is reasonably practical, depending upon power requirements.
> In fact, the manufacturer of the VFD I installed in my garage about
> 25 years ago mandated that the input be supplied via a 5 KW isolation
> transformer. Such a transformer can as easily be a step-up
> transformer as the unity-ratio transformer that I installed.
>
> The latter is very risky and/or expensive for a several reasons. (1)
> You need a 3-phase transformer. (2) You are subjecting the
> transformer to the high rise-time switching transients that can lead
> to insulation breakdown unless its insulation is specifically
> designed to withstand them. (3) You CAN NOT use a transformer on the
> output of a VFD that could possibly be set to lower frequency than the
> transformer is designed for. The transformer would saturate, leading
> to very large currents damaging either (or both) the VFD or the
> transformer.
>
> I have been using a solid-state VFD to drive my 3-phase geared-head
> drilling machine, bench grinder, and Bridgeport milling machine with
> complete satisfaction for more than two decades. Before I installed
> the VFD, I changed the motor on my first milling machine (a Rockwell)
> to single-phase. It worked fine, but changing the motor on the
> Bridgeport was not an option, and the variable speed of the VFD is
> extremely valuable, especially for drilling large holes with the
> drilling machine. No more burned drills!
>
> Ignoring the high voltage ken o requires, I strongly recommend
> investing in a VFD instead of buying or building a rotary converter,
> primarily to get the variable speed capability. I think that careful
> shopping and not going for bells and whistles like an LCD readout or
> programmability will get you a VFD good for up to about a 1 HP motor
> for well below US$ 100.
>
> The threat of a VFD to motors not designed for "Inverter Duty," is
> from the fast transients from the IGBTs or MOSFETs. A couple of
> things can happen. One is insulation punch-through. An equally
> damaging, but less obvious phenomenon is capacitively-coupled current
> spikes through the ball bearings leading to erosion of the balls and
> races and ultimate failure. This is not an academic issue to me, as
> the electronically controlled motor in my new, high-efficiency
> clothes washing machine failed in less than a year. A post-mortem
> clearly revealed severe bearing erosion. In replacing the motor, I
> lifted the motor ground to interrupt the path for
> capacitively-coupled transients to ground via the motor bearings. No
> subsequent failures for 5 years. (I know that this is a serious
> violation of safety codes, but the motor is fully enclosed and I hung
> BIG warning sign on it explaining exactly what had been done. )
>
> awright
>
In that case you had a crappy VFD + motor implementation. And the VFD in
the washer is a piece of crap. do not buy that brand for years and scream
at the manufacturer, the press , and all your friends, associates,
neighbors, and acquaintances. Selling short lived pieces of crap must be
punished.
--
JosephKK

.

Relevant Pages

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  • 3-phase power at home
    ... I need 550 volts to tun this baby," slipped under everybody's radar. ... transformer capable of transforming the full power demanded by the ... doing it for a 550 volt motor may well be. ... the manufacturer of the VFD I installed in my garage about ...
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