Ignoramus3531 wrote:
On 2007-10-04, gwes wrote:
Here's a money-grubbing question from a hard-case tinkerer:

Leadin:
In order to run a 3-phase motor drive from single phase input,
the user must derate the unit by 50%. This is because the input
rectifier and filter capacitors are too small to handle the increased
peak current of a single phase feed.

Usually, the derating factor is 0.66.
Not according to several manufacturers' web sites. They say 50%.
Work out the peak currents in the rectifiers and the capacitors.


There's a "simple" way around this: feed the input with DC (or feed
the internal DC bus).

You cannot feed a three phase rectifier with DC and utilize all three
inputs. So you have to feed the bus (as you say), which is easy only
if DC terminals are available.

Furthermore, it's often advisable to isolate the input of a motor
drive with a transformer. This gets large, heavy, and if you have to
buy it new, expensive.

Very expensive. But the need for it rare.
Depends - for transformer, read filter reactors. Again the
highly non-sinusoidal input waveform causes problems when you're
drawing 20A because the harmonic currents (like 180Hz and 300Hz)
are pretty high and the rest of your appliances may not like them.

The question:
If I sold a relatively inexpensive fully-isolated power-factor-corrected,
low-harmonic-distortion input multi-kilowatt 310-335V power supply for
220-240V motor drives, would anybody be interested. I'd price it so that
the solution would be cheaper than buying an over-rated drive and a
reasonably priced used transformer.

I would suspect that people needing isolation, would have three phase
available.

Perhaps. Isolation is a good thing - otherwise you can get very
substantial ground currents from the transformer frame even on a
lowly 1 or 2 HP motor. Doesn't do the bearings any good, for instance.
This is true with a 3-phase feed as well.
It -will- work without isolation. It works better and longer with it.


geoff