Harvesting energy from 3-phase

[Massoud]

upsidedown@downunder.com wrote in
news51qkehqpl3uq403fkk6oiqkjip008liei@4ax.com:

On Thu, 8 Aug 2019 21:15:03 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, August 8, 2019 at 1:55:57 AM UTC-7, Clive Arthur wrote:
A customer has a three-phase supply drawing from 50A to 100A at
nominally 60Hz. He wants to tap some power from this without any
electrical contact, so probably using say three current transformers
with each phase going through one toroid.

[intended for 10W of output power]

Why? You can (six diodes) rectify the three-phase, and filter it,
and make a switchmode isolated power supply, where does the
'electrical contact' become a problem?

A 'current transformer' gives very low voltages, into a short
circuit, it's a terrible choice of power-delivery component.
It's useful for METERING, where power transfer
(and impedance matching) is not required.

There are current transformers from 1.5 VA to 60 VA and standard
secondary currents of 1 A and 5 A. Thus the 1.5 VA version could
deliver 1.5 V resp. 300 mV. The 60 VA models could deliver 60 V resp.
12 V.

The high power and hence high voltage is useful if the distance
between the current transformer to the actual measurement device is
large due to the voltage losses due to the wiring.

Anyway sturdy wires must be used to handle any short current peaks due
to short circuit in the high power line. Long wires are typically
encountered when th CT is outside and the instrument is indoors.

Current transformer's secondary should never be open. Zs=N^2*Zp so if Zs
is infinite, current line becomes hi impedance and destroys the
transformer. Consider off mode of you gadget.