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Boost Electrical Engineering 3 phase converter, 1993



Jan 1, 1970
Converting single phase to 3 phase to run a lathe from domestic supply.
Hopefully just replacing the 415V 20A contact ratings 4 pole relay but I'd
like to know a bit about how it works , in general terms, as both coil and
power useage contacts are u/s.

2 pairs of contacts paralleled to bring up to 40 amps rating. But I assume
over time with bias to one set closing before the other one pair failed and
then the second pair failed soon after but why the o/c coil , would there be
a thermal fuse buried in the housing somewhere, lots of heat was developed
from the bad contacts

I assume the converter is peincipally an autotransformer to create the other
phase in conjunction with some capacitance. There is a boost function that
is manual biased switch at start of motor or automatic if the lathe cuts
through a hard spot, this brings in more capacitance via the relay it seems.
There is also a reed relay with a few turns of the power amperage passing
around it , presumably the auto function ,but the mystery is a small TO3
size bridge rectifier and electrolytic for a 100V or so DC supply for an
offset or something


Jan 1, 1970
this textually sounds the sort of thing

"FIG. 1 is a 3-winding induction motor supplied from a single-phase power
supply with a phase control consisting of a capacitor connected to a tap on
an autotransformer."


"The current in line L2 goes through a coil around a reed-relay so that the
reed-relay contacts close when the current exceeds several times full load
current, and the contacts open when the current is less than twice full load
current. The coil is selected so that the contacts open when the motor shaft
speed is greater than 80% of rated speed. The reed-relay contacts control a
solid-state-switch which inserts the starting capacitor CS between terminals
TC and T4. "

not Solid State Seitch in this one


Jan 1, 1970
Sort of working order but will not drop out of boost mode. The voltage over
the relay drops but not enough , it would seem , to make this replacement
relay and so different coil demand, drop out. Will have to do some on site
fudge adjustment to the original 6K8 dropper for this relay and check that
strange bridge rectifier and offset DC function as no ac or dc without a
load motor