"Spehro Pefhany " Spehro Pefhany wrote
in message ...
On Fri, 06 Feb 2004 18:58:18 GMT, the renowned "Toolbert"
wrote:


If you or a friend can get into some experimenting, you can build
hysteresis
into a DPDT relay by using one set of contacts and some power resistors
wired to add a "boost" to the coil voltage once the relay pulls in.
Resistance and power rating would depend on the particular coil and
current.
Seems like something that ought to work but that is only ni theory.
Bob

I have to make a 10HP rotary converter.. what do you suggest for the
capacitors and what are the potential relay on/off voltages? I might
whip up something electronic to switch a regular contactor if someone
can tell me what it should do.

The contact opens when the coil voltage passes ~ 170 volts and closes when
it drops below 90 volts. In a rotary phase converter that is connected
across the 3rd leg to neutral not phase-to-phase. Keep in mind the 3 phase
neutral is offset from the single phase neutral. 3rd leg to single phase
neutral is typically 200 volts.

I don't know if the 170 / 90 is optimal. With an undersized idler the 3rd
leg will drop low enough to kick in the relay when starting a large motor.
That isn't good for the start capacitors but it does give a useful boost to
the load - engaging the start capacitors to get the load motor up to speed
faster. If you build something electronic, probably better to build it as a
one-shot that can't kick in again until the entire unit is power cycled.

Capacitors just use the r.c.m. guidelines ... 15 uF per hp per phase run, 70
uF per hp start I think. I've built or collaborated on four 7.5 hp
converters with good results using 92 uF run capacitors (one each side) and
500 uF start capacitors (two 250 uF in parallel).

Bob