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From: ESV@psuvm.psu.edu (ANDREW COLL esv@psuvm.psu.edu)
Newsgroups: sci.electronics
Subject: Ac motor control question
Message-ID: <90256.174441ESV@psuvm.psu.edu>
Date: 13 Sep 90 21:44:41 GMT
Organization: Penn State University
Lines: 32

Greetings everyone...

I just moved into a new house and had to replace a broken pump.
I'm having a small problem controlling the motor.  The pump
is actually a wastewater pump for a basement sink, it pumps water
from both basement sinks of a duplex house, 4 feet up into the stack.
There is a check valve on the pump outlet to prevent wastewater from
the stack from entering the pump and so on.  My problem is this:
The pump is supposed to come on automatically, via a float switch on
the inlet pipe, one foot away from the pump.  When water flows into
the pipe, the float switch turns on and starts the pump.  The pump then
sucks all of the water out of the drain line, causing the float
switch to turn off again.  About a half a second later, enough water
has flowed into the pipe again to turn the pump back on.  So now, the
pump is oscillating between on and off, about fifteen times a minute.
This can't be terribly good for the lifetime of the pump and no amount
of adjusting float switch will correct the problem.  Moving the switch
farther away from the pump is impractical because the plumbing in this
house is at least 40+ years old and its far more likely that I'll break
something if I try to modify the drain system.
My only idea so far was to use the float switch to trigger a 555 timer
set up as a retriggerable one-shot with a variable duration between
30 seconds and 5 minutes (whatever I find works best).  I would probably
operate the motor with relay driven by a power transistor.  My questions
are:  1. Does this sound reasonable   and  2. Should I use the relay
or opt for some kind of SCR based circuit to operate the motor   and
3. Does anyone have a better idea?  :-)

Many thanks....

Andrew Coll       ESV@PSUVM.BITNET
                  ESV@PSUVM.PSU.EDU