Bruce in Bangkok wrote in
:

Larry,

I am in the process of replacing the anchor windlass power relay. It
had a "starter relay" installed and after 5 years I am replacing it
with a similar relay but am wondering whether putting a diode across
the contacts would lengthen the life of the relay (the current one
apparently arced so bad that the contacts would weld and I'd have to
frantically dive down below to flip the circuit breaker before the
anchor hit the stops).

If a diode would help approximately what voltage and current rating
diode would one use -- 12VDC motor rated 3/4 HP. ?


Bruce-in-Bangkok
(correct Address is bpaige125atgmaildotcom)


No. Think back to old gas engines with points and coils (your motor is
a coil). To keep the points from arcing and burning up, we put a
CAPACITOR across the points to absorb the collapsing field pulse trying
to keep the points conducting...making a big arc and welding their
surfaces to the temperature of the sun.

So, let's put a CAPACITOR across these contactor contacts to eliminate
the arcing when they open. I suggest using a non-polarized 600VDC, 1UF
"orange drop" (the plastic coating is colored orange). Any
manufacturer's cap at this, or higher, voltage range and from .5uf to
10uf would be just fine as we don't care about points and ignition
timing on the windlass. This cap does NOT GO TO GROUND! It goes from
the battery contact of the contactor to the motor contact ACROSS the
points it's protecting. The car points cap went to ground because one
of the contacts was ground.

Now, when the relay deenergizes, its points start to open. The running
motor creates an inductive kick from its collapsing fields, just like
the spark coil did, and tries to continue the current. The voltage
across the points, without the cap, increased a LOT to several hundred
volts and welded the opening surface of the contacts. But, alas, this
motor-generated kick will BYPASS the points because the cap will charge
to battery voltage in a few microseconds holding the current up, then
dropping the current slower (not much microseconds) while the points get
so far apart they can't arc...only a fraction of a mm at this voltage
level. The points won't arc at all if the cap is big enough. Problem
solved.

Now, about that DIODE! You CAN use that diode you wanted, too! The
motor is a coil, just like a relay coil. When we cut current to a coil,
the coil "kicks" from the collapsing field. IF we put a diode ACROSS
the coil so it is REVERSE biased when the coil is energized (cathode
ring to + and anode (no ring) to -), nothing happens when the motor is
running...reverse biased diodes are an open circuit. When the contactor
opens, this motor becomes a temporary generator from the collapsing
field with the OPPOSITE polarity across it. The diode will conduct for
a few milliseconds as the field collapses and even provide a little
dynamic breaking to bring the motor to a halt a little faster. This is
called a "free wheeling diode" and you'll find them across all relay
coils that use electronics as controllers to protect the controller's
output transistors from the inductive kick of the relay coil collapsing.
You could do that. Make the diode a 10A 50V PIV (Peak inverse voltage)
so we don't blow it easily. (A 10A diode will handle 100A for 50ms,
typically). 6A diodes are easy to come by, that would be a good one,
too. This isn't really necessary with the cap but if you must...(c;

As this contactor is pretty sealed up, I'm not worried too much about
corrosion, which arcing of an OPEN relay with EXPOSED contacts CLEANS
the contacts every time it arcs. If this were an open relay, I'd want a
little arcing, which will happen anyways even with the cap, like the old
points in your old motor did to clean its points.

This concludes this chapter of Boatyard Electrical Engineering. Read
pages 241 through 292 for next session. Class........DISMISSED! (Ring
bell with cap across bell points to absorb shock and arcing.)