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Bruce L. Bergman
 
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Default Wiring my Marathon motor

On Thu, 9 Mar 2006 09:04:39 -0500, "Proctologically Violated©®"
wrote:

Good info, but here's a Q:
Isn't a contactor w/ OLP just a fused switch, w/ mebbe remote control?
In principle, couldn't a simple fused knife switch suffice?


NO! Do it right the first time and save yourself a whole lot of
grief. A proper motor starter is critical for all three-phase motor
applications, and a smart thing to do for any single phase motors over
roughly 3 to 5 HP. A motor starter is much cheaper than the motor
going up in smoke repeatedly.

The contactor half of a motor starter has a magnetic coil for fast
action, so the contacts spring open and closed fast. They have angled
alloy contact points (usually coin silver) that are designed to be
operated millions of times under load, and self-clean by a wiping
action when they close. They have magnetic arc chutes arranged that
any arcing on opening is directed out and away from the contacts to
quench quickly. The contacts do get cruddy from wear but still work
by design (self cleaning, remember?) but when they wear out and fail
to work they are easily replaceable.

Fused switches are simple copper alloy contact points that operate
relatively slowly, and Are Not Designed To Be Operated Repeatedly
Under Load. They may have some sort of arc chute arrangement, but
it's more to keep the switch from self-destructing if opened under
load accidentally or in an emergency. Burn the contacts up, and you
have to replace the whole thing.

The Overload Protection monitors the starting and running current on
all three phases, which is critical if one of the contact sets goes
bad or you blow a fuse on one phase.

A simple knife switch doesn't know or care about a single-phasing
problem. If the motor blows before the other two fuses, oh well...
If the motor is seriously overloaded or in a locked rotor condition,
the fuses will react much more slowly than the Starter Overload -
again, the motor may pop before the fuses do.

And a simple knife switch won't drop out if there's a power failure
as a safety feature like a properly wired Motor Starter does, so when
the power is restored the equipment will start right back up again -
doesn't matter that you have your arm in there trying to figure out
why it stopped... We'll call you "Lefty". If your head was in there,
we'll call you 'decedent'.

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
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