On 2014-01-26, bud-- wrote:
On 1/25/2014 7:10 AM, Mike Marlow wrote:
DaveC wrote:
Looking to provide switching for 1-phase, 220 AC, 1/2 hp motor that
doesn't include use of a contactor. I appreciate the need for precise
current limit (LRA) that a branch breaker can't provide.

[ ... ]

Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses be
sufficient to protect this motor?

To the 1/4 A out of what rating? 1/4 A out of a 1/4 A fuse is
a pretty big error. 1/4 A out of a 50 A fuse is a lot less.

In other words, what kind of percentage? And the curves for
fuses don't say *when* it will blow, they simply say that it must
tolerate full load amps for N hours, and for each increment above that,
they have to blow within a certain time. They can be quicker, but not
slower.

How are you going to find fuses that precisely matched? You
can't really *know* that they are that precisely matched until you have
tested them -- by which time they are now blown, and thus useless,
except perhaps as a poor insulator. :-)

May or may not, depending on the fuse characteristics. The fuse has to
be slow-blow enough to not open when the motor starts (starting current
is about 6x running current). But it has to open within the i-squared-t
rating of the motor (as in Jim's post).

The "overloads" in motor starters are matched to motor starting. And the
settings in are much finer steps (or are continuous) compared to fuses.

Many 1/2HP motors have internal overload protection ("thermally protected").

Some of those are thermal fuses (cooked off by the heat in the
windings) buried in the windings. When that goes, you can't access it
to replace it -- except by re-winding the motor. They don't re-set on
their own.

Enjoy,
DoN.

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