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.

So, I choose to not have a contactor but instead, this, which
provides the power-fail protection:

http://www.amazon.com/Woodstock-D4160-110-Volt-Paddle-
Switch/dp/B005W17HYY/ref=pd_sim_hi_1/191-4569239-2528519

This has a relay (rated 1/2 hp) that drops out on power fail, and has
external connections for additional e-stop mushroom switch. But no
provision for motor protection.

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

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").


Look at that switch again - it's a 110v switch, not a 220v switch. Hate to
see you order the wrong one. You could fuse the circuit behind the switch
but you'd have no way to ensure that both legs will drop out at the same
time the way that a breaker does. You would not want to find yourself in a
situation where one leg drops out but the other does not.

In the US, using the NEC, fuses are allowed. And a "controller" only has
to open one of the supply wires. (You are right about the 110V rating on
the amazon switch.) A "disconnect" has to open all the live supply
conductors.


What is the application for this motor that requires so much more precision
than the circuit protection that a breaker would provide?


Application is important. But a breaker provides little overload
protection for a motor. For a hard-wired motor circuit (like central air
conditioning) the breaker does not usually provide overload protection
(that is provided at the motor). The breaker can be significantly larger
than the rating of the wires, and provides short circuit protection. A
breaker is large so it does not trip on the motor starting current.