Sorry Charlie, only the best get to be StarKist.
The motor and overload are in series and see
exactly the same current.
BTW, at least 95% of all 3 phase motors are protected
by an overload relay that is art of the magnetic motor
starter used to control the motor.
Not necessarily true either. I didn't look in detail at theat switch's
specifications, but I believe it had current-sensing heaters in it
matched to the motor size, similar to what a magnetic starter with overload
There is always an easy solution to every human problem -- neat,
plausible, and wrong." (H L Mencken)
On 4/14/2014 10:31 PM, firstname.lastname@example.org wrote:
It has nothing whatever really to do with predicting the motor
temperature, per se, it's simply overcurrent motor protection (as
opposed to overcurrent _circuit_ protection that is the function of the
breaker/fuse). Sizing is based on FLA and motor rating of how long is
allowed before protection kicks in.
The devices have colloquially been called "heaters" at least in the US
(I don't know about the frozen north jargon :) ) as they're just thermal
links. Some are resettable bimetal strips (the builtin motor variety
for example) whereas typical magnetic starter heaters are fusible links.
That is, yes, the motor ratings are generally based on what the internal
insulation and all will stand as temperature overloads but that is
protected against by the current draw and manufacturers' ratings with
the presumption of proper installation, cooling, maintenance including
removal/prevention of dust buildup, etc., etc, etc., ... so that by
limiting the current draw within the expected time the internal
temperatures will not reach danger levels.
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