mick wrote:
On Wed, 31 Mar 2010 13:24:28 -0800, Robert Baer wrote:

I have this circuit that will "trip" a PIC (as a controller), but it had
no delay so i added a resistor.
SPICE circuit is attached.
The added resistor seems to give abou a 2 second delay to trip from
initial input drive.
But i know nothing about motors except that they work and have an
initial surge current which probably depends on load.
The idea is to shut down a machine by turning off the motor and other
things if the motor has an excess load (gears seize, etc).
I do not know if that excess should be set to more or less that the
surge current; that is to say what is a "safe" method / setting?

snip


Personally I wouldn't be tempted to use a PIC for this, unless it's just
a programming exercise?

Motors are usually installed with a thermal or electronic overcurrent
device mounted on the contactor. There is nothing to stop you taking a
"motor tripped" signal from the contact on this, of course. The
overcurrent relays are adjustable over a limited range so that they can
be trimmed to suit the particular motor & application. They are also
available with different tripping curves, allowing fans, for example, to
have a longer start time than, say, pumps (because of the inertia of the
blades).

As a rule of thumb, allow 5-7 times full-load current for a 3-phase motor
and about 8-9 times flc for a single phase as normal starting current
(direct-on-line starting, ordinary induction motor). The PIC should
adjust it's trip delay depending on the overcurrent detected. As the
current increases the delay should reduce, until there is a very short
trip to deal with "locked rotor" situations (not instantaneous as this
will happen at the beginning of the start curve anyway).

Nowadays many motors use electronic soft-start or variable speed drive
units, incorporating all the protection (and sometimes a contactor) in
the same unit. These build a software model of the motor and compare it
with the real one, giving far better protection than anything else.

Not often I see a post that fits in with my (previous) job... ;-)


The motors that will be used have / do not come with external sensors
or contactors, hence this sensor "trick" i came up with.
The PIC is also being used for a number of equipment related things
(a sample): forward / reverse; run / jog; tank lo level, high level,
extreme level detection; if too cold, a heater goes on for a settable
predeterimed time before tank transfer pump (one of the motors) is run;
transfer motor timeout (as it is not rated for continuous duty so want
to protect it from burning out).
Adjustable trip delay is something not known / thought of; a
different kettle of fish.