Searching for an unusual relay

[Ross Herbert]

:On Sun, 15 Mar 2009 23:54:15 GMT, Ross Herbert <[email protected]>
:wrote:
:
:>On Sun, 15 Mar 2009 07:15:27 -0500, John Fields
<[email protected]>
:>wrote:
:>
:>:On Sun, 15 Mar 2009 07:28:51 GMT, Ross Herbert <[email protected]>
:>:wrote:
:>:
:>:
:>:>Without modification to the motor itself there can be no means by which
:>control
:>:>of an external device or protection method is possible. The only way I
cansee
:>:>out of your dilemma is for the external motor protector to have priority
over
:>:>the internal temperature overload function of the motor.
:>:
:>:---
:>:View in Courier:
:>:
:>:Like this?
:>:
:>:.AC>---+------+
:>:. | O COM
:>:. | |- - - - - - -[COIL]K2
:>:. S1| O NO | <-O NO | |
:>:. | O START | | |
:>:. | | | |
:>:. +----------+ | |
:>:. | T1 | |
:>:. +---P||S---+ |
:>:. R||E |
:>:. +---I||C------+
:>: |
:>:. +----|----+
:>:. | | |
:>:. | O|S2 |
:>:. | O| |
:>:. | | |
:>:. | [MOTOR] |
:>:. | | |
:>:. +----|----+
:>:. |
:>:.AC>--------------+
:>:
:>:If the thermal cutout switch, S2, in the motor assembly is closed, then
:>ressing S1 (a normally-open momentary switch) will connect the motor to
:>:the mains through the primary of T1, a current transformer. When that
:>:happens the motor will start to turn and a voltage will be induced in
:>:the secondary of the transformer which will be placed across the coil of
:>:K2, a normally-open relay with an AC coil.
:>:
:>:When the contacts of K2 close they will be in parallel with the contacts
:>f S1 and will provide a redundant connection of the motor and the
:>:transformer to the mains.
:>:
:>:Therefore, when S1 is released, the connection to the mains will be
:>:maintained by the closed K2 contacts.
:>:
:>:However, should S2 open because of motor overheating, (or any other
:>:reason) the current in the coil of K2 will be interrupted, the relay
:>:contacts will open, and the connection to the mains for the motor and
:>:K2's coil will be broken.
:>:
:>:Once that happens, the connection to the mains will remain broken even
:>:if S2 closes once the motor cools off, pressing S1 being the only way to
:>:begin the cycle anew.
:>:
:>:
:>:JF
:>
:>
:>Yes John, that's exactly the way I see it. The motor protection current limit
is
:>dialled up on the external motor starter/protector so that it is below that
:>required to heat the motor up to the internal temp setting for S1 to trip.
:
:---
:There's no need for any of that since once the motor's internal thermal
:switch (S2) opens up the motor will be disconnected from the mains, and
:will remain disconnected, until S2 closes _and_ S1 is momentarily
:closed.
:
:JF

Your idea is most likely a good solution. However, is such a device available
off the shelf? I think not. I doubt the OP wants to muck around building a
device which may or may not be approved by his employer/client no matter how
well it works.

Using a readily available off the shelf, motor contactor/protector, will provide
an approved method of allowing manual motor restart after an overload. As an
example; most submersible pump motors will have thermal overload protection
inbuilt similar to the OP's motor and a motor contactor/protector works fine in
that situation.

http://www.cessco.us/files/pumps-electric-submersible-0108-brochure.pdf

Note that the more sophisticated controllers have adjustable current setting as
a feature.