Searching for an unusual relay

John Fields wrote in news: snipped-for-privacy@4ax.com:

The schematic above is functionally what I ended up doing. The idea of using a transformer to drive the relay had not crossed my mind. It does provide a low impedance that can be placed in series with the motor.

Fortunately the 80 C ambient rating on the components to be used was droped. A prototype was delivered this afternoon. Digikey has a current sensor in stock with an AC switch output.

Have a good day, Robert

On Mon, 16 Mar 2009 00:44:33 -0500, John Fields wrote:

:On Sun, 15 Mar 2009 23:54:15 GMT, Ross Herbert :wrote: : :>On Sun, 15 Mar 2009 07:15:27 -0500, John Fields

:>wrote: :>

:>:On Sun, 15 Mar 2009 07:28:51 GMT, Ross Herbert :>: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 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 :>:pressing 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 :>:of 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.

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