On Thu, 12 Mar 2009 14:42:04 GMT, Robert wrote:
:Ross Herbert wrote in :news: snipped-for-privacy@4ax.com: : :>:Most of the overload relays I looked at are thermal. When heated a :>:set of contacts open. This works fine for shutting down when aced :>:with a sustained overload. I needed contacts that opened when the :>:current, or heating, fell too low. Compared to the usual power power :>:levels for such devices, mine are quite low. I deeded to switch at :>:less than an amp. :>: :>:Fortunately, those in control have backed away from 80 C ambient. I :>:am back into a temperatur range that can be met with commonly :>:available items. An electronic current sensor, a relay to handle the :>:load, and a momentary switch to get things started. :>: :>:Having explained the application many times, I may not have done so :>:here. Unfocused conversations and corispondance eats up a lot of :>:time. A small motor with internal, automatically resetting, thermal :>:protection has been used for a long time. The hazard of having it :>:restart is now understood. A field installable way of requiring :>:operator intervention for a restart is needed. When a requirement of :>:80 C ambient was added, an electromechanical device came to mind. :>:While they may have been common at one time, that time has past. :>: :>:I offer my thanks to those who offered thoughts and information on :>:this subject. While ranking the responses is inappropriate, the term :>:amperometric does stand out. :>: :>:A good day to all, :>:Robert H. :> :> :> It sounds to me as if you are looking for a standard motor overload :> protector which has both overcurrentand thermal overload tripping. I :> am certain that these devices do not automatically reset once tripped :> - you have to manually press a button to restart. :> eg. :>
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:> : :The motor being used is not being changed or modified. It is a small AC :motor. It has internal over temp that automatically resets. It is the :automatic restarting that is the problem. A way of externally preventing :the restart is needed. : :The item you pointed out looks like it trips with overloads and over :current. I need to trip in response to the motor shutting down from the :internal protection opening up. : :Fortunately those who imposed the 80 C ambient rating have yeilded. :Specification by a comitee that has only a single member familar with the :technology can be a challenge. : :Thanks, :Robert H. :
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.
Since the motor temperature is a function of mechanical load and the current being drawn by the motor, you can use a motor protector such as the one I mentioned and set the overload current (yes, this value is adjustable) to some value below that at which causes the motor to exceed its overload temperature. That way the external protector will have priority and will trip before the motor temperature is reached and then you have manual control via the external protector.