Preventing AC motor restart.

Hello All,
An AC motor with internal thermal protection shuts down and then restarts when the motor cools down. This automatic restart presents a hazard.
I am on a scavenger hunt for an off the shelf item that can be wired into power feed to the motor that would prevent the restart without operator intervention. Does anyone have any suggestions?
Thanks, Robert H.
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On Mon, 09 Mar 2009 17:11:53 +0000, Robert wrote:

I _don't_ know what this would be called for real, I _do_ strongly suspect that it already exists as a bit of industrial kit:
A relay that works off of the motor current would do what you want; basically by building a latching relay with the motor as part of the circuit. Basically you'd take a low voltage relay that has a wide current range, and rig it so that it stays closed as long as the motor is pulling current but opens as soon as the motor stops.
Were I looking for something like this, I'd start my web searching with "motor starters" (then I'd ask here...).
Hopefully someone who knows what you should be ordering will tell you the right name.
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wrote:

Sustaining relay? The relay coil is in series with the start switch, the contacts in parallel with the switch. When the relay energizes and the contacts shuts, it "sustains" itself after the start switch is opened. When power is interrupted the relay de-energizes, the contacts open, and so the motor won't restart when power is restored.
--
Rich Webb Norfolk, VA

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From my younger days I recall relays that had coils wound out of heavier wir with fewer turns. The assumption was that the coil was placed in series and the contacts switched at an current level. A long time ago.
From another news group, the term "amperometric relay" came up.
Googling "sustaining relay" has some interesting hits. The scavenger hunt continues.
Thanks, Robert H.
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Robert wrote:

Not all thermal cutouts operate as you described, some require manual reset. Replacing the motor would be expensive enough to warrant some of the fixes already described, but can you replace the cutout?
Jerry
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Robert wrote:

This is a standard motor starter. Off the shelf from any relay vendor.
Line Neutral | --- startpb Tol stop pb | |-o o------------|/|------o__o-----------------()-|run coil | | | |--||--| | | sustaining contact | | | |--||-------------------------------------(Motor)-|
Jeff
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jeff wrote:

Indeed. How does one arrange for the thermal cutout to activate the stop PB?
Jerry
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My thoughts are to sense the motor current and use its presence to sustain power to the motor. A momentary switch would be used to start. If the thermal protection opened the motor winding, there would be no current and the momentary switch would be used to restart.
This can be accomplished with a current sensor and solid state relay from Digikey for $60. Unfortunately, the electronics does like the ambient temperature that I have been told to use.
An electromechanical relay with a coil heavy, and low impedance, enough to be in series with the motor would open if the motor stopped drawing current. Old technology that has the possibility of working at higher temperature.
Old references to to motor control devices that would shut down a motor under no or low load have been seen. They were used to keep the motor speed from climbing.
Thanks for the drawing. What is Tol ?
Have a good day, Robert H.
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Robert wrote:

It's Thermal Over Load. All well and good, but it's not the one built into the motor, If you can rewire the motor's thermal cut-out to work like in the diagram, fine. If you need to cut and tape, you'll lose UL approval.
Jerry
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<snip>

For some reason XNews did not quote you signature line. So here it is. "Engineering is the art of making what you want from things you can get."
And people wonder why I keep a junk box.
Robert H.
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Robert wrote:

According to spec, sig is is to be preceded by a line that consists of two dashes and a space. My newsreader conforms. When I reply to such a message, the tag line and all that follows are not quoted. When I receive such a message, they are gray rather than black. Apparently, XNews doesn't even send them. In this, I removed the space after the two dashes. Let's see what happens.

:-)
Jerry -- Engineering is the art of making what you want from things you can get.
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As you can see, XNews now quotes it.
Robert H.
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One doesn't. Use the same basic technique that many (most? all?) electric clothes dryers use, which is to incorporate a centrifugal switch in series with the sustaining contact.
L N
o start stop o | --- | o-----o o----------o-------------o o--------(K)-----o | | --- | | | | | | | | K || M || | | o-----||-------||--' | | || || | | | | | | Tol | | K || _ | '-----||------( M )------o_/ \o---------------------' || (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
This assumes that the thermal overload is already stuffed into the motor controller and it's required/desired that it stay there without being rewired at all.
The start button needs to be held long enough for the motor to come up to speed and shut the centrifugal switch. Once that happens, the sustaining contacts and centrifugal switch keep it all running. On a thermal overload fault, the motor slows, releasing the centrifugal switch. When the thermal overload clears, the motor won't restart.
--
Rich Webb Norfolk, VA

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wrote:

This is a new one for me. I have seen centrifugal switches used to disconnect starting windings but had not run across any that close contacts when up to speed. This could be usefull in many ways. Unfortunately I am dealing with a retro fit.
AACircuit ?? Something new to use. :-) Are there any other programs for producing ASCII schematics? Linux?
Thank you, Robert H.
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You may be able to set one up with a belt/pulley arrangement, although I'd bet (haven't searched on it) that somebody must make a "split-ring" style that can mount on existing motor shafts. If nothing else, as repair parts.
The one in my dryer (which I have to tear into this weekend to find out where the >klunk-klunk< noise is coming from) uses a "regular" centrifugal switch to take out the starting winding but there's another set of contacts that shut when it's up to speed to enable the heaters to come on. No motor means no blower, so no heaters allowed. This gizmo: <http://www.partadvantage.com/catalogimage.php/15002041.gif?geometry (1000x1000)>

Yes, very handy. Might run under Wine.
--
Rich Webb Norfolk, VA

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Due to the hoops to be jumped through for changes on an approved product, they want an entirely external soltution.
A split tach sensor was used at a different employer. An optical TX/RX and some reflective tape on the shaft might also work.
The last dryer I was into had a vane in the air flow attached to a microswitch.
AACircuit will be tried on Crossover ( built upon Wine ). If it has any problems, a win98se license is run in a virtual machine to take care of such needs. XNews is run on it due to some display problems. It boots much quicker than XP and consumes less resources.
Robert H.
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That could be the ticket. An edge-triggered, retriggerable multivibrator that's active as long as it is getting pulses from the pickup (a Hall sensor might fit, too) but falls to inactive when the edges are lost or appear at less than, say, 80% rated speed.
--
Rich Webb Norfolk, VA

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<snip>

I tried it under Crossover which is based on Wine. A couple of errors come up, not in English. The window on the right side that displays components is blank. No time was spent at fixing things.
It seams to run nicely in a win98se virtual machine that is used for such things.
Thank you, Robert H.
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On Wed, 11 Mar 2009 13:57:24 +0000, Robert wrote:

Huh. It works just fine under regular old Wine on Ubuntu 8.10.
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That seams to be the way things go for me. Things run under wine, but have some issues that the display. When I have tried, things are the same with Crossover and directly with wine. XNews, there is a problem resizing columns and redrawing when a window is resized. Weatherscope, the legend window could be seen only when the mouse pointer was over it and the pull down menues could only be seen after pulling them down. Weatherscope now works with the latest versions of the program, Wine, and Crossover.
Many others do not seam to have these problems. My experience is the same across multiple computers and Linux distributions. The one common ting that comes to mind is the use of Nvidia video card with Nvidia driver.
Thanks for the information. A note has been made. It is the kind of thing that gets looked at late at night when I want something entirely differnet.
Have a good day, Robert H.
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