Preventing AC motor restart.

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.

--
http://www.wescottdesign.com

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

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.

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
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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

jeff wrote:

Indeed. How does one arrange for the thermal cutout to activate the stop
PB?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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.

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
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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.

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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

As you can see, XNews now quotes it.

Robert H.

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

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.

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

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.
  

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

<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.

 

On Wed, 11 Mar 2009 13:57:24 +0000, Robert wrote:


Huh.  It works just fine under regular old Wine on Ubuntu 8.10.

--
http://www.wescottdesign.com

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.