[GreenKeys] Smoke and Fire

Tue Jan 9 13:28:21 EST 2018

With a 3A fuse protecting it, that 120V coil can continuously source ~360W
of heat before the fuse starts to think about letting go. Now take a look
at the coil, and compare it - size, shape and mass - to a 360W power
resistor.

360W is a lot of heat rejection for a little coil that lacks any kind of
serious heat-sinking. Heck, even half that sounds like it would be a major
problem.. are we all sure that a 3A fuse is correct for the application?

On Tue, Jan 9, 2018 at 11:11 AM, Wayne Durkee <teletypeparts at comcast.net>
wrote:

> Good info. Thanks Bruce.
>
> Wayne
> KB1FDW
>
>
> On January 9, 2018 at 11:57 AM Bruce Gentry <ka2ivy at verizon.net> wrote:
>
> I am not yet familiar with the coil and magnet you mention in a 33, but
> have decades of experience with coils, magnets and relays of all sorts in
> elevator controllers. If the coil in question was disigned for 120 volts,
> and designed to be energized for a reasonably long time-several minutes or
> more- then the failure was almost certainly an intra-winding short. These
> occur when a turn or so of the winding shorts out, almost always deep
> inside the winding where heat can not escape easily. One shorted turn
> creates heat, which causes more shorts, a vicious cycle begins and
> continues until the winding burns up and opens. Age is the main contributer
> to this, useage heats the coil and over the years the insulation fails. In
> addition, older insulating materials were not as robust as newer ones can
> be. Other considerstions can overheat the coil as well. Over voltage will
> cause overheating, and under voltage can as well if the plunger or armature
> the coil is pulling does not move all the way to it's seated position. AC
> magnet coils draw far more current if the plunger or armature (clapper) do
> not move all the way and "seal".  As for not blowing the fuse, it's purpose
> is only to keep the device from starting a fire in the building. There is
> no consideration in most cases to preserve the equipment from damage as
> long as whatever happens inside does not start a fire outside. The safety
> codes are especially loose  for equipment that is not typically kept and
> operated in a residential setting.
>
>        Bruce Gentry, KA2IVY
>
> On 1/8/18 6:18 PM, Michael Zahorik wrote:
>
> Well...... it seems like only a few hours ago, I was talking about how
> well my ASR33 unit was working. This evening I was reading a paper tape
> into my PDP8E and half way through the read, I noticed smoke and fire
> inside my ASR33. Turns out the distributor trip magnet caught fire. Looking
> at the circuit I find that it is very simple. This coil is 120 VAC,
> protected by a 3 amp slow blow fuse (which did not blow) and the only
> control that I see is the tape tight/out or reader switch off contact. I
> have removed the coil and it appears to have burnt in mid coil, I can see
> red wire at the top and bottom. This coil reads open and I see no short to
> the core. This magnet has a number on it, 1800853. While the magnet is out,
> re energized the unit and there are no problems, smoke and/or fire that is.
> I can read 120 VAC at J4 pin 11 & 12, when the unit is in either local or
> line and there is paper tape in the reader. If the tape is removed or the
> Local/Line switch is turned off the 120 VAC goes away. I'm betting the coil
> somehow shorted internally either to itself or the core (ground). What are
> my options on finding a replacement magnet? Thanks for the help.
>
> Mike Zahorik (414) 254-6768
>
>
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