[Milsurplus] Fwd: Re: ART-13 dynamotor woes

[Bruce Gentry]

-------- Original Message --------
Subject: 	Re: [Milsurplus] ART-13 dynamotor woes
Date: 	Sun, 10 Feb 2013 16:33:09 -0500
From: 	Bruce Gentry <ka2ivy at verizon.net>
To: 	Meir WF2U <wf2u at ws19ops.com>



On 2/10/13 12:30 PM, Meir WF2U wrote:
>
>  I'm scratching my head, because the field coils are fed from the primary 28
>  VDC, so the insulation is not breaking down from high voltage. If therewas
>  a manufacturing fault in the field coil installation, the short would have
>  existed from the get-go.
>  Should I venture and attempt to take the whole dynamotor apart and remove
>  the field coils to see if I can find the short?
>
>
>  Chances are the field coil had a turn of wire pinched somewhere when itwas installed, the coil was pulled too tight around the pole piece, or it was caught under a corner of the pole piece. This can go un-noticed fordecades until  getting it hot for a few hours causes  it to finally puncture the insulation mechanically. Inductive kicks caused by a connection to the field being loose, such as a crystallized solder connection between the two field windings, can also do it. A transient puncture will usually be intermittent, it could be the reason the dynamotor was surplused inthe first place. I would recommend dismantling the dynamotor, removing the pole pieces, and examining the coils. The spot where the short is should not be too hard to find. If it's a pinch and the wire is not partiallycut through or badly mutilated, re-insulating the winding with tape capable of standing the moderate heat and clamping action of the pole piece is probably all you will need. If it's worse, try to bridge the damage with a piece of new wire of as close to the same size as you can get. It is also permissable in a shunt winding to unwind one turn and splice the break. This will also let you relocate the splice to a place where the winding is not confined or under clamping. If the short is from a transient puncture, see how badly the wire is burned. Usually, it will be to only oneor two wires, put some insulating varnish on the spot and re-insulate the winding.  Originally, windings were wrapped with friction tape or clothtape, I have had excellent results on small motors - 2 HP or less- usingtwo layers of Mylar packing tape cut into 3/4 inch wide strips. It will stand heat and voltage far greater than the original, and is thinner, so fitting the coil back into place on and under the pole piece will be easier. I have done this many times  with good and lasting results.

       Bruce Gentry KA2IVY
>  Any thoughts?
>
>
>
>  73, Meir WF2U
>
>  Landrum, SC
>
>  ______________________________________________________________
>  Milsurplus mailing list
>  Home: http://mailman.qth.net/mailman/listinfo/milsurplus
>  Help: http://mailman.qth.net/mmfaq.htm
>  Post: mailto:Milsurplus at mailman.qth.net
>
>  This list hosted by: http://www.qsl.net
>  Please help support this email list: http://www.qsl.net/donate.html
>