MIL-DTL-917 Electric Power Equipment, Basic Requirements ForThis specification covers the basic requirements applicable to the design, materials, and construction of naval shipboard electric power equipment \(exclusive of communication equipment, information technology \(IT\) equipment, and combat systems [fire control and weapons control equipment]\). Unless otherwise specified herein, the electric equipment developed for shipboard use under the requirements of this specification are limited to a maximum 450 volt \(V\) alternating current \(AC\) and a maximum 1,000 V direct current \(DC\). This equipment is intended for use on surface and submarine naval vessels \(see 6.2\).

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MIL-DTL-917F(SH)

3.8.2 Encapsulants and compounds used on coils and windings. Encapsulants and winding compounds

(paste-like material usually applied by brush or spatula) are characteristic in that they cure to a solid state by catalyst

reaction plus heat. Typical materials are the polyester and epoxy resins. These materials shall not, either in original

application or as a result of aging, have any deleterious effect on the insulation materials to which they are applied

and shall not cause corrosion or deterioration of any adjacent parts. These materials shall not crack or flow at the

temperatures encountered during normal operation. The conditions under which these materials are applied and

used shall in all respects conform to the manufacturer's recommendations. Neither the material itself nor the

process by which it is applied shall have a deleterious effect on the operation of the assembly or the overall

equipment.

3.9 Varnish treatment. Fabrication operations, such as welding, machining, drilling, and tapping, shall have

been completed prior to varnish treating. The windings and coils shall be clean and dry. The drying shall be

accomplished by prebaking the windings to remove all moisture and to cure any uncured winding parts. The

windings or coils, prior to core insertion, shall then be allowed to cool to a temperature not below 50 �F (10 �C)

above room temperature, then immersed in the varnish until bubbling ceases, allowed to drain, and then baked at the

temperature and for the time specified by the varnish manufacturer. A minimum of three dips and bakes shall be

used. If the VPI process is used, a minimum of one treatment shall be applied. Pre-impregnated coils receive one or

more varnish or resin treatments prior to assembly in the slot or on the pole piece. Post-impregnated coils receive

all varnish or resin treatments after insertion in the slot.

3.9.1 Baking ovens. The baking ovens shall be rated at least 347 �F (175 �C) for Classes 90, 105, 130, and

155 windings; and at least 500 �F (260 �C) for Class 180 and higher temperature windings. The capacity of the

baking ovens shall be sufficient to maintain the appropriate temperatures at a full exhaust rate of two air changes per

minute. In addition, baking ovens used to cure VPI treated windings shall be equipped with a rotisserie for rotating

the apparatus during curing.

3.9.2 Baking time specified. Baking time for curing varnished windings shall be as specified by the resin or

varnish manufacturer on the basis of time after the winding has reached the specified temperature as determined by

attached thermocouples.

3.9.3 Typical treating guidelines. The treating checklist in table XIV may be used as a guide in processing

electrical windings. The baking times and temperatures vary depending on the type and grade of varnish or resin

used and the size of the winding being processed. The varnish or resin manufacturer's recommended treating

schedule should be followed as far as preheat time and temperature, baking time and temperature, and piece

temperature are concerned.