ON THE PROSPECTS FOR USING EPOXY COMPOUND FOR ENCAPSULATING WINDINGS OF ELECTRICAL MACHINES

G. A. SIPAILOV, R. M. KESSENIKH, Yu. P. POKHODKOV

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

Hot-curing epoxy compounds possess a number of valuable properties that make it possible to use them for encapsulating the windings of electrical machines operating under severe climatic conditions. Abroad, epoxy compounds have found wide application in various branches of electrical engineering [1, 2, 3]. By varying the formulation and curing conditions of these compounds, a material with specified properties can be obtained.

The use of an epoxy compound as an encapsulating material for low-voltage electrical machines has considerable advantages in comparison with various impregnating varnishes, including organosilicon ones [4].

At present, for operation under marine and tropical climatic conditions, electrical machines are used mainly with multilayer insulation of the windings, impregnated with organosilicon varnishes and encapsulated with an epoxy compound. The principal drawback of such a design is its high cost, due to the presence of a large quantity of moisture-resistant materials (multilayer slot insulation) and to the need for an additional impregnation operation.

In the present work a new design of winding insulation based on a perforated insulating material is proposed and investigated (Fig. 1). A distinctive feature of this design is that, in order to reduce the quantity of materials used and to improve the conditions for penetration of the compound into the winding in the slot portion, instead of several layers of electrical insulating materials a single layer of perforated electrical insulating material is used, serving as a spacer between the slot walls and the winding conductors. In addition, the design makes it possible to dispense with wedges, insulation of the end portions, and banding (for armature windings), since their functions can be performed by the epoxy compound.

The investigations were carried out on models representing actual armatures of DC machines with a power of up to 1 kW. As the slot insulation of the armature winding we used perforated pressboard 0.2 mm thick. The perforations consisted of holes 2.5 mm in diameter; the distance between holes was 0.5 mm (Fig. 1). The wire was PEV-2, 0.93 mm in diameter.

In the general case, the wire and the material of the perforated layer may be selected with due regard for the compatibility of the insulation in order to provide the required heat-resistance class.

Fig. 1. Armature with a winding encapsulated in epoxy compound and perforated frame insulation.
1 — epoxy compound;
2 — blank for slot insulation.

Ultimately, taking into account the future creation of new synthetic electrical-insulating materials, there is a possibility of selecting such a compound and such a material for the perforated layer that, during curing, will enter into a chemical reaction and form a homogeneous frame insulation.

Speaking of winding designs with epoxy insulation, it may be noted that abroad, in the last few years, fairly considerable attention has been paid to such designs [5, 6, 7]. Designs are known for machines intended for marine and tropical service with windings encapsulated in epoxy compound. In this case, however, preliminary impregnation is most often practiced. Also known [8] is a conductor-insulation design for sections of high-voltage electrical machines, in which individual bars wrapped with glass tape are insulated with perforated micafolium and then impregnated with epoxy compound. In this case, however, the purpose of the perforation is only to promote penetration of the compound into the micafolium, without reducing the quantity of materials (the diameter of the perforation holes is 0.1 mm).

The design proposed by us is simpler. Before encapsulation, the perforated electrical-insulating material (electrocardboard) serves as a mechanical separator between the winding and the magnetic core. The low initial viscosity of the compound at a temperature of 140°C makes it possible to impregnate the winding thoroughly and to fill all perforation holes. This is facilitated by the small volume of solid insulation in the machine slot, which after perforation is reduced by approximately a factor of two.

After the compound has cured, the winding in the slot portion is insulated from the slot walls either by a layer of epoxy compound or by a layer of electrocardboard impregnated with the compound.

To avoid damage to the conductor insulation during placement at the points where it exits the slot, the end edges of the electrocardboard are not perforated.

The use of an epoxy compound for casting makes it possible to dispense with wedges; after the winding has been laid in the slots, the electrical cardboard closes the conductor tightly and remains in this position owing to its rigidity. After casting, the function of the wedges is performed by the layer of compound located in the slot above the winding.

The use of this winding-insulation design for armatures of electrical machines makes it possible to dispense with the use of bandages, under-bandage insulation, and insulation of the end parts from the shaft. The functions of these materials are performed by the epoxy compound. Insulation of the end parts from the shaft is provided as follows: when the winding is laid in the armature slots, split bushings are fitted onto the shaft in the region of the end parts, providing a clearance between the winding and the shaft. Immediately before casting these bushings are removed, and after casting their place is occupied by the epoxy compound. We made mock-ups of windings with the described insulation design; their behavior under various climatic conditions was studied simultaneously with a whole series of other designs.

In particular, the following were tested:

1. Armatures with slot insulation consisting of micanite, electrical cardboard, and PEV-2 wire, impregnated with silicon-organic varnish EFZ-BSU (composition A).

2. Armatures with insulation of composition A, additionally cast with an epoxy compound (composition B).

3. Armatures with slot insulation made of perforated electrical cardboard, with impregnation and casting by an epoxy compound (composition V). Composition of the compound: ED-6 resin, phthalic anhydride, quartz sand, dibutyl phthalate.

Figures 2 and 3 present comparative dependences of the change in insulation resistance of the compositions considered on the time of exposure under conditions of elevated marine humidity and seawater. The table gives the results of changes in capacitance \(C\), \(\operatorname{tg}\delta\), and \(U_{\text{bd}}\) of the insulation after 72 hours of exposure in seawater.

Elevated marine humidity \((\varphi = 100\%)\) and seawater have a harmful effect on multilayer insulation impregnated

Fig. 2. Dependence of the logarithm of the insulation resistance of compositions on exposure time under conditions of 100% relative marine humidity.
Curve 1 — composition A.
Curve 2 — composition B.
Curve 3 — composition V.

![Graph: dependence of \(\lg R\) on \(t\) (hours), with curves labeled 1, 2, and 3.]

Fig. 3. Dependence of the logarithm of the resistance of the composite insulation on the time spent in seawater.
The designations of the curves are the same as for Fig. 2.

with silicone-organic varnish; the insulation resistance drops by almost 3 orders of magnitude (Figs. 2 and 3), and other indicators—\(C\), \(\tg\delta\), \(U_{\mathrm{pr}}\)—also deteriorate (see table). This deterioration is apparently due to the presence of pores in the varnish film formed during evaporation of the solvent.

Table

The electrical properties of the insulation of composition B, under prolonged exposure to the same conditions as for the insulation of composition A, are more stable. The insulation of composition C is not inferior in the stability of its electrical properties to the insulation of composition B. This is promoted by the monolithic nature of the capsule made of epoxy compound and the good quality of the ground insulation, consisting of 55% epoxy compound.

Test data for armatures with insulation of composition C over the service life make it possible to assign this insulation to class E. The service life of the winding insulation of composition C at various temperatures is: \(105^\circ\mathrm{C}\)—31,600 hours, \(120^\circ\mathrm{C}\)—14,900 hours, \(130^\circ\mathrm{C}\)—9,800 hours.

The winding design based on perforated insulation and epoxy compound has significant advantages over other designs of potted windings.

1. A significant reduction in the range of materials used will reduce the cost of the product and lead to a considerable reduction in operations for preparing insulating materials and placing them in the machine slots.

1. Reducing the volume of solid insulation in the slot of the machine and increasing its surface area will make it possible to shorten the drying time of the insulation and, in a number of cases, to dispense with vacuum treatment.

2. The large surface area of the solid insulation, due to perforation, will promote better impregnation of this insulation.

Thus, both in terms of electrical properties and in service life, the insulation of the armature windings of low-voltage electrical machines, consisting of a single layer of perforated electrical cardboard impregnated and encapsulated with epoxy compound, can successfully replace multilayer slot insulation consisting of moisture-resistant materials impregnated with organosilicon varnishes and encapsulated with epoxy compound.

LITERATURE

1. K. I. Chernyak, Epoxy Compounds and Their Application, Sudpromgiz, 1963.

2. Review of insulation paper presented of ATEE summer general meeting, Insulation, 6, No. 8, 1960.

3. R. Houwink, A. Staverman. Chemistry and Technology of Polymers. Khimiya Publishing House, 1966, volume two, part two.

4. M. I. Kessenikh, Yu. P. Khokhlov, A. V. Petrov. On the possibility of using epoxy compounds for encapsulating windings of electrical machines. Proceedings of the Siberian Branch of TsBTI, 1966.

5. F. I. Davidson. Application and techniques for encapsulation wish epoxy insulating resins, Insulation, No. 1, 1956.

6. C. W. Parc. Epoxy resin casting of gry type high voltag component, Electrical engineering, No. 12, 1956.

7. M. Richter, B. Bartakova. Tropicalization of Electrical Equipment, Gosenergoizdat, 1962.

8. Swedish patent class 21 Hd 53, (H02K) No. 185290.