Dielectric Strength

The usual figure given for dielectric strength is the breakdown value in rms volts at 60 cycles in a 1-minute test. It is not possible to operate class A insulation anywhere near this value because of the cellular structure of all organic materials. Even after these materials are treated with varnish, many small holes exist throughout a coil structure which ionize and form corona at voltage far below breakdown. With class A insulation (organic materials), the designer must be governed more by resistance of the insulation to corona over a long period than by breakdown strength of the insulation in a 1-minute test. For example, a 20-mil thickness of treated cloth will withstand 10,000 volts for 1 minute. However, corona starts at 1,250 volts, and operation at any higher voltage would puncture the insulation in a few weeks. It is much wiser to keep a reasonable margin, say 20 to 30 per cent, below the corona limit than to use a fraction of the 1-minute breakdown test. Approximate voltages at which corona is audible are plotted in Fig. 37 as a function of insulation thickness.

Fig. 37. Corona limit for treated cloth and paper.

Differences in hearing ability between persons make a corona measurement desirable. This is done by means of the standard NEMA circuit of Fig. 38.⁽¹⁾

Fig. 38. Standard NEMA radio-influence measuring circuit.

With the transformer connected as shown, receiver output meter is adjusted to half-scale by a volume control potentiometer in the receiver. Next, the transformer is replaced by a modulated 1-mc signal generator, the output of which is varied until the noise meter output is again half-scale. The signal generator output in microvolts is read on an attenuator; this is then a measurement of the corona present.

Class B insulation can be worked much closer to the ultimate dielectric strength, but the latter is less a factor in determining size than creepage distance to the core. For mica an approximate working voltage rule is 100 volts rms per mil thickness.

Insulated coils in air are subject to a two-dielectric effect that is peculiarly troublesome. If the path of electric stress is partly through solid material and partly through air, the air may be overstressed because it has the lower dielectric constant (unity, compared with 3 to 5 for most coil materials). If this condition exists, it is usually impracticable to increase the air distance and so reduce the volts per inch to a value below the corona limit. The addition of more solid insulation over the whole coil may make it too large. Often the only feasible remedy is to fill the air space with more solid material, either in the form of filling compound or strips of insulation like micarta or press-board.

It is important, when dealing with insulation voltage, to make a distinction between test voltage and operating voltage. Of the two, operating voltage is the better value to specify.

(1)	See "Radio Influence Characteristics of Electrical Apparatus," by P. L. Bel-laschi and C. V. Aggers, Trans. AIEE, 57, 626 (November, 1938).