Why is the rated operating voltage not specified on the inductance data sheet

Introduction         Many electronic components, including capacitors, resistors, and integrated circuits, have specified rated voltages, but inductors are rarely specified. This article explains why the rated operating voltage of inductors is usually not given.         There are some challenges in determining the rated voltage of an inductor through testing or calculation. Inductance does not support DC or low-frequency operating voltage unless it is a high inductance value (usually>1 mH). It is difficult to verify the working voltage through testing and it should depend on the actual application. The different manufacturing methods of inductors and process stresses such as bending wires make it impractical to calculate the theoretical rated voltage.         This article elaborates on these issues in order to facilitate the selection of the most suitable inductance for specific applications.

Definition of rated voltage

The rated voltage of an inductor refers to the maximum voltage applied to the terminal, and it will not generate arcing or insulation breakdown. When the voltage exceeds the maximum rated voltage, it will cause insulation breakdown between turns and short circuit, or insulation breakdown between windings and iron cores or frames.

Calculate

Calculating the theoretical voltage limit of an inductor is extremely difficult, if not impossible, due to numerous construction variables such as the number of turns, copper wire insulation type and thickness, coil layering, and bending or forming across the lead wire.

For example, there are some standard testing methods to determine the rated voltage of insulated copper wire, which is usually several hundred volts even if it is not over a thousand volts. This rated value is easily affected by the coil forming process, which is a necessary process for winding inductors.

Even in a strictly controlled manufacturing environment, copper wire can still be subjected to stress, resulting in cracks or defects. At a voltage far below the rated value of the copper wire "catalog", a small scratch on the copper wire can cause an arc or breakdown. It is not practical to calculate the rated values that include these variables.

Test method

High voltage (high potential) testing is not sufficient to determine the operating voltage of an inductor. High voltage testing is used to determine the isolation from primary to secondary within a transformer, and can be used to determine the isolation from winding to shell or winding to ground. However, traditional high-voltage testing cannot be used to determine the rated voltage from inductor terminal to terminal.

For inductor operation, V=L × di/dt explicitly implies that large DC voltage (or AC line voltage) cannot be applied to the terminals of the inductor.

Illustrate with examples

The rated DC resistance of the power inductor MS1038-103 of Baoheng 10 μ H is 0.035 Ohms. If a traditional 500Vdc high voltage test is performed on the terminals of the inductor, it is like applying 500V to a copper wire with a resistance of 0.035 Ohms. According to Ohm's Law, applying 500 V to 0.035 Ohms requires a current source greater than 14000 Amps.

Although this may seem impractical, it does illustrate the problem. The voltage breakdown test is actually used to determine when high voltage will cause a short circuit. For traditional high-voltage testers, inductance is already a short-circuit circuit!

In order to test insulation breakdown or arcing, it is necessary to induce voltage pulses, spikes, or oscillations in the inductor. This type of induced voltage or oscillation test can be conducted. This technology has been widely accepted and there are commercial testers available. However, the induced voltage depends on the storage and maintenance of resonance energy in the inductor. Generally, an inductor value greater than 1 mH is effective, which excludes most inductors except those designed for power frequency applications.

The induced voltage that an inductor can withstand depends on the type of copper wire insulation, the number of coils and layers, as well as other factors such as proximity to the magnetic core, distance between terminals, and insulation material such as CTI (compared to leakage trace index).

Therefore, even within a series of inductors, each inductance value will have a different rated voltage.

Furthermore, it is very difficult to infer and predict actual insulation breakdown in specific applications based on induced voltage testing. Voltage arcing or breakdown varies with environmental factors such as dust or humidity, as well as changes in working conditions such as pulse waveforms. Compared to constant voltage or slowly increasing voltage, signals with sharp rise times generally generate arcs at lower voltages.

Conclusion

From this perspective, choosing an inductor seems almost impossible. The good news is that although all these factors make it difficult to test or calculate the rated voltage of an inductor, there is no need to test it. Most applications require inductors to operate at very low operating voltages, typically only a few volts. Remember, most inductors work by suppressing current rather than by high voltage induction. Almost all commercial inductors have no problem working at several volts or even tens of volts. Therefore, for most applications, the rated voltage of the inductor is not a problem. For high-voltage induction applications with high line voltage or windings, it is necessary to consider the possible breakdown of inductive windings.

For these situations, it is recommended to discuss specific applications with the inductor designer/manufacturer and develop a testing plan to ensure the suitability of the inductor.