The reliability and service life of winding operation largely depend on the performance of the insulation material. The basic requirements for insulation material performance include electrical properties, heat resistance, and mechanical properties. This article provides a brief introduction to the electrical properties of insulation materials. These electrical properties include breakdown strength, insulation resistivity, dielectric constant, and dielectric loss.
1. Breakdown strength of insulating materials
The breakdown voltage is calculated by dividing the thickness of the insulation material at the breakdown point by the breakdown voltage, expressed in kilovolts per millimeter. Breakdowns in insulation materials can be broadly categorized into three types: electrical breakdown, thermal breakdown, and electrical discharge breakdown.
● Electrical breakdown. Under the influence of a strong electric field, charged particles inside the insulation move violently, colliding and ionizing, destroying the molecular structure, and eventually breaking down. This is called electrical breakdown. The electrical breakdown voltage increases linearly with the thickness of the material. In a uniform electric field, unless the duration of the impulse voltage is less than 10 seconds, the electrical breakdown strength is generally independent of the voltage application time.
● Thermal breakdown. Under the influence of an alternating electric field, heat is generated inside the insulating material due to dielectric loss. If this heat cannot be dissipated in time, the internal temperature of the material will rise, leading to the destruction of the molecular structure and breakdown. This is called thermal breakdown. The thermal breakdown voltage decreases with increasing ambient temperature. As the material thickness increases, heat dissipation conditions worsen, and the breakdown strength decreases. As the frequency increases, dielectric loss increases, and the breakdown strength also decreases.
● Discharge breakdown. Under the action of a strong electric field, the bubbles contained inside the insulating material discharge due to ionization; impurities are also heated and vaporized by the electric field, producing bubbles, which further develops the bubble discharge, leading to the breakdown of the entire material, which is called discharge breakdown.
Breakdown of insulating materials often occurs simultaneously in all three forms mentioned above, making them difficult to separate completely. Impregnating insulating materials with insulating varnish or adhesive can improve the electric field distribution and thus increase electrical breakdown strength, as well as improve heat dissipation and thus increase thermal breakdown strength.
2. Insulation resistivity
When an insulating material is subjected to voltage, a small leakage current always flows through it. Part of this current flows inside the material; the other part flows through the surface. Therefore, insulation resistivity can be divided into volume resistivity and surface resistivity. Volume resistivity characterizes the internal conductivity of the material, measured in ohm-meters; surface resistivity characterizes the surface conductivity of the material, measured in ohms. The volume resistivity of insulating materials is typically in the range of 10⁷ to 10¹⁹ m·m. The resistivity of insulating materials is generally related to the following factors.
● As temperature increases, resistivity decreases exponentially.
Water promotes the dissociation of polar molecules, thus the insulation resistivity decreases with increasing humidity, and this effect is more sensitive to porous materials (such as insulating paper). Hydrophilic substances such as polar materials easily form a continuous water layer on their surface, thereby reducing surface resistance; non-polar materials such as ceramics and polytetrafluoroethylene do not easily form a continuous water layer on their surface, and therefore have less impact on their surface resistance.
● Impurities in insulating materials mostly produce conductive ions, which can also promote the dissociation of polar molecules, causing the resistivity to drop rapidly.
● Under the influence of a high electric field, the migration force of ions increases, thus reducing the resistivity.
3. Dielectric coefficient of insulating materials
The relative permittivity of an insulating material represents the movement of charges within the material under the influence of an electric field, i.e., the degree of polarization. Generally, it decreases gradually with increasing electric field frequency; it increases with moisture absorption; and due to the influence of temperature on polarization, it exhibits a peak value at a certain temperature.
4. Dielectric loss of insulating materials
When insulating materials are exposed to an electric field, energy loss occurs due to leakage current and polarization. The magnitude of dielectric loss is generally expressed as power loss or the tangent of the loss angle.
Under DC voltage, there will be instantaneous charging current, absorption current, and leakage current. When AC voltage is applied, the instantaneous charging current is reactive current (capacitive current); the leakage current is in phase with the voltage and is active current; the absorption current has both reactive and active current components. These are the main factors affecting the dielectric loss of insulating materials.
●Frequency. At a constant temperature, the loss tangent peaks at a certain frequency, at which point the dielectric loss P per unit volume increases the fastest.
Since different frequencies have different dielectric losses, a certain frequency must be selected when measuring the loss tangent. Usually, the dielectric loss tangent of the materials used in motors is measured at the power frequency.
●Temperature. At a constant frequency, the loss tangent peaks at a certain temperature, at which point the loss generated by the absorbed current is greatest. In the low-temperature region, both the leakage current and the active component of the absorbed current are very small, so the loss tangent is very small; in the high-temperature region, the loss generated by the absorbed current disappears, and the loss is determined by the leakage loss.
Some organic insulating materials may exhibit several peaks in their loss tangent at different temperatures or frequencies. Therefore, in high-frequency or high-voltage electrical equipment, appropriate insulating materials should be carefully selected based on the relationship curves between the loss tangent and temperature and frequency to avoid peak loss tangent values at the operating frequency and temperature, thereby preventing accelerated aging or thermal breakdown of the material.
● As the electric field strength increases, the loss tangent also increases. When the voltage increases to a certain value, local ionization will occur in the air bubbles inside the dielectric or at the electrode edges, and the loss tangent will suddenly increase significantly. This voltage value is called the initial ionization voltage. In engineering, the measurement of the initial ionization voltage is often used to check the air gaps inside the insulation structure in order to control the insulation quality.
In addition, some insulating materials should also be considered for electrical properties such as corona resistance, arc resistance, and resistance to tracking.
For motors, the most important electrical performance requirements for insulation materials are breakdown electric field strength and insulation resistance. However, the requirements for other electrical performance characteristics vary depending on the type of motor. For example, the insulation of high-voltage motors requires low dielectric loss and good corona resistance; and the electric field distribution between the core and conductors must also be considered.
