Electrical performance requirements for motor insulation materials
The reliability and service life of winding operation largely depend on the performance of insulation materials. The basic requirements for the performance of insulation materials include electrical performance, heat resistance, and mechanical performance. This article provides a brief introduction to the electrical performance of insulation materials for Hengli Motors. The electrical properties of insulation materials include breakdown strength, insulation resistivity, dielectric constant, and dielectric loss.
1. Breakdown strength of insulation materials
Divide the thickness of the insulation material at the breakdown point by the breakdown voltage, expressed in kilovolts per millimeter. The breakdown of insulating materials can be roughly divided into three forms: electrical breakdown, thermal breakdown, and discharge breakdown.
● Electric breakdown. Under the action of a strong electric field, charged particles inside the insulation move violently, collide and dissociate, damage the molecular structure, and eventually break down, which is called electrical breakdown. The 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 breakdown strength is generally independent of the duration of the voltage applied.
● Thermal breakdown. Under the action of alternating electric field, heat is generated inside the insulation material due to dielectric loss. If it cannot be dissipated in time, the temperature inside the material will rise, causing molecular structure damage and breakdown, which is called thermal breakdown. The thermal breakdown voltage decreases as the temperature of the surrounding medium increases, the material thickness increases, the heat dissipation conditions deteriorate, and the breakdown strength decreases; When the frequency increases, the 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 an electric field, generating bubbles, which further develop the discharge of the bubbles, leading to the breakdown of the entire material, known as discharge breakdown.
The breakdown of insulating materials often occurs in all three forms simultaneously, making it difficult to distinguish them completely. Immersing insulation materials with insulating paint or adhesive solution can improve electric field distribution and enhance electrical breakdown strength, as well as improve heat dissipation conditions and enhance thermal breakdown strength.
2 Insulation resistivity
Under the action of voltage, insulation materials always have a small leakage current passing through. Part of this current flows through the interior of the material; Part of it flows through the surface of the material. Therefore, insulation resistivity can be divided into volume resistivity and surface resistivity. Volume resistivity characterizes the internal electrical conductivity of materials, measured in ohms per meter; Surface resistivity characterizes the electrical conductivity of a material's surface, measured in ohms. The volume resistivity of insulating materials is usually in the range of 107~1019 m · m. The resistivity of insulation materials is generally related to the following factors.
As the temperature increases, the resistivity decreases exponentially.
Water can promote the dissociation of polar molecules, so the insulation resistivity decreases with increasing humidity, which has a more sensitive impact on porous materials such as insulating paper. Hydrophilic substances such as polar materials can easily form a continuous water layer on the surface, reducing surface resistance; Non polar materials such as ceramics and polytetrafluoroethylene are not easily able to form a continuous water layer on their surface, thus having a relatively small impact on their surface resistance.
Impurities in insulation materials mostly generate conductive ions, which can also promote the dissociation of polar molecules, leading to a rapid decrease in electrical resistivity.
Under high electric field strength, the migration force of ions increases, resulting in a decrease in electrical resistivity.
3. Dielectric constant of insulation materials
The relative dielectric constant of insulating materials represents the degree of polarization, which refers to the movement of charges inside the insulating material under the action of an electric field. Generally, it gradually decreases with the increase of electric field frequency; Increase with moisture absorption of the material; Due to temperature affecting polarization, a peak will occur at a certain temperature.
4. Dielectric loss of insulation materials
Under the action of an electric field, insulating materials experience energy loss due to leakage and polarization. Generally, the magnitude of dielectric loss is represented by loss power or loss tangent.
Under the action of DC voltage, instantaneous charging current, absorption current, and leakage current will be generated. When an AC voltage is applied, the instantaneous charging current is the reactive current (capacitor current); The leakage current is in phase with the voltage and is the active current; Absorbing current involves both reactive and active current components. The main factors affecting the dielectric loss of insulation materials.
● Frequency. When the temperature remains constant, the loss tangent reaches a peak at a certain frequency, at which point the dielectric loss value P per unit volume increases the fastest.
Due to the different dielectric losses at different frequencies, a certain frequency must be selected when measuring the tangent of the loss angle. Generally, the materials used in motors are measured for their dielectric loss tangent at the power frequency.
● Temperature. When the frequency remains constant, the loss tangent reaches its peak at a certain temperature, at which point the maximum loss is caused by the absorbed current. In the low-temperature region, the active components of leakage current and absorption current are both very small, so the loss tangent is very small; In the high temperature zone, the loss caused by absorbing current disappears and 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 insulation materials should be carefully selected based on the relationship curve between loss tangent and temperature and frequency, to avoid peak loss tangent at operating frequency and temperature, and to prevent accelerated aging or thermal breakdown of materials.
The electric field strength increases. The tangent of the loss angle also increases accordingly. When the voltage increases to a certain value, local ionization occurs at the edges of bubbles or electrodes inside the medium, and the tangent of the loss angle suddenly increases significantly. This voltage value is called the initial ionization voltage. In engineering, the measurement of initial free voltage is often used to check the air gap inside the insulation structure and control the insulation quality.
In addition, some insulation materials should also consider electrical properties such as resistance to corona, arc, and leakage marks.
The electrical performance requirements for insulation materials in motors are primarily based on the breakdown electric field strength and insulation resistance. Depending on the type of motor, the requirements for other electrical performance are not exactly the same. For example, in the insulation of high-voltage motors, the insulation material is required to have low dielectric loss and good corona resistance; And the electric field distribution between the iron core and the conductor must be considered.
