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Electrical properties required for motor insulation

The reliability and service life of the motor winding operation depend to a large extent on the properties of the insulating material. The basic requirements for the properties of insulating materials include electrical, thermal and mechanical properties. Electrical properties of insulating materials include breakdown strength, insulation resistivity, dielectric constant, and dielectric loss.

1 insulation material breakdown strength
The thickness of the breakdown at the insulation material is divided by the breakdown voltage, expressed in kilovolts per millimeter. The breakdown of insulating materials can be roughly divided into three types: electrical breakdown, thermal breakdown and discharge breakdown.
● Electrical breakdown. Under the action of strong electric field, the charged particles inside the insulation move vigorously, and the collision is free, destroying the molecular structure, so that the final breakdown is called electric breakdown. The electrical breakdown voltage increases linearly with the thickness of the material. In a uniform electric field, unless the impulse voltage is shorter than 10 seconds, the electrical breakdown strength is generally independent of the voltage action time.

● Thermal breakdown. Under the action of alternating electric field, heat is generated inside the insulating material due to dielectric loss. If it cannot be dissipated in time, the internal temperature of the material will rise, causing breakdown of the molecular structure and breakdown, which is called thermal breakdown. The thermal breakdown voltage decreases as the temperature of the surrounding medium increases, the thickness of the material increases, the heat dissipation conditions become worse, 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 in the insulating material are discharged by ionization; the impurities are also vaporized by the electric field to generate bubbles, so that the bubble discharge is further developed, resulting in breakdown of the entire material, which is called discharge breakdown.
The breakdown of insulating materials is often the same as the above three forms, and it is difficult to separate them. Impregnating the insulating material with insulating varnish or glue can improve the electric field distribution and increase the electrical breakdown strength, and can also improve the heat dissipation conditions to improve the thermal breakdown strength.
Motor insulation material, high voltage motor insulation paper, motor winding insulation material

2 insulation resistivity
Under the action of voltage, there is always a small leakage current flowing through the insulating material. A portion of this current flows through the interior of the material; a portion flows through the surface of the material. Therefore, the insulation resistivity can be classified into volume resistivity and surface resistivity. The volume resistivity characterizes the internal electrical conductivity of the material in ohm-meters; the surface resistivity characterizes the conductance characteristics of the material surface in ohms. The volume resistivity of the insulating material is usually in the range of 107 to 1019 m·m. The electrical resistivity of insulating 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, and it is more sensitive to porous materials (such as insulating paper). A hydrophilic substance such as a polar material easily forms a continuous water layer on the surface to lower the surface resistance; a non-polar material such as ceramics, polytetrafluoroethylene or the like does not easily form a continuous water layer on the surface, and thus has little influence on the surface resistance thereof.
● The impurities in the insulating material mostly produce conductive ions, which can promote the dissociation of polar molecules and cause the resistivity to drop rapidly.
● Under the action of high electric field strength, the migration force of ions increases, and thus the resistivity decreases.
3 dielectric material dielectric coefficient
The relative dielectric constant of the insulating material indicates the state in which the internal charge of the insulating material moves under the action of an electric field, that is, the degree of polarization. Generally, it decreases step by step as the frequency of the electric field increases; it increases as the material absorbs moisture; and because of the influence of temperature on polarization, a peak occurs at a certain temperature.
4 dielectric loss of insulating materials
Under the action of an electric field, the insulating material generates energy loss due to leakage and polarization. The loss power or loss tangent is generally used to indicate the dielectric loss.
Under the action of the DC voltage, the instantaneous charging current, the sinking current and the leakage current flow will pass. When an alternating voltage is applied, the instantaneous charging current is a reactive current (capacitor current); the leakage conducting current is in phase with the voltage and is an active current; and the sinking current has both a reactive current component and an active current component. The main factor affecting the dielectric loss of insulating materials.
● Frequency. When the temperature is constant, the loss tangent peaks at a certain frequency, and the dielectric loss value P per unit volume increases the fastest.
Since there are different dielectric losses at different frequencies, a certain frequency must be selected when measuring the loss tangent. Generally, the materials used in the motor are generally measured as the dielectric loss tangent of the power frequency.
● Temperature. When the frequency is constant, the loss tangent shows a peak at a certain temperature, and the loss caused by the absorbed current is the largest. In the low temperature region, the active components of the leakage current and the absorption current are small, so the loss tangent is small; in the high temperature region, the loss due to the absorption current disappears, which is determined by the leakage loss.
For some organic insulating materials, the loss tangent may have several peaks at different temperatures or frequencies. Therefore, in high-frequency or high-voltage electrical equipment, the appropriate insulation material should be carefully selected according to the loss tangent and temperature and frequency curves to avoid loss tangent peaks at the operating frequency and temperature to prevent accelerated aging or thermal shock. wear.
● The electric field strength increases. The loss tangent also increases. When the voltage increases to a certain value, the bubble inside the medium or the edge of the electrode will be partially free, and the loss tangent will suddenly increase significantly. This voltage value is called the initial free voltage. Engineering always uses the measurement of the initial free voltage to check the air gap existing inside the insulation structure to control the insulation quality.
In addition, some insulating materials should also consider electrical properties such as corona resistance, arc resistance, and leakage tracking.

The electrical performance requirements of the motor for the insulating material are the most important for breakdown electric field strength and insulation resistance. Depending on the type of motor, other electrical performance requirements are not exactly the same. For example, the insulation of high-voltage motors requires less dielectric loss and corona resistance. The electric field distribution between the core and the conductor must be considered.

ZTELEC offers different grades of insulation materials, and motor manufacturers are welcome to visit the factory.

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