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Minimum insulation resistance of 6kV engine. Measurement of insulation resistance of winding of asynchronous motor

Minimum insulation resistance of 6kV engine. Measurement of insulation resistance of winding of asynchronous motor

Electrical insulation resistance measurement is the most frequent measure during electrical work. The main purpose of this type of measurement is to determine the overall usefulness of electrical conductors, electronically, electrical and electrical equipment.

Insulation resistance depends on various factors. These include ambient temperature, air humidity, and insulation materials. The resistance unit is ohm. When measuring the insulation resistance, this value is typically kilo ohms (1 kOhm) and mega ohms (1 megohm).

Insulation resistance is most often measured in electrical cables, electrical wiring, electric motors, circuit breakers, power transformers and switchgear. The main tool for measurement is meghmmeter (meg-ohmmeter). Megohm meters are two main types of electrons with a manual dial and a digital display.

During the measurement, the meter meter generates the test voltage. The standard voltage of MegaHome is 100V, 250V, 500V, 1000V, and 2500V. The most commonly used voltages are 1000V and 2500V, and 500V is less.
Megammetric examination

Before making any measurements, you should verify the penetrability of the devices used. To do this, perform two control measurements. The first measurement is taken with a short meg meter line between each other. In this case, the measured value must be zero. The second control measurement is performed using open lines. The measured resistance value must be infinitely large.

Safety during measurement

Safety precautions must be observed when measuring insulation resistance. First, it is strictly forbidden to use a defective Megohmmeter. Second, you should check the electrical cable, motor or electrical equipment for no voltage with an indicator or indicator before taking measurements. If there is no voltage, briefly ground the appropriate part of the powered cable, motor or electrical equipment during operation to remove any residual charge. Measures to remove charge must be performed after each measurement.

Measurement of insulation resistance of electric power cable and wiring

The insulation of the electric cables and wires is first checked at the factory and then checked before installation and after completion of the installation. The number of measurements depends on the number of cores in the cable or wire.
Power electrical cables and wires are 3-core, 4-core and 5-core. The three cores are either phase, zero, and ground wire or three-phase "A", "B", or "C". Four cores are 3 phase + zero (ground wire or PEN bonded core). The five wires are three-phase, neutral and ground.

The insulation resistance measurement of a three-core cable or wire is performed as follows. Each of the three conductors is contrasted with two different grounded conductors. The result is three measurements. You can also check the resistance between each two conductors first and then check the resistance between each residence and "ground". In this case, six measurements are obtained.

For 4-core or 5-core electrical cables (wires), the measurement procedure is similar to that for 3-core conductors, but with a slightly larger number of measurements.

Measurements are performed within 1 minute so that the measured values ​​match reality. The amount of insulation resistance of electrical conductors must be within the limits of the main standard. Typically it is 0.5MΩ or 1MΩ for low voltage 220V or 380V cables.

In the case of an electric motor, the insulation of the stator winding is checked. Currently, three-phase electric motors with squirrels for an operating voltage of 380V are the most common.

These engines have three stater winding, which are interconnected by a triangular pattern or star circuit. Connections are made either inside the engine casing or in the engine junction box ("borne"). Since in the first case the winding can not be separated from each other, the measurement results are reduced to measuring the insulation of all three winding connected with the motor housing. In a second variant, the winding can be separated from each other, and then an insulation test between the winding as well as an insulation test of each winding with respect to the metal body of the motor is carried out. Each measurement is performed within 1 minute. The final value of the quantity must also comply with state regulations.

Very powerful high voltage electric motors are often used in production. Measurement of the insulation resistance of winding of such motors often leads to determining the absorption coefficient. From the determination of the moisture content of the windings. To do this, the value is fixed after 60 seconds after the 15 second measurement. The value of the absorption coefficient is the ratio of the resistance of R60 to the resistance of R15. This value should not be less than 1.3.
Measurement of insulation resistance of power transformer

The only device that converts the current voltage from one value to another is the transformer. In fact, power supply can not be produced without a transformer. Each transformer must pass a high voltage test before commissioning. The insulation resistance of the winding must be measured before performing the high voltage test.

Since the transformer has a primary winding and a secondary winding (winding), the insulation of each winding should be isolated from other windings and grounded during measurement. Also measures between primary and secondary windings.

It is often necessary to determine the moisture content of the transformer windings. In this case, the absorption coefficient is determined just like a high voltage motor.

The insulation resistance of a motor winding with rated voltage up to 500V is measured by a 1000V magnetometer (stator winding) and 500V (phase rotor winding).

Measurements of the insulation resistance of the winding between the winding of the induction motor and the housing are made at low temperature. For asynchronous motors with phase rotors, insulation resistance must be measured separately for stater winding and rotor windings.

When inspecting the winding insulator for the housing, one of the probes is applied to the metal surface (preferably the housing ground position) of the motor housing and the second is applied to the output terminal or surface of the conductor of the corresponding winding whose insulation resistance is to be measured . In addition to measuring the insulation resistance of each winding for the case, it is necessary to check the insulation between them (Figure 1).

Figure 1 - Method of measuring insulation resistance of induction motor winding:

a) the insulation resistance of the phase for the housing and two different grounded phases (at accessible zero point);

b) Insulation resistance between windings (0

c) the insulation resistance of the windings assembled to the case (inaccessible zero);

L - clamp "line"; "З" - Clip "Earth".

Mega-meter readings should be measured for 60 seconds after application of voltage R 60.

In some cases, the insulation resistance must be measured twice. The test winding must be discharged before the re-measurement or insulation test is completed and the high voltage potential must be removed. Otherwise, it may cause personal injury when touching the terminals of the winding. Also, if such a discharge is not made in the motor housing, a large error in the over-evaluation direction of the meg-ammeter will inevitably appear. When the insulation resistance measurement of all motor wingdings is completed, the insulation resistance meter should be checked again.

For stater wingdings of asynchronous motors up to 660V, the insulation resistance shall be at least 1 MΩ at 10-30 ° C and at least 0.5 MΩ at 60 ° C. The winding of the phase rotors shall not be normalized to the insulation resistance.

Insulation resistance measurement of winding
Double winding transformer

Insulation resistance measurements of transformer winding are performed using a mega- no meter at a voltage of 2500 V with an upper limit greater than 10,000 MΩ. The measurement of the two-winding transformer is carried out alternately for the high and low voltage winding with respect to the case, with the remaining winding connected to the case and grounded between the winding of different voltages (Fig. 2).

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