(1) The structure and working principle of megger
To measure insulation resistance, a megohmmeter is used. The megger is also called an insulation megger, or megger for short. Commonly used meggers include the ZC-7, ZC-25, and other models. Its scale is in megohms, which is why it is called megohmmeter, and because it is composed of a manually operated high voltage DC generator and a magnetoelectric system protractor. . It is composed, which is why it is also called a shaking table.
The magnetoelectric ratiometer is a special form of magnetoelectric instrument, the appearance of which is as shown in Figure 1-39a. Its fixed part consists of a permanent magnet, a polar palm and an open-gate annular magnetic core. The shape of the polar palm is particular in order to make the air gap between the magnetic core and the polar palm uneven, making itif the magnetic. unequal field distribution in the air gap. The moving part consists of two moving coils arranged at a certain angle to each other. The coil and the pointer are fixed on the same axis. The ends of both sets of coils are connected to the external circuit via hanger wires, but the hanger wires only conduct electricity and do not produce a reaction force. The hand-cranked DC generator can generate high DC voltage and serve as a measurement power supply for the instrument. Figure 1-39b shows the structural principle of the megger.
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Figure 1-39 The appearance and structural principle of the oscillator
(a) Appearance (b) Structural principle
1. Cylindrical notched iron core 2. Polar palm 3. Moving coil 14. Moving coil 25. Permanent magnet 6. Pointer
(2) Use of the megometer
① Calibrate the meter. Before measurement, the megometer should be tested for open circuits and short circuits to check whether the two connecting wires are open and the pointer should point to "∞", then short circuit the two connecting wires. The pointer should point to "0". Those that meet the above conditions are good, otherwise they cannot be used.
②The device under test is disconnected from the line. Large-capacity equipment should also be discharged
③ Use a megger with an appropriate voltage level
④ When measuring insulation resistance, generally only use the “ L” and the “E”. Terminals. When measuring the insulation resistance of the cable to ground or the leakage of the equipment under test is serious, use the "G" terminal and connect the "G" terminal to the shielding layer or shell, then shake - it clockwise. . The speed of actionation should be slow to fast. When the rotation speed reaches approximately 120 revolutions per minute (ZC-25 model), continue rotating at a constant speed and take a reading after 1 minute. not stopped.
⑤ After reading, remove the wires by shaking slowly, then unload the equipment under test. The discharge method involves removing the ground wire used during measurement and shorting it with the equipment. currently being tested. Simply click (not the megometer to unload).
Learn to use megometer
The instrument used to measure large value resistance and insulation resistance is called a megometer, and now the new product is generally called megometer or insulation resistance tester, sometimes. This is called a megger. Among them, the appearance of the old megger is shown in the right picture below:
The main structure of the megger (insulation resistance meterement) is composed of a manual generator, an electromagnetic meter head without mechanics. reaction and external terminals (L: line segment, E: earth terminal, G: shield terminal). Newer insulation resistance testers generally resemble digital multimeters.
The working principle of megometer (insulation resistance meter) is as shown in the figure below. Its magnetoelectric meter has two moving coils located at a certain angle to each other. of a notched cylindrical iron core is fixed on a rotating shaft with the needle, forming the moving part of the watch head. It is placed in a permanent magnet. The air gap between the magnetic poles of the magnet and the iron core is unequal. . Since the pointer does not have a shock absorber spring, the pointer can remain in any position when the instrument is not in use.
Shake the handle and gDC generator can produce current. Among them, a current I1 flows in the loop between coil 1 and the measured resistance Rx, and the other current. I2 flows through coil 2 and through the resistance loop Rf, assuming that the resistance of coil 1 is R1 and the resistance of coil 2 is R2, according to Ohm's law:
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I1=U÷(Rc+R1+Rx), R2=U÷(Rf+R2)
The excited coil in the magnetic field is affected by the force of the magnetic field, forcing coil 1 to generate a rotational torque M1 and coil 2 to generate a rotational torque M2, since the two coils are wound in opposite directions, the directions of M1 and M2 are opposite and the resulting moment of the two couples causes the pointer to deviate. When M1 = M2, the pointer is stationary and what it points at this time is the insulation resistance value of the device under test.
When Rx is disconnected (i.e. X=∞ (infinity)), I1=0, M1=0, thepointer deviates to the left under the action of M2, and finally points towards the mark. At the scale Rx=∞, if Rx is shorted (i.e. Rx=0), I1 will be the largest and M1 will be the largest at that time, causing the pointer to deviate in the clockwise, and the pointer points to the point Rx=0 on the scale. According to this principle, the quality of the megger can be tested.
First, let's talk about megger selection. For equipment with a voltage rating below 500 volts, choose a 500 volt or 1000 volt megger (megger) with. a nominal voltage of 500 volts. For voltages above 1,000 volts, use a megger with a range of 1,000 volts to 2,500 volts.
How to use the megohmmeter
The megohmmeter (megohmmeter) has three measuring terminals and one line terminal (L) . , the other is the ground terminal button (E) and the other is the shield terminal buttonage (G). Generally, when measuring the insulation resistance of photos or power lines to ground, only terminals L and E are used, and the wiring is as follows Shown on the left. Terminal “L” is connected to the “fire” or “phase” terminal of the device under test, and terminal “E” is connected to the “earth” terminal of the device under test. Use the “G” terminal when measuring cable insulation resistance to ground or when the leakage current of the device under test is large. The right figure below shows the wiring diagram for measuring cable insulation resistance.
Once the lines are connected, you can turn the watch generator crank clockwise so that the voltage put out by the generator rotor can be used for measurement. The crank rotation speed should be slow to fast. When the rotation speed reaches a certain value, the rotation speedn must be kept uniform and stable. (Typically, the rotation speed of a regular megger is around 120 rpm). When the speed of the megger generator stabilizes, the pointer on the dial also stabilizes. At this time, the value indicated by the needle is the measured insulation resistance value.
Notes
Before measurement, the megohmmeter (megohmmeter) should be tested for open circuits and shorts circuits to check if the meter is in good condition? If both connecting wires are open circuited and the handle is gently shaken, the pointer should point to "∞". If both connecting wires are shorted again, the pointer should be "0", indicating that the meter is open circuit. in good condition. There is an error in the table.
The equipment under test must be disconnected from the power supply and the capacitive equipment must be completely discharged pto ensure personal safety and accurate measurement.
During the telemetry process, no one can work on the device under test.
When measuring motors, transformers, cables and capacitors with large capacitances, there must be a certain charging time. The larger the capacity, the longer the charging time generally. , a megger is used to turn the meter. The reading after one minute serves as a reference. Unload immediately after the measurement is completed to ensure safety. The discharge method involves removing the ground wire used during measurement from the megger and shorting it to the object to be measured.
It is prohibited to measure insulation during lightning or on equipment with high voltage conductors nearby. The measurement can only be carried out when the equipment is not charged and cannot be charged due to the induction from other power sources.
Do not touch the measuring part of the equipment or the meter terminals until the meter stops rotating. When removing wires, do not directly touch exposed parts of the wires.
The megometer should be calibrated regularly: the calibration method is to directly measure the standard resistance with a certain value and measure whether it has a measurement error and whether it is within the authorized limits. range.
The working principle of the megometer (insulation resistance meter)
There are two coils connected to the 'needle of the megometer, Connect the additional resistor R in the meter in series; connect the other resistor in series with the resistor R to be measured, then connect them together to the hand generator. When the generator is started by hand, current flows through both coils at the same time.emps, causing torques in opposite directions on the two coils, and the hands of the watch follow the movement of the two couples. The angle of deviation is determined by the ratio of the two currents. The additional resistance remains unchanged, so the current value depends only on the size of the resistance to be measured.
Principles of selection of megometers (insulation resistance meters)
(1) Selection of rated voltage levels . Generally speaking, for equipment with a voltage rating below 500 V, a 500 V or 1000 V megger should be used; For equipment with a rated voltage above 500V, a 1000V~2500V megometer should be used.
(2) Resistance range selection. There are two small black dots on the scale of the megger dial, and the area between the small black dots is the precise measurement area. Therefore, when selectingection of a table, the insulation resistance value of the equipment under test must be within the precise measurement area.
Use of a megometer (insulation resistance meter)
(1) Calibration of the meter. Before measurement, the megometer should be subjected to open circuit and short circuit test to check whether the megometer is in good condition. Open the two connecting wires, shake the handle, the pointer should point to "∞", then short circuit the two connecting wires, the pointer should point to "0", if the above conditions are met, it is good, otherwise it cannot be used.
(2) the equipment under test is disconnected from the line and the large capacity equipment must be unloaded.
(3) Select a megger with a constant voltage level.
(4) When measuring insulation resistance, generally only the "L" and "E" terminals are used, but when measuringre of the insulation resistance of the cable to ground or the current leakage of the equipment under test. In more serious cases, use the "G" terminal and connect the "G" terminal to the shielding layer or shell . After the circuit is connected, you can turn the shake handle clockwise. Shaking speed should be slow to fast. When the rotating speed reaches about 120 revolutions per minute (ZC-25 type), continue to rotate at a constant speed and take. reading after 1 minute. Take the measurements by shaking and don't stop to read.
(5) Remove the wires and unload. When the reading is complete, shake slowly while removing the wires, then discharge the device under test. The discharge method involves removing the ground wire used during the measurement from the megometer and short-circuiting it with the device under test (so as not to discharge the megometer).
Megometer measurement steps (meterinsulation resistance)
The use of the megometer is divided into the following steps:
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(1) Power failure, power test and discharge of the equipment under test
(2) Select the appropriate equipment; depending on the equipment tested The megometer;
(3) Check the megger, which is divided into open circuit inspection and short circuit inspection. An open circuit means both tests. the pens do not touch and the grip and pointer rotate at a constant speed, see.To infinity; short circuit check is to rotate the flip-flop at a constant speed, the two test leads are in contact, and the pointer points to zero;
(4) First; connect segment E to the ground, and rotate the seesaw at a constant speed to reach 1 minute, then connect segment L to the line;
(5) After. the measurement is finished, releasedrge.
Precautions for megometer (insulation resistance meter)
(1) It is prohibited to measure during lightning or near high temperatures. Voltage Equipment Insulation resistance can only be measured when the device is not loaded and no electricity is induced.
(2) During the shaking test, no one may work on the equipment under test.
(3) The meter wires cannot be twisted together, but must be separated.
(4) It is strictly prohibited to touch the megger with bare hands before it stops rotating or before the device under test is discharged. When removing the wires, do not touch the metal parts of the cables.
(5) At the end of the measurement, large capacity equipment must be unloaded.
(6) Its accuracy must be checked regularly.
The instructions onway to use the megger are as follows:
1. Before measurement, the megger should be kept in a horizontal position, hold the meter body with your left hand, and shake the megger with your right handle, the rotation speed is about 120 rpm, and the pointer should point towards infinity (∞), otherwise it means that the megger is defective
2. Before measurement, the power supply to the electrical device and the circuit under test must be cut off and the relevant components must be be inspected. Perform a temporary grounding discharge to ensure the safety of people and the megger and the accuracy of the measurement results. ;
3. The wiring must be correct when measuring. The megger has three terminals (L, E, G). When measuring loop resistance to ground, the L end is connected to the exposed conductor of the loop and the E end is connected to the ground wire or la metal shell when measuring the insulation resistance of the loop, the first and; the last ends of the loop are connected to L and E respectively; cable insulation is measured when measuring resistance, in order to prevent leakage current on the cable surface from affecting the measurement accuracy, the cable shielding layer should be connected to the terminal G;
4. The test lead exiting the megger terminal must be well insulated between the leads and between the leads and ground to avoid affecting the measurement. precision;
5. When shaking the megger, do not touch the megger terminals and the circuit under test with your hands to avoid electric shock;
6. megohmmeter, do not short circuit the terminals to avoid damage
Megger is also called megger, which is a portable uncharged electrical instrument for measuring the insulation resistance of equipment and liness electrics. Megohmmeters are different from multimeters. The power supply inside the megger uses a hand generator that can produce hundreds of volts to thousands of volts. The meter voltage levels are: 500, 1000, 2500 and 5000 V.
Low voltage electrical equipment is usually measured with a 500 V level indicator. Master the use of a megger is the most basic skill of electriciansOne of them, the editor of Xianji.com, will explain in detail the correct use of the vibrating table and the precautions to take.