How to use the watch

The instrument used to measure high-value resistance and insulation resistance is called a shaker. Now new products are usually called insulation resistance testers or insulation resistance meters, sometimes also called megohmmeters. The shape of the old rocking watch is shown below:
The main structure of the rocking meter (insulation resistance meter) is composed of a hand-cranked generator, an electromagnetic head without mechanical reaction, and external terminals (L: line segment, E: grounding terminal, G: shielding terminal). A new type of insulation resistance tester usually has a shape similar to a digital multimeter.
The working principle of the rocking meter (insulation resistance meter) is shown in the following figure. Its magnetoelectric head has two movable coils at an angle to each other, which is installed outside a notched cylindrical iron core and fixed with the pointer On a rotating shaft, the movable part constituting the watch head is placed in a permanent magnet, and the air gap between the magnetic pole of the magnet and the iron core is uneven. Since the pointer has no damping spring, the pointer can stay in any position when the meter is not in use.
By shaking the handle, the DC generator can output current. Among them, one current I1 flows into the loop of the coil 1 and the measured resistance Rx, and the other current I2 flows into the loop of the coil 2 and the additional resistance Rf. Set the resistance of the coil 1 to R1 and the coil 2 The resistance is R2, according to Ohm's law:
I1 = U ÷ (Rc + R1 + Rx), R2 = U ÷ (Rf + R2)
The energized coil in the magnetic field is subjected to the magnetic field force, so that the coil 1 generates a rotating torque M1, and the coil 2 generates a rotating torque M2. Since the two coils are wound in opposite directions, the direction of M1 and M2 is opposite. The combined torque of the two torques makes the The pointer is deflected. When M1 = M2, the pointer is still. At this time, the pointer indicates the insulation resistance value of the device under test.
When Rx is disconnected (ie X = ∞ (infinity)), I1 = 0, M1 = 0, the pointer deflects to the left under the action of M2, and finally points to the scale Rx = ∞, if Rx is shorted (ie Rx = 0) At this time, I1 is the largest and M1 is the largest, so that the pointer is deflected clockwise, and the pointer points to the standard scale Rx = 0. According to this principle, the quality of the shaking table can be checked.
Let me talk about the selection of the lower shaker. For equipment with a rated voltage of less than 500 volts, choose a 500 volt or 1000 volt shaker (megaohm meter), and a rated voltage above 500 volts to choose a 1000 volt to 2500 volt megohmmeter.
Types of shake table
1. Ground shake meter
Measurement of grounding resistance usually uses a ground shaker. The working principle of the ground shaker uses an AC bridge, so the output of the shaker is AC. This is mainly because during the measurement of the grounding resistance, a battery will be formed between the grounding wire to be measured and the ground, which will cause errors when measured with DC. At the same time, the DC conductor will also polarize the ground conductor and increase the contact resistance.
2. Megohm shaker
The megohm shaker is usually used to measure the insulation resistance. The megohm shaker is different from a multimeter. The power supply in the megohmmeter uses a hand-cranked generator that can generate hundreds of volts to thousands of volts. The voltage levels of the meter are: 500, 1000, 2500, 5000V. Low-voltage electrical equipment is generally measured with a 500V-rated meter. The generators in the megohm shaker generally emit alternating current, which becomes DC output after rectification.

The working principle of shaking table
There are two coils connected to the hands of the shaking table, one is connected in series with the additional resistance R in the table; the other is connected in series with the measured resistance R, and then connected to the hand crank generator together. When the generator is shaken by hand, current flows through the two coils at the same time, and torques in opposite directions are generated on the two coils. The hands deflect a certain angle with the magnitude of the combined torque of the two torques. The angle depends on the ratio of the two currents, the additional resistance is constant, so the current value only depends on the size of the resistance to be measured. Selection principle of shaking table
(1) Selection of rated voltage level. In general, equipment with a rated voltage below 500V should use a 500V or 1000V shake meter; equipment with a rated voltage above 500V should use a 1000V ~ 2500V shake meter.
(2) Selection of resistance range. There are two small black dots on the dial scale of the shaking table. The area between the small black dots is the accurate measurement area. Therefore, when selecting the table, the insulation resistance of the device under test should be within the accurate measurement area.
Use of shaker
(1) School table. Before the measurement, the shaker should be subjected to an open and short circuit test to check whether the shaker is in good condition. Open the two connecting wires, shake the handle, the pointer should point at "∞", and then short-circuit the two connecting wires, the pointer should point at "0". Those who meet the above conditions are good, otherwise they cannot be used.
(2) The device under test is disconnected from the line, and the device with large capacitance must be discharged.
(3) Select a shaker whose voltage level meets.
(4) When measuring the insulation resistance, generally only use the "L" and "E" terminals, but when measuring the insulation resistance of the cable to the ground or the leakage current of the device under test is serious, the "G" terminal should be used, and "G" is terminated with a shield or enclosure. After the line is connected, you can turn the crank in a clockwise direction. The shaking speed should be from slow to fast. When the speed reaches about 120 revolutions per minute (ZC-25 type), keep the rotation at a constant speed. The reading will be taken after 1 minute, and the Do not stop reading while shaking.
(5) Disconnect and discharge. After the reading is completed, remove the wire while shaking slowly, and then discharge the device under test. The discharge method is to remove the ground wire used in the measurement from the shaker and short it with the device under test (not the discharge of the shaker).
Shaker measurement steps
The use of shaker is divided into the following steps:
(1) Power failure, power inspection, and discharge of the equipment under test;
(2) Select the appropriate shaker according to the equipment under test;
(3) Check the shaker, it is divided into open circuit check and short circuit check, open circuit is that the two test leads are not in contact, rotate the handle at a constant speed, the pointer points to infinity; short circuit check is to rotate the handle at a constant speed, the two test pens touch, the pointer points to zero ;
(4) Ground segment E first, rotate the handle at a constant speed to reach 120 revolutions per minute, then connect segment L as a line
(5) After measuring, discharge.
Notes on shaking the watch
(1) It is forbidden to measure the insulation resistance during lightning or near high-voltage equipment. It can only be measured when the equipment is not energized or induced.
(2) During the shake test, no one can work on the device under test.
(3) Do not twist the watch wires together, but separate them.
(4) It is strictly forbidden to touch with the hand before shaking the watch or before the device under test is discharged. When removing the wire, do not touch the metal part of the lead.
(5) At the end of the measurement, discharge the large-capacity equipment.
(6) Check its accuracy regularly.

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