What does a megohmmeter do? Is a megger a megohmmeter?

Commonly known as a shake meter, it is a commonly used measuring instrument. Megohmmeters are mainly used to check the insulation resistance of equipment, household appliances or electrical lines to ground and between phases to ensure that these equipment, electrical appliances and lines are working in normal condition and to avoid accidents such as electric shock, casualties and equipment damage. Most megohmmeters are powered by hand-operated generators, so they are also called megohmmeters. Its scale is in megaohms (mω).

1. Structure

The two coils are fixed on the same axis and perpendicular to each other. One coil is connected in series with the resistor r, the other coil is connected in series with the resistor rx under test, and both are connected in parallel to DC.

2. Working principle

During measurement, the currents passing through the coil are i1=u/(r1+r), i2=u/(r2+rx), where r1 and r2 are the coil resistances. The coil is affected by the magnetic field and produces two torques in opposite directions. , t1 = k1*i1*f1(α), t2 = k2*i2*f2(α). f1(α) and f2(α) are respectively the functional relationship between the magnetic induction intensity at the location of the two coils and the deflection angle α.

The movable part of the meter is deflected by the torque until the torques produced by the two coils are balanced.

When the torques generated by the two coils are balanced, there is t1 = t2

That is k1*i1*f1(α) = k2*i2*f2(α)

The above formula shows that the deflection angle α is related to the ratio of the currents in the two coils, so it is called a current ratio meter.

Conclusion: 1. The deflection angle α has a certain functional relationship with the measured resistance rx, so the α angle can reflect the size of the measured resistance.

2. The deflection angle α of the instrument has nothing to do with the power supply voltage u, so the rotation speed of the hand generator does not affect the reading.

Megohmmeter, commonly known as megohmmeter, is used to measure large resistance and insulation resistance. Its measurement unit is megaohm (mω), so it is called megohmmeter. There are many types of megohmmeters, but their functions are roughly the same.

1. Selection of megger

It is stipulated that the voltage level of the megohmmeter should be higher than the insulation voltage level of the object under test. Therefore, when measuring the insulation resistance of equipment or lines with a rated voltage below 500v, you can use a 500v or 1000v megohmmeter; when measuring the insulation resistance of equipment or lines with a rated voltage above 500v, you should use a 1000~2500v megger; measurement When installing insulators, a 2500~5000v megger should be used. Under normal circumstances, when measuring the insulation resistance of low-voltage electrical equipment, a megohmmeter with a range of 0~200mω can be used.

2. Measurement method of insulation resistance

The megohmmeter has three terminals. The two larger terminals at the upper end are marked "ground" (e) and "line" (l) respectively. The smaller terminal at the bottom is marked "protection ring" ( or "shielding") (g).

(1) Insulation resistance of line to ground

Connect the "ground" terminal (i.e., e terminal) of the megger to the ground reliably (usually to a grounding body), and connect the "line" terminal (i.e., l terminal) to the line under test, such as As shown in Figure 17a. After the connection is completed, shake the megohmmeter clockwise. The speed will gradually increase. Keep it at about 120 rpm and then shake it at a constant speed. When the speed is stable and the pointer of the meter is also stable, the value indicated by the pointer is the insulation resistance of the object being measured. value.

In actual use, the two terminals e and l can also be connected arbitrarily, that is, e can be connected to the object under test, and l can be connected to the ground body (i.e., grounded), but terminal g must not be connected incorrectly.

(2) Measured insulation resistance

Connect the megohmmeter's e terminal to the chassis (that is, ground), and connect the l terminal to the winding of a certain phase of the motor, as shown in Figure 17b. The measured insulation resistance value is the insulation resistance value of a certain phase to ground. .

(3) Measure the insulation resistance of the cable

When measuring the insulation resistance of the conductive core of the cable and the cable shell, connect the terminal e to the cable shell, connect the terminal l to the wire core, and connect the terminal g to the insulation layer between the cable shell and the core.

3. Note on use

(1) Open circuit and short circuit tests should be conducted before use. Make the l and e terminals disconnected, shake the megohmmeter, the pointer should point to "∞"; short-circuit the l and e terminals, turn slowly, the pointer should point at "0". Both of these items meet the requirements, indicating that the megohmmeter is good.

(2) When measuring the insulation resistance of electrical equipment, you must first cut off the power supply and then discharge the equipment to ensure personal safety and accurate measurement.

(3) The megohmmeter should be placed in a horizontal position when measuring, and the megohmmeter should be pressed firmly to prevent it from shaking during shaking. The shaking speed is 120 rpm.

(4) The lead wire should be a multi-stranded soft wire with good insulation properties. The two leads should not be twisted together to avoid inaccurate measurement data.

(5) After the measurement, the object under test should be discharged immediately. Do not touch the measurement part of the object under test or remove the wires with your hands before the handle of the megger stops rotating and the object under test is not discharged to prevent electric shock.

A megger measures insulation resistance, high voltage, and low current; a grounding megger measures ground resistance, low voltage, and high current.

The difference between a type insulated megohmmeter and a hand-operated insulated megohmmeter:

1. Electronic insulated megohmmeter: each meter has 2 or more rated voltages;

Hand-cranked watch: only one

2. Electronic insulation megger: Stabilizes itself to generate a rated voltage, and the output voltage is stable;

Hand-operated insulated rocker: 120 rpm artificially generates a rated voltage, and the output voltage is stable when the rotational speed is relatively stable.

3. Electronic insulation megohmmeter: convenient for testing, high precision and high degree of automation;

Hand-operated insulated rocker meter: artificially caused large accuracy errors, extremely inconvenient to operate

4. Electronic insulation megger: very convenient for measuring various insulation parameters r15s, r60s, r10min, absorption ratio, and polarization index;

Hand-cranked insulation rocker: It takes 1 minute to measure the absorption ratio and 10 minutes to measure the polarization index;

5. Electronic insulation megohmmeter: not afraid of short-circuit testing, not afraid of current counterattack of the product under test, and automatically discharges the product under test;

Hand-operated insulated meter: does not have this function.