The megaohmmeter (from megaohm and -meter ; the outdated name is a meter ) is an electrical measuring instrument designed to measure large values of resistance. It differs from an ohmmeter in that a relatively high voltage is applied to the measured circuit when measuring the resistance (in most models - 100, 500, 1000 or 2500 volts ).
Megger - the outdated name of the megohm meter. In accordance with GOST 2.105, the documents do not allow the use of turns of colloquial speech, technicisms, arbitrary word formations.
In devices of earlier designs, in order to obtain test high voltages, a built-in electromechanical constant voltage generator with a manual drive from the handle is usually used, the generator operates on the principle of a dynamo machine . Currently, an electronic inverter with a rectifier , powered by batteries or replaceable galvanic cells built into the device, is used as a source of constant high test voltage in megohm meters.
As an indicator in the megohm meters of the earlier designs, dial gauges were used.
Usually a megohmmeter is used to measure the insulation resistance of high-voltage power cables .
The megohmmeter is also used to measure the high resistance of insulating materials ( dielectrics ) of wires and cables, electrical connectors, inter-winding resistance of transformers , windings of electrical machines and other devices, as well as to measure the surface and volume resistances of insulating materials. The measured resistances calculate the coefficients of absorption (moisture) and polarization (aging insulation).
Measurement of insulation resistance with a megohm meter
Insulation resistance characterizes its state at a given time and may vary due to the influence of external conditions, since it depends on a number of factors, the main influencing factors are the temperature and humidity of the insulation at the time of the measurement.
In GOST 183-74, the norms for the permissible minimum insulation resistance are not standardized, since there are no absolute criteria for the minimum permissible insulation resistance. They are usually established in the standards for specific types of machines or in technical specifications for products or materials with the obligatory indication of the temperature at which measurements should be taken, and methods for recalculating the measured resistance, reduced to standard conditions, if the measurements were carried out at a different winding temperature.
Measuring the resistance of the insulation of the windings establishes the possibility of testing the insulation by operating high voltage without the risk of electrical breakdown of insulation that is in good working condition but which has a high humidity.
Measurements are carried out with a megohmmeter, the test voltage of which is selected depending on the nominal operating voltage of the test insulation. For devices with a rated voltage up to 500 V (660) V, 500 V megohm meters are used, for devices with a voltage up to 3000 V, 1000 V megohm meters are used, and for devices with a nominal voltage of 3000 V and more, megohm meters are 2500 V and higher.
The degree of moisture of the insulation is judged not only by the resistance value at the time of measurement, but also by the nature of the change in the megohm meter reading during the measurement process, which is usually carried out for 1 minute. In this case, the readings of the instrument are recorded 15 seconds after the application of the test voltage (sufficient time to establish the readings), this resistance is indicated by R15 "and at the end of the measurement - 60 seconds after the start - the designation R60". The ratio of these readings R60 "/ R15" is called the absorption coefficient (KA). Its value determines the ratio of the polarization current to the leakage current through the dielectric - winding insulation. With wet insulation, the spacecraft is close to 1. With dry insulation, the R60 value is 30-50% more than the R15 value.
The megohmmeter also measures the insulation resistance of thermal converters built into electrical machines, and the insulation resistance of wires connecting thermal converters with external terminals.
The insulation resistance of thermocouples is measured relative to the case of the device and relative to the windings of the machine. This insulation is not designed to work at high voltages of the windings of the machine, so the measurement of its resistance should be carried out with an instrument with a rated voltage not higher than 250 V.
In addition to the insulation resistance of the windings during the on-site testing of the machine, the insulation resistance of the bearings is also measured, which is installed to prevent the currents of bearings from flowing in machines with stand bearings [ clarify ] .
Thus, the insulation resistance of different windings of the same machine with different nominal voltages, for example, the stator and rotor windings of a synchronous motor , should be measured with different megohm meters with different nominal voltages, or with a megohm meter with switchable test voltage.