Motor insulation resistance is one of the very important parameters. It is important enough for the normal operation of an electrical device, and therefore it must be measured with a certain frequency. The main purpose of the measurements is to check the insulation condition and determine the suitability of the machine for subsequent testing or operation.
Why is insulation testing necessary?
It is important to understand here that those materials that are used as an insulating winding for an electric motor are essentially not pure dielectrics. All of them conduct electric current to a greater or lesser extent. It largely depends on their physical and chemical properties.
In addition to the fact that these factors influence the insulation resistance index, here it is also necessary to take into account the fact that such a characteristic as humidity plays a very important role. In addition, mechanical damage, as well as various possible contaminants and dust, can adversely affect this characteristic. Due to all these factors, an operation such as measuring resistance is an integral part of the working process of an electric motor.
Verification Overview
It is necessary to check the insulation resistance of the electric motor at a time when the machine is in a practically cold state, that is, before it starts to work. There are several more specific conditions that must be met in order for the testimony to be true. Firstly, the insulation resistance of the windings of an electric motor at a rated winding voltage of up to 500 V is measured using a megaohmmeter of 500 V. If the rated operating voltage of the winding is more than 500 V, then it is necessary to change the device to a more powerful one, up to 1 kV.
Sometimes, in order to measure the insulation resistance of an electric motor, i.e. its windings, it is necessary to use sufficiently powerful measuring equipment. Most often this applies to those cases when the rated operating voltage of the electrical equipment itself is up to 6 kV. In this case, you need to use a 2.5 kV megaohmmeter, which additionally has a motor drive or a static AC rectification circuit.
Insulation measurement in relation to various parts
When it comes to measuring the insulation resistance of an electric motor, it must be understood here that it is determined in relation to something. If measuring work is carried out in relation to the machine body or windings, then they must be carried out alternately for each circuit.
Measurement of the insulation resistance of the electric motor, or rather its windings with a three-phase current, which are usually paired in such connections as a star or a triangle, is carried out immediately for the entire winding in relation to the housing, and not alternately, as described previously.
Water cooled winding
Measurement of insulation resistance of an electric motor, which has a winding with direct water cooling, should be carried out using a megohmmeter with built-in shielding. Here you need to pay attention to the fact that the clamp, which is connected to the screen, must be connected to the catchment. Collectors themselves should not have any metal connection with an external distillate winding power system.
After all measurements in the circuit have been completed, it is necessary to discharge it. For this, an electrical connection is used with the grounded casing of the machine. If the rated operating voltage of the windings is 3 kV or more, then the electrical connection time with the housing should be as follows:
- electrical equipment, the power of which is up to 1000 kW (kV * A) - a duration of at least 15 seconds for a complete discharge;
- if you need to discharge a machine whose power exceeds 1000 kW, then the time should be increased to 1 minute or more.
The same megaohmmeter can also be used for discharge. If you use a device with a power indicator of 2.5 kV, then the time for discharging any electric motor, regardless of its power, is at least 3 minutes.
Rotor and stator resistance
Permissible insulation resistance of the electric motor is one of its main indicators, which indicate the state of the insulating winding of both the rotor and the stator of the electric motor. It is worth mentioning that the measurement work on the stator winding is always accompanied by the determination of such an indicator as the absorption coefficient.
It is possible to measure the insulation resistance of the rotor only on synchronous equipment, as well as on electric motors with a phase rotor. In this case, the voltage must be 3 kV or more, or the power must be above 1 MW. For such equipment, the insulation resistance should be at least 0.2 megohms. The norm of insulation resistance of the electric motor will increase with the growth of its operational characteristics. It is worth mentioning here that the absorption coefficient is also determined only if there is a voltage of more than 3 kV or a power of more than 1 MW.
Preparation of the instrument for measurement
In order to successfully carry out all measurements, it is necessary to prepare the equipment.
First you need to charge the battery or the battery if you are using a MIC-2500 megohmmeter. After that, it is necessary to set the value of the test voltage. If, for example, the ESO202 pointer device is used for measurement, then it should be located strictly horizontally. For the same device, before starting work, it is necessary to set not only the voltage value, but also the required measurement limit, set the scale. After this, you need to check the operability of the measuring apparatus. To do this, close the measuring probes of the device with each other and begin to rotate the handle of the generator. The frequency of rotation should be 120-140 revolutions per minute. With these parameters, the arrow of the device should show "0". After this, the probes open, and the handle must again begin to rotate at the same speed. In this case, the device should show a resistance of 10 4 MΞ©.
Preparing the electric motor for inspection
In addition, before proceeding to check the insulation resistance of the electric motor, it is necessary to open its input device, which is called boron. After that, the insulators should be thoroughly wiped from any dirt and dust. Only then can the measuring device be connected according to its circuit.
During direct resistance measurement, it is necessary to take readings from the device every 15 seconds. The actual value of the resistance of the winding is the value that will be removed 60 seconds after the start of the procedure. And the ratio of the values ββtaken in 60 seconds to the value obtained after 15 seconds is called the absorption coefficient, which was mentioned earlier.
Measurement results
If the electric motor has a rated operating voltage of 0.4 kV, that is, it is part of a group of devices with a voltage of up to 1000 V, then a megaometer measuring 2.5 kV in one minute is considered a high voltage test.
To avoid a negative result when measuring the stator winding resistance of a synchronous motor, it is necessary to short-circuit and ground the rotor winding. If this is not done, then the negative result will be that during the measurement the rotor insulation will be damaged.