Views:2 Author:Site Editor Publish Time: 2020-06-21 Origin:Site
Insulated bearings are mainly used for motor products. With the development of the domestic motor industry, the bearing manufacturers in China also invest a lot of capital in the research and development of insulated bearings. The development of insulating bearing technology not only covers the development of basic bearing technology, but also solves the current corrosion of motor bearing shaft in bearing technology.
1. Purpose and Scope of Inspection
In order to ensure the insulation performance, it is necessary to measure the insulation of the insulated bearing, including the insulation resistance, shaft voltage and shaft current of the bearing. The purpose of measuring motor shaft voltage is to know the magnitude of motor shaft current. The purpose of measuring motor shaft current is to directly get the value of motor bearing current. Because the damage of the shaft current mainly occurs in the motor powered by the variable frequency power supply and the ordinary large-sized and medium-sized high-voltage motors. Therefore, this inspection is generally only carried out in the initial trial production of these types of motors or in the improvement of the design of some motors whose shaft current has a serious impact on the bearings.
2. Measuring Method of Insulation Resistance
When measuring, the bearing can be carried out separately, but it is best to install the bearing on the rotating shaft. Insulation resistance meter with 250V specification is generally selected. Connect the E-end of the instrument to the rotating shaft. After the insulating part of the bearing (such as the outer ring) is covered with aluminum foil, fix the aluminum foil with bare copper wire and then connect it to the L-end of the instrument. Or install the insulated bearing into the bearing housing or motor end cover bearing chamber. Measure the insulation resistance between the shaft and the bearing housing or motor end cover when there is no electrical path between the bearing housing or motor end cover and the shaft where the insulated bearing is installed. The measurement operation is the same as the measurement of insulation resistance of motor and winding to ground.
3. Measuring Method of Bearing Current
For the motor with spherical plain bearings, install an insulating ring between the bearing at the non shaft extension end of the motor and the casing (put a dry insulating piece between the bearing and the shaft) or use an insulating bearing to ensure the good insulation of the motor bearing. Connect the ammeter in series to the metal parts contacting with both sides of the bearing insulation layer, operate at no-load under the rated voltage and frequency, and measure the current value, that is, the shaft current. If the motor with sliding bearing and rolling bearing cannot be measured according to the above method, the current transformer can be placed on the shaft to measure the shaft current.
1. The Cause and Harm of the Formation of Motor Shaft Voltage and Bearing Current
In the operation of the motor, any unbalance in the stator and rotor magnetic circuit or the phase current around the shaft can produce the flux linkage of the rotating system. When the shaft rotates, these flux chains can produce potential difference at both ends of the shaft, which is called shaft voltage. The shaft voltage can excite the circulating current in the loop (closed circuit) formed by the shaft and the casing through the bearings at both ends, which is called the shaft current. The magnitude of the bearing current is related to the structure of the motor, the power of the motor, the amplitude of the driving voltage, the rise time of the pulse, the length of the cable and other factors. The higher the power of the motor, the higher the driving voltage, the steeper the rising edge of the driving voltage and the shorter the cable, the greater the bearing current.
2. Measures to Block Shaft Current - Use Insulated Bearing
In order to avoid the damage of shaft current to the bearing, effective measures should be taken to isolate the shaft current. For large motors with independent bearing pedestals at both ends, isolation gaskets made of insulating materials can be placed between the bearing pedestals and metal pedestals. For ordinary motors with assembled bearings and housings, insulated bearings are generally used at one end (often arranged at the non-spindle extension end). For the occasions with high requirements, the insulating bearing is installed at both ends. The insulating bearing used is generally a method of coating an insulating layer to the outer ring. In some cases, an insulating layer is added to both the inner and outer rings.
Generally speaking, it is very difficult to completely eliminate the potential difference inside the bearing. However, if we can prevent or greatly reduce the current through the bearing, we can prevent the bearing from galvanic corrosion. At present, all kinds of insulated bearings are designed for this purpose. Select the bearing insulation method according to the type of voltage generated.