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Insulated bearings 6044-M
1. What is an insulated bearing?
The so-called insulating bearing refers to a general term that can block the flow of current and the bearing itself has insulating properties. Its insulation performance is usually ensured by coating a layer of insulating material on the outer or inner ring of the bearing through a special process, or its rolling elements are made of ceramics. The name and code of insulated bearings adopt the international bearing code. The outer ring insulated bearing code is vl0241, and the inner ring insulated bearing code is vl2071.
2. Advantages of insulated bearings
1. Inner or outer ring coating
Insulated bearings use a special spraying process to spray high-quality coating on the outer surface of the bearing. The coating has strong adhesion to the substrate and good insulation performance, which can avoid electrical corrosion of the bearings due to induced current, prevent current damage to grease, rolling elements and raceways, and extend the service life of the bearings. The process is constantly improving. In insulated bearings, the surface of the outer or inner ring has a 100μm thick coating that can withstand voltages up to 1000V DC. The special spraying process creates a coating with uniform thickness and strong adhesion, which can be further treated to prevent moisture and humidity.
2. Avoid damage
Insulated bearings prevent damage caused by electrical corrosion. Therefore, they ensure that the motor runs more reliably than ordinary bearings. It is more cost-effective and reliable than other insulation methods such as shaft or housing insulation. The overall dimensions and basic technical characteristics of insulated bearings are the same as non-insulated bearings and are 100% interchangeable. Suitable for motors and generators, especially variable frequency motors.
3. Ceramic rolling elements
This kind of bearing uses ceramic rolling elements (balls or rollers) made of Si3N4 material; it has excellent insulation properties, its capacitance is basically 40pf, and its insulation properties are better than coated bearings. At high temperatures, its DC impedance can also reach the G ohm range and has good insulation properties; ceramic rolling elements have excellent wear resistance and low lubrication requirements. Especially suitable for high-speed, low-friction, low-temperature operation; runs well under dry friction conditions; for small rolling bearings, ceramic rolling element insulated bearings have good economic performance.
3. Classification of insulated bearings
The main categories of electrically insulated bearing products are: electrically insulated deep groove ball bearings, electrically insulated angular contact ball bearings, electrically insulated cylindrical roller bearings, hybrids of insulated bearings with oxide coatings on the inner or outer rings and electrically insulated ceramic rolling elements. bearings.
1. Under what circumstances are electrically insulated bearings used?
Even if the production of motors is strictly controlled during the manufacturing process, it is difficult to completely avoid magnetic asymmetry in the armature and rotor. This asymmetry creates a voltage between the stator and thus a current through the bearings, especially in AC motors. This current will cause corrosion pits, melt marks, electrical corrosion file lines, discoloration, wear and other damage on the surfaces of bearing raceways and rolling elements. Of course, this phenomenon will also appear on motors using variable frequency and variable frequency technology. In order to block the current flowing through the bearing, it is necessary to cut off its circuit, so insulated bearings and insulated bearing chambers came into being.
2. Insulation inspection of insulated bearings
(1) Purpose and scope of inspection
In order to ensure the insulation performance, insulation measurements of the insulated bearings are required, including the bearing's insulation resistance, shaft voltage and shaft current. The purpose of measuring the motor shaft voltage is to understand the motor shaft current; the purpose of measuring the motor shaft current is to directly obtain the current value flowing through the motor bearings.
Because damage from shaft current mainly occurs on motors driven by variable frequency power supplies with frame numbers of 280 and above and ordinary large and medium-sized high-voltage motors. Therefore, this kind of inspection is generally only carried out for the first trial production of this type of motor or the design improvement of some motors where the shaft current has a serious impact on the bearings.
2) Measurement of insulation resistance of insulated bearings
When measuring, the bearing can be measured separately, but it is best to install the bearing on the rotating shaft before proceeding. Generally, a 250V insulation resistance meter is used. Connect the e-end of the instrument to the rotating shaft; wrap the insulated part of the bearing (such as the outer ring) with aluminum foil, tie it with bare copper wire to fix the aluminum foil, and then connect it to the l-end of the instrument; or install the insulated bearing into the bearing seat or motor end cover. In the seat, when there is no electrical path between the bearing seat or motor end cover and the motor, measure the insulation resistance between the shaft and the bearing seat or motor end cover. Install the insulated bearing on the rotating shaft.
(3) Measurement method of bearing current
For motors using rolling bearings, the shaft current should be measured as shown below. Install an insulating ring between the bearing at the non-shaft extension end of the motor and the casing (insert an insulating sheet between the bearing and the rotating shaft) or use an insulating bearing to ensure good insulation of the motor bearing.
(4) Measurement method of bearing current
Connect the ammeter in series to the metal parts in contact with both sides of the bearing insulation layer, run it without load at rated voltage and rated frequency, and measure the current value, that is, the shaft current.
If motors using sliding bearings and rolling bearings cannot be measured according to the above method, a current transformer can be placed on the shaft to measure the shaft current.
4. Application of insulated bearings in motors
1. Causes and hazards of motor shaft voltage and bearing current
During the operation of the motor, any imbalance in the stator and rotor magnetic circuits or around the axis phase current will produce flux linkage in the rotating system. When the shaft rotates, these flux links produce a potential difference across the shaft, called shaft voltage. The shaft voltage can stimulate circulation in the loop (closed circuit) formed by the shaft and the casing through the bearings at both ends. This current is called axis current. like
In addition, there is a large amount of residual magnetism in the rotor core. For wound rotor motors, if two or more windings are short-circuited to the rotor core or shaft, shaft voltage and shaft current will also occur.
The size of the bearing current is related to the motor structure, motor power, driving voltage amplitude, pulse rise time, cable length and other factors. The greater the power of the motor, the higher the driving voltage, the steeper the rising edge of the driving voltage, the shorter the cable, and the greater the carrying current.
2. Measures to block shaft current - use insulated bearings
In order to prevent the shaft current from burning the bearings, effective measures should be taken to isolate the shaft current.
For large motors with independent bearing seats at both ends, an isolation gasket made of insulating material can be placed between the bearing seat and the metal base. For ordinary motors in which the bearing and casing are integrated, an insulated bearing is generally used at one end (often arranged at the non-main shaft extension end). For occasions with high requirements, insulated bearings are installed at both ends. The insulated bearings used generally add (usually coat) an insulating layer to the outer ring. In some cases, the method of adding an insulating layer to both the inner and outer rings is also used.
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