Power Transmission

Power Transmission

It consists of power generation, power supply (transmission, substation, distribution), power consumption facilities, and secondary facilities such as adjustment control and relay protection and safety automatic devices, metering devices, dispatch automation, and power communication required to ensure its normal operation. constitute a unified whole.

The power system is an electric energy production and consumption system composed of power plants, transmission and transformation lines, power supply and distribution stations, and electricity consumption. Its function is to convert primary energy in nature into electrical energy through power generation devices, and then supply the electrical energy to each user through power transmission, transformation and distribution. In order to realize this function, the power system also has corresponding information and control systems at each link and at different levels to measure, adjust, control, protect, communicate and dispatch the electric energy production process to ensure that users obtain safe and high-quality electric energy. 

The main structure of the power system includes power sources (hydropower stations, thermal power plants, nuclear power plants and other power plants), substations (step-up substations, load center substations, etc.), transmission and distribution lines and load centers. The power points are also connected to each other to realize power exchange and regulation between different regions, thereby improving the safety and economy of power supply. The network composed of transmission lines and substations is usually called the power network. The information and control system of the power system consists of various testing equipment, communication equipment, safety protection devices, automatic control devices, monitoring automation and dispatching automation systems. The structure of the power system should ensure the reasonable coordination of electric energy production and consumption on the basis of advanced technical equipment and high economic benefits.

Insulated bearings

The electrical insulating bearing adopts a special spraying process, and the outer surface of the bearing is sprayed with a high-quality coating. The coating has a strong bond with the substrate and has good insulation performance, which can avoid the electric corrosion of the bearing by the induced current and prevent the current from causing damage to the rolling elements. and improve bearing life. The process has been continuously improved. In insulated bearings, there is a 100μm-thick coating on the surface of the outer or inner ring, which can withstand voltages up to 1000VDC. A special spraying process creates a coating of uniform thickness and extremely strong adhesion, which is further treated to make it impervious to moisture and humidity.

Inspection of insulation of insulated bearings

1. The purpose and scope of the inspection

In order to ensure the insulation performance, it is necessary to measure the insulation of the insulated bearing, which includes the insulation resistance, shaft voltage and shaft current of the bearing. The purpose of measuring the motor shaft voltage is to understand the magnitude of the motor shaft current; the purpose of measuring the motor shaft current is to directly obtain the value of the current flowing through the motor bearing.

Because the damage of shaft current mainly occurs in motors powered by variable frequency power supply with frame size of 280 and above and ordinary large and medium-sized high-voltage motors. Therefore, this inspection is generally only carried out for these types of motors during the initial trial production or for individual motors that are found to have a serious impact on the bearing due to shaft current during use.

2. Measurement of insulation resistance of insulated bearings

When measuring, the bearing can be carried out alone, but it is best to carry out the measurement after the bearing is installed on the rotating shaft. Generally, the insulation resistance meter of 250V specification is selected. Connect the E end of the instrument to the rotating shaft; after covering the insulating part of the bearing (such as the outer ring) with aluminum foil, bind the aluminum foil with bare copper wire to fix it, and then connect it to the L end of the instrument; or install the insulating bearing into the bearing seat or motor end In the bearing chamber of the cover, measure the insulation resistance between the rotating shaft and the bearing seat or the motor end cover when there is no electrical path between the bearing seat or the motor end cover and the rotating shaft on which the insulating bearing is installed. The measurement operation is exactly the same as measuring the motor and winding insulation resistance to ground.

3. Measurement method of bearing current

For motors using rolling bearings, the shaft current is measured. Install an insulating ring between the bearing of the non-shaft extension end of the motor and the casing (a dry insulating sheet is placed between the bearing and the rotating shaft) or use an insulating bearing to ensure that the motor bearing is well insulated.

Connect the ammeter in series to the metal parts in contact with the two sides of the bearing insulating layer, run under no-load at rated voltage and rated frequency, and measure the current value, that is, the shaft current.

For motors using sliding bearings and rolling bearings, if the above method cannot be measured, the shaft current can be measured by placing a current transformer on the shaft.

Product category

Electrically insulated deep groove ball bearings

Electrically insulated angular contact ball bearings

Electrically Insulated Cylindrical Roller Bearings

Inner or outer ring insulated bearings with oxide coating

Hybrid bearings with electrically insulating ceramic rolling elements

Advantages

Electrically insulated bearings can avoid damage caused by electrical corrosion, so they can be used in motors to ensure more reliable operation than ordinary bearings. It is more cost-effective and reliable than other insulation methods, such as shaft or housing insulation. The outer dimensions and basic technical characteristics of insulated bearings are the same as those of non-insulated bearings, so they are 100% interchangeable. It is suitable for motors, generators, especially variable frequency motors, which are more widely used.

Application of Insulated Bearings in Motors

1. Causes and hazards of the formation of motor shaft voltage and bearing current

During motor operation, any imbalance in the stator and rotor magnetic circuits, or in the phase currents around the shaft, can create a rotating system flux linkage. When the shaft rotates, these flux linkages can generate a potential difference across the shaft, which is called the shaft voltage. The shaft voltage can excite a 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.

In addition, the rotor core has a lot of residual magnetism. For a wound rotor motor, if two or more windings are short-circuited to the rotor core or the rotating shaft, shaft voltage and shaft current will also be generated.

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, and the length of the cable. The greater the power of the motor, the higher the drive voltage, the steeper the rising edge of the drive voltage, and the shorter the cable, the greater the bearing current.

2. Measures to block shaft current - use insulated bearings

In order to avoid the burn damage to the bearing caused by the shaft current, effective measures should be taken to isolate the shaft current.

For large motors with independent bearing housings at both ends, a spacer made of insulating material can be placed between the bearing housing and the metal base. For a motor with an ordinary bearing and a casing assembled into one body, an insulating bearing is generally used at one end (often arranged at the non-spindle extension end), and for occasions with higher requirements, an insulating bearing is installed at both ends. The insulating bearing used is generally a method of adding (generally coating) an insulating layer to the outer ring. In some occasions, the inner and outer rings are both additionally insulating.