1. Causes of corona
Corona is generated because an uneven electric field is generated by an uneven conductor. When the voltage rises to a certain value near the electrode with a small curvature radius around the uneven electric field, a discharge will occur due to free air, forming a corona. Because the electric field at the periphery of the corona is very weak and no collision dissociation occurs, the charged particles at the periphery of the corona are basically electric ions, and these ions form the corona discharge current. Simply put, corona is generated when a conductor electrode with a small radius of curvature discharges into the air.
2. Causes of corona in high-voltage motors
The electric field of the stator winding of the high-voltage motor is concentrated at the ventilation slots, linear exit slots, and winding ends. When the field strength reaches a certain value at a local location, the gas undergoes local ionization, and blue fluorescence appears at the ionized location. This is the corona phenomenon. .
3. Dangers of corona
The corona produces thermal effects and ozone and nitrogen oxides, which increase the local temperature in the coil, causing the adhesive to deteriorate and carbonize, and the strand insulation and mica to turn white, which in turn causes the strands to become loose, short-circuited, and the insulation ages.
In addition, due to poor or unstable contact between the thermosetting insulating surface and the tank wall, spark discharge in the gap in the tank will be caused under the action of electromagnetic vibration. The local temperature rise caused by this spark discharge will seriously erode the insulation surface. All this will cause great damage to the motor insulation.
4. Measures to prevent corona
(1) Generally, the insulation material of the motor is made of corona-resistant material, and the dipping paint is also made of corona-resistant paint. When designing the motor, the harsh working conditions must be considered to reduce the electromagnetic load.
(2) When making the coil, wrap anti-sun tape or apply anti-sun paint.
(3) The slots of the core are sprayed with low-resistance anti-blooming paint, and the slot pads are made of semiconductor laminates.
(4) After the winding insulation treatment, first apply low-resistance semiconductor paint on the straight part of the winding. The length of the paint should be 25mm longer on each side than the core length. Low-resistance semiconductor paint generally uses 5150 epoxy resin semiconductor paint, whose surface resistance is 103~105Ω.
(5) Since most of the capacitive current flows from the semiconductor layer into the core outlet, in order to avoid local heating at the outlet, the surface resistivity must gradually increase from the winding outlet to the end. Therefore, apply high-resistance semiconductor paint once from the vicinity of the winding exit notch to the end of 200-250mm, and its position should overlap with the low-resistance semiconductor paint by 10-15mm. High-resistance semiconductor paint generally uses 5145 alkyd semiconductor paint, whose surface resistivity is 109 to 1011.
(6) While the semiconductor paint is still wet, wrap a half layer of 0.1mm thick dewaxed glass ribbon around it. The dewaxing method is to put the alkali-free glass ribbon into the oven and heat it to 180~220℃ for 3~4 hours.
(7) On the outside of the glass ribbon, apply another layer of low-resistance semiconductor paint and high-resistance semiconductor paint. The parts are the same as steps (1) and (2).
(8) In addition to anti-halation treatment for the windings, the core also needs to be sprayed with low-resistance semiconductor paint before coming off the assembly line. The groove wedges and groove pads should be made of semiconductor glass fiber cloth boards.
Post time: Sep-17-2023