Rotating motor

There are many types of rotating electric machines. According to their functions, they are divided into generators and motors. According to the nature of voltage, they are divided into DC motors and AC motors. According to their structures, they are divided into synchronous motors and asynchronous motors. According to the number of phases, asynchronous motors can be divided into three-phase asynchronous motors and single-phase asynchronous motors; according to their different rotor structures, they are divided into cage and wound rotor types. Among them, cage three-phase asynchronous motors are simple in structure and manufactured. Convenience, low price, reliable operation, the most widely used in various motors, the largest demand. The lightning protection of rotating electrical machines (generators, adjusting cameras, large motors, etc.) is much more difficult than that of transformers, and the lightning accident rate is often higher than that of transformers. This is because the rotating electric machine has some characteristics different from the transformer in terms of insulation structure, performance and insulation coordination.
(1) Among the electrical equipment of the same voltage level, the impulse withstand voltage level of the insulation of the rotating electrical machine is the lowest.
The reason is: ①The motor has a high-speed rotating rotor, so it can only use solid medium, and cannot use solid-liquid (transformer oil) medium combination insulation like a transformer: during the manufacturing process, the solid medium is easily damaged, and the insulation is Voids or gaps are prone to occur, so partial discharges are prone to occur during operation, leading to insulation degradation; ②The operating conditions of motor insulation are the most severe, subject to the combined effects of heat, mechanical vibration, moisture in the air, pollution, electromagnetic stress, etc. , The aging speed is faster; ③The electric field of the motor insulation structure is relatively uniform, and its impact coefficient is close to 1. The electric strength under overvoltage is the weakest link. Therefore, the rated voltage and insulation level of the motor cannot be too high.
(2) The residual voltage of the lightning arrester used to protect the rotating motor is very close to the impulse withstand voltage of the motor, and the insulation margin is small.
For example, the factory impulse withstand voltage test value of the generator is only 25% to 30% higher than the 3kA residual voltage value of the zinc oxide arrester, and the margin of the magnetic blown arrester is smaller, and the insulation margin will be lower as the generator runs. Therefore, it is not enough for the motor to be protected by a lightning arrester. It must be protected by a combination of capacitors, reactors, and cable sections.
(3) The inter-turn insulation requires that the steepness of the intruding wave is strictly limited.
Because the inter-turn capacitance of the motor winding is small and discontinuous, the overvoltage wave can only propagate along the winding conductor after it enters the motor winding, and the length of each turn of the winding is much larger than that of the transformer winding, acting on two adjacent turns The overvoltage is proportional to the steepness of the intruding wave. In order to protect the inter-turn insulation of the motor, the steepness of the intruding wave must be strictly limited.
In short, the lightning protection requirements of rotating electrical machines are high and difficult. It is necessary to fully consider the protection requirements of the main insulation, inter-turn insulation and neutral point insulation of the winding.