Induction motor start why current large motor maintenance

When the induction motor is in the stopped state, from the electromagnetic point of view, just like a transformer, the stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed rotor winding is equivalent to the secondary coil of the transformer short-circuited. There is no electrical connection between the stator windings and the rotor windings, but only magnetic connection. The magnetic flux is closed through the stator, air gap and rotor core. When the closing moment, because the rotor inertia has not yet turned up, the rotating magnetic field cuts the rotor winding at the maximum cutting speed -- synchronous speed, so that the rotor winding induces the highest possible potential. Therefore, a large current flows through the rotor conductor, which generates magnetic energy to offset the stator magnetic field, just like the secondary magnetic flux of a transformer to offset the role of the primary magnetic flux. The stator increases the current automatically in order to maintain the original flux corresponding to the power supply voltage at this time. Because the current of the rotor is very large at this time, the stator current also increases greatly, even up to 4~7 times the rated current, which is the reason for the large starting current. Why small after starting: with the motor speed increases, the stator magnetic field cutting rotor conductor speed decreases, the rotor conductor induction potential decreases, the rotor conductor current also decreases, so the stator current used to offset the influence of the rotor current generated by the magnetic flux that part of the current also decreases, so the stator current from large to small, until normal.

In order to effectively eliminate low-efficiency motors and speed up the promotion of high-efficiency motors, the new version of the national standard "Energy Efficiency Limits and Energy Efficiency Grades of Small and Medium-sized three-phase asynchronous Motors" issued by the Standardization Administration was implemented in September 2012, which has a great impact on the small and medium-sized motor industry. At present, the Y, Y2, Y3 three-phase asynchronous motor which is high-volume production in our country will be prohibited, and the YX3 series high-efficiency three-phase asynchronous motor which enjoys national benefit engineering will probably no longer get policy subsidy. High efficiency motor research and development and promotion is like an arrow in the string, the industry attention is unprecedented.

In motor medium electrical equipment, insulation material is the weakest link. Insulation materials are especially vulnerable to high temperatures, which can accelerate aging and damage. Different insulation materials have different heat resistance performance, the use of different insulation materials of electrical equipment, its ability to withstand high temperature is different. Therefore, the general electrical equipment has specified its maximum working temperature.