| Squirrel cage induction motors are widely used in industrial and agricultural pro-duction because of their simple structure,low cost and high reliability.Medium and large squirrel cage motors are usually made up of the deep groove or the double squirrel cage,using skin effect to increase starting torque and reduce starting current.Although skin effect can greatly improve the starting performance of the motors,it also brings some negative effects.Considering the skin effect,the rotor broken bar model would be laboriously established and the broken bar of the outer cage is hard to identificat,not to mention the number of broken bars.Moreover,the skin effect affects the accuracy of the no-load test of the squirrel cage motor.In this dissertation,the layered method is used as the basic method to solve the skin effect.On this basis,the modeling of double squirrel cage motor considering skin effect,the steady and transient analysis of broken rotor bars are studied.Meanwhile,the effect of skin effect on no-load test and the method to improve the accuracy of no-load test without increasing cost are also studied.The steady state current distribution of double squirrel cage motor rotor is studied by using the layered method,and the calculation formulas for the current distribution of common-end-ring and independent-end-ring double squirrel cage motors are unified.The distribution law of rotor current under different slip rates is quantitatively analysed.It was found that the independent end-ring type had little effect on the rotor current distribution of double squirrel-cage motors,which provids a theoretical basis for simplifying the rotor model of double squirrel cage motors.The rotor model of double squirrel cage motor is simplified reasonably,ignoring the influence of independent end-rings.In this way,the common end-rings'model and independent end-rings' model are unified by using the common end-rings structure for analyzing.The concept of equivalent deep groove is put forward.The impedance of upper and lower cages are replaced by equivalent deep groove impedance.The impedance values of different equivalent deep groove states are calculated by layered method.The rotor multi-loop equation of double squirrel cage motor based on equivalent deep groove is established,of which the dimension is equivalent to that of a single squirrel cage motor.Using these equations,the steady-state rotor current distribution of double squirrel cage motor with broken bars is analyzed.It was found that,when a upper cage of double squirrel cage motor was broken,the current of the upper cage in the slot adjacent to the broken strip was higher than that in other slots,and the current of the lower cage at the broken strip was obviously higher than that of the lower cage at the non-broken strip.And the current of the lower cage at the center of the broken strip was the largest.When a lower cage was broken,the current of the upper cage at the broken strip was higher than that at the non-broken strip,and this would be more obvious with the increase of the slip rate.It was found for the first time that the groove total current and the upper and lower cage current outside the broken bar position were all distributed in a sinusoidal regularity with a period of 1 pole,of which the peak was at 0.5 pole,1.5 pole,2.5 pole and 3.5 pole away from the broken bar,the trough was at 1 pole,2 pole and 3 pole away from the broken bar.The amplitude of the sinusoidal distribution was larger than that of the rated operation when the rotor was blocked.It was positively correlated with the fault characteristic ratio,and reflected the unbalance degree of the rotor current.When a upper cage bar was broken,the most likely position of the next broken bar would be the upper cage bars in the two grooves adjacent to the broken slot,and the next but one would be the upper cage bar at the sinusoidal distribution wave crest.When a lower cage bar was broken,the most likely position of the next broken bar would be the upper cage bars in the broken slot.The fault characteristic ratio of upper cage broken bars increased with the slip rate,while that of lower cage broken bars decreased with the slip rate.In order to study the starting process of double squirrel cage motors with broken rotor bars,a hierarchical method is proposed to transform the rotor model of double squirrel cage motors with skin effect into a multi-squirrel cage rotor model without skin effect.For the first time,the multi-loop transient equation of double squirrel cage motor considering skin effect is prposed,and is applied for the transient analysis of start-up process of no-load test of double squirrel-cage motor drive system.The time-frequency analysis of stator starting current of double squirrel-cage motor is proposed based on the Synchro Extracting Short-time Fourier Transform.Moreover,the amplitude-frequency plane angle are defined to distinguish the early broken bars of upper or lower cages and the number of continuous broken bars in upper and lower cages.When the amplitude-frequency plane angle is negative,the broken bars of lower cages will occurre in double squirrel cage motors.When the amplitude-frequency plane angle is positive,the broken bars of upper cages will occurre in double squirrel cage motors,and the more the number of continuous broken bars in the upper cage,the bigger the amplitude-frequency plane angle.The time-frequency plane angle which can roughly judge the number of upper cage broken bars.The more the upper cage broken bars,the bigger the time-frequency plane angle.The effect of skin effect on no-load test is studied.It was found that skin effect would increase the slip rate and overload coefficient of the motor obtained by no-load test,and would decrease its efficiency.The power factor of the motor tested by circle diagram method would increase,and by equivalent circuit method it would decrease.In view of the above problems,a virtual frequency conversion software algorithm is proposed to reform the original on-load testing.Three kinds of explosion-proof motors with different specifications were tested without load by using the no-load test system based on virtual frequency conversion technology.The test results showed that the virtual frequency conversion technology can improve the accuracy of no-load test without increasing hardware cost. |
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