Zhao Haisen

A Design Method for Cage Induction Motors With Non-Skewed Rotor Bars

To overcome the influence of the skewed rotor bars on the operating performance of the cage induction motors, this paper presents a design method for non-skewed small and medium cage induction motor based on the time-stepping finite element method, in which the approaches of asymmetrical rotor slots, closed slots, reasonable slot combination, and air-gap length are adopted and studied systematically. With this method, the cage induction motors with non-skewed rotor bars can be realized. Simultaneously, the slot harmonic, which is caused by slot opening, can be eliminated effectively and the starting performance can also be improved. The design example on a 5.5 kW cage induction motor is presented and the experimental validation is also performed.

The influence of wye and delta connection on induction motor losses taking slot opening and skew effect into account

As a basic theoretical research for the ultra efficiency motor design, in this paper, the influence of wye and delta connection on induction motor losses is focused on. The Multi-slice field-circuit coupled time-stepping finite element method (T-S FEM), taking slot opening and skew effect into account, is used to predict the losseswith wye and delta connection under different load conditions respectively. It is shown that the copper loss is higher when delta connection is adopted and the iron losses of wye connection may be more than that of the delta connection.

The research on residual voltage of asynchronous motors and its impact on power restoration

The residual voltage of asynchronous motors after AC dump(RVACD) could have result in inrush current and inrush torque when asynchronous motors were reclosed or reswitched on the alternate power source, which would have cause damage to motors or bring power source problems such as mal-operation of feeder protection and voltage sag, etc. In this paper we built the space vector based model of motors to research the RVACD so as to thoroughly analyze the varying laws of electrical qualities and magnetic field after dumping, deduce the analytical expression of residual voltage of one motor, and get the solution for the group of asynchronous motors. The inrush current and torque are presented for power source to be restored at different moment after dumping.

Research on a novel wye-delta soft start method of three-phase induction motor

This paper proposes a wye-delta soft start method of the three-phase induction motors and its control device based on SCR switches. Comparing with traditional wye-delta start, this method can suppress inrush current effectively both in the initial stage with wye connection and the transforming stage from wye to delta. With the sequence trigger control strategy of SCRs, the smooth transition of the magnetic flux and lower inrush current can be realized. Furthermore, this paper also discusses the best transforming point from wye to delta connection on the basis of comparison of start current wave at various slip of motor. And the method is verified by the simulation on a 2.2kW induction motor.

Influence of Different Practical Models on the First Swing Stability of Turbine Generators

Abstract There are two popular practical models of synchronous generators used in power system simulation, both based on Park equations, defined as assumed A and assumed B respectively, according to the different assumptions. To study the influence of the different models on the first swing stability calculation precision, the physical essence of the two assumptions are revealed in theory, and it is obtained that assumed A takes account of the mutual leakage flux linkage between field and damping windings and neglects the self-leakage flux linkage of damping winding, the opposite of assumed B. Taking a 300-MW turbine generator as an example, first swing stability limits calculated by two practical models are compared; simulation results are verified by the time-step finite-element model. The influence of line reactance and excitation system on first swing stability limits is studied. Results show that the first swing stability limit calculated by the practical model with assumed A is closer to the result of the time-step finite-element model, since assumed A takes account of the larger mutual leakage flux linkage. Therefore, the practical model with assumed A is more accurate. The result provides reasonable reference to select synchronous generator models in power system simulation.

Time Step Finite Element analysis for synchronous generator's asynchronous operation during loss of field

In order to study synchronous generators asynchronous operation during loss of field, the Field-Circuit Coupled Time-Stepping Finite Element Model (T-S FEM) is built, in which the saturation of magnetic circuit, magnetic distortion, and harmonic field and so on are considered. The simulation modeling is verified by testing. The physical process of the synchronous generators asynchronous operation during loss of field is analyzed. On that basis, take a single-machine infinite-bus for example, two conditions of the synchronous generators asynchronous operation during loss of field are simulated, one of which is excitation opened circuit, and the other is excitation shorted circuit. The difference under excitation opened circuit between T-S FEM and PSD-BPA simulation is bigger than excitation shorted circuit.

Loss and Air-gap Force Analysis of Cage Induction Motors With Non-skewed Asymmetrical Rotor Bars Based on FEM

Non-skewed asymmetrical rotors bars can be used to eliminate the slot harmonic fields and the resulting noise and vibration in cage induction motors. However, it is difficult to determine the spatial distribution of the asymmetrical rotor bars and the slot combinations, which can affect the loss and the air gap force significantly. With a 5.5 kW, 4-pole induction motor as an example, this paper studies the influence of the above factors on the loss and the air gap force by time-stepping finite element method, and the experiments are performed to validate the calculations.

Piecewise variable parameter loss model of laminated steel and its application in fine analysis of iron loss of inverter-fed induction motors

Due to the coaction of supply harmonics and harmonic fields inside the motors, the mechanism and distribution characteristics of iron losses become more complicated in inverter-fed induction motors. Therefore, accurate prediction and fine analysis of iron loss are very important at the design stage of high-efficiency inverter-fed induction motor. In order to predict the iron losses accurately, this paper proposes a piecewise iron loss model whose parameters vary with the amplitude and frequency of flux density, and two additional flux density terms are introduced to the classical iron loss model considering the nonlinearity of magnetic material and harmonic fields. With this model, the iron losses are calculated for an inverter-fed 5.5kW induction motor. The results reveal the distribution characteristics of hysteresis and eddy current losses in stator and rotor cores, and acquire the characteristics of additional iron loss caused by harmonic fields. By the comparison of predicted and measured no-load iron loss under different supply voltages and switching frequencies, the proposed model and analysis results are validated.

Time-stepping finite element analysis on iron loss distribution of cage induction motors

To study iron loss distribution characteristics in stator and rotor cores of cage induction motors, this paper establishes a computational model, with skew rotor bars considered, based on Time-Stepping Finite Elements Method (T-S FEM). Taking a 5.5kW cage induction motor as an example, time variations and loci of the flux density at different core locations is analyzed with no-load and full-load conditions, then the loss density and iron loss distribution in different locations in stator and rotor cores is derived. It is shown that high loss density appears in the middle of the tooth top for both the stator and rotor cores. Furthermore, iron loss distributions in different core areas are compared. On the stator side, iron losses mainly distributes at the boundary between teeth and yoke, in yoke and in tooth bodies, respectively occupying a proportion of 32%, 28% and 17.5% of the total calculated iron loss with no-load condition and 28.6%, 23.8% and 15.4% with full-load condition. On the rotor side, iron losses in the tooth top area caused by harmonic fields contributes 15% and 19% to the overall iron loss of the machine, with no-load and full-load conditions respectively. Experiments are performed on a special prototype motor, and the effectiveness and correctness of the above analysis is verified. The achievements could provide necessary technical supports for developing optimal design to reduce iron losses.

Time-stepping finite element analysis on the influence of rotational flux on local core losses of AC electrical machines

Purpose – The purpose of the paper is to study the influence of rotational flux on local core losses of the motors, which is difficult to distinguish the iron losses caused by rotational flux in certain electric machines through experiment. Therefore, time-stepping finite element method (T-S FEM) is used to consider the rotational flux and to predict the local core losses. Design/methodology/approach – Time-stepping finite element method. Findings – It is found that, in stator side, rotational flux mainly exists in tooth root and yoke area near the bottom of stator slot, those area is about 45 percent of total stator core area; in rotor side, due to slot harmonic field, the rotational flux mainly exists in the tip of tooth. Originality/value – Through analyzing the magnetization characteristics at different positions in stator and rotor cores by T-S FEM, the influence of rotational flux on local core losses of AC electric machines is studied.