Natural Commutation of Current-Source Thyristor Inverters by Cage-Rotor Induction Machines

A new type of brushless cage-rotor induction machine has recently been developed [1]-[3] which is capable of providing enough counter electromotive force (EMF) when interfaced with a current-source inverter (CSI) to naturally commutate the thyristors in a high-voltage dc-link adjustable-frequency converter. The basis of this system is that by stator electromagnetic design, the induction motor may have the terminal characteristics of a dc-field synchronous machine yet still retain the torque and constructional features of a conventional induction machine. Four-quadrant operation has been demonstrated on 35- and 150-hp cage motors, and most important, leading power factor operation of the entire unit at full load is compatible with high power natural commutation of thyristor inverters. The single rotating machine has two distinct and different stator pole-pitches which must be excited from a common CSI. The analysis determines the magnitude of the CEMF available from the asynchronous condenser winding and the consequent operation of the machine in both natural commutation and in a dc-link current pulsing mode for startup.     

PERFORMANCE ANALYSIS OF LOAD COMMUTATED INVERTER FED CAGE INDUCTION MOTOR DRIVE

ABSTRACT

An approach, to analyse the performance of a variable speed cage induction motor drive fed from load commutated inverter (LCI), is presented. Induction motor with an appropriate capacitor across its stator terminals constitutes the load on the inverter. A fully controlled bridge converter, supplied by a variable dc voltage source, commutated with the back emf of the motor, acts as an inverter. An algorithm to compute the motor performance, is developed using equivalent circuit representation of the system. Strategies for the selection of control variables, in order to achieve the desired speed range for satisfactory performance of the drive are given. The performance of a 3-hp, cage induction motor fed from LCI is computed, using the developed algorithm for wide range of speed; and is verfied experimentally.     

Synchronous Motor Railcar Propulsion

Development of ac motor drives for rail transit car has centered on the induction motor with pulsewidth modulated (PWM) inverter control. Interest in the induction motor as a replace for the series dc traction motor stems from the simplicity of the squirrel cage rotor of the induction motor. In this article, the short-comings of PWM inverter-induction motor transit car drive are examined. It is shown that the synchronous, or brushless dc, motor drive can provide performance exceeding both the PWM inverter-induction motor and the conventional dc motor in the transit car application.     

Application of Gate Turn-Off Thyristors in 460-V 7.5-250-hp AC Motor Drives

A 7.5-250-hp range of variable-frequency ac motor drives has been under development at the Westinghouse R & D Center since early in 1981. Design and performance details for prototype equipment rated for 10, 20, and 50 hp will be discussed. The drive system comprises a three-phase thyristor controlled ac/dc converter in conjunction with a dc/ac adjustable-frequency inverter using gate turn-off thyristors (GTO's). The output of the inverter is a six-step voltage wave in the range of 6-120 Hz, suitable for driving three-phase ac induction or synchronous motors. Details of the inverter design, including means to protect the gate turn-off thyristors from output line-to-line or line-to-ground short circuits, will be given special attention.     

Field Weakening Alternative

The most common machines for traction are induction and PM synchronous machines. The latter are efficient and small, but they are more expensive and less rugged than induction machines. Fault behavior is easier to manage with an induction machine. For induction machines, the back electromotive force vanishes rapidly when the stator voltages are removed. As shown later, the efficiency of a rerated induction machine can compete with that of a PM machine when both are operated over a wide speed range. The size of any electromechanical device is related to the internal force it generates. It is important to recognize that the size and mass of a motor in a traction system are based on the target torque level. Any induction machine can support a wide frequency and speed range. When operated by an inverter, the name- plate frequency rating of the motor becomes meaningless, as it can be operated at arbitrary speed by adjusting the inverter fundamental frequency. By keeping the voltage-to- frequency ratio constant, the flux level in the machine is constant, meaning that nominal torque can be obtained at any given frequency that supports this constant ratio. The speed and output power of the machine then change linearly with frequency. It works within reason, as long as physical limits on the currents (overheating) and the flux densities (core or tooth saturation) are maintained. According to the National Electrical Manufacturers Association (NEMA), the admissible maximum operating speed for a general-purpose induction machine is twice its synchronous speed.     

PWM-CSI induction motor drive with phase angle control

The authors describe a pulse-width-modulated current-source inverter drive system using an induction motor. Its dominant feature is that it provides adequate control of either torque or speed over a wide range without requiring a shaft position or speed sensor. The capacitor-loaded current-source invertor system has the advantages of simplicity, low switching frequency, four-quadrant operation, overcurrent protection, and low harmonic content in the motor current and voltage. The choice of maximum value for the capacitor depends on the degree of nonlinearity that can be tolerated in the torque-stator current relation, whereas its minimum value depends on the need for a low impedance path for inverter current harmonics. The control strategy is based on the concept of controlling the stator phase angle of the induction motor. The stator voltage is sensed, and the current-source inverter is used to inject current into the motor with the desired phase angle     

Current-Source Converter for AC Motor Drives

The employment of current-source concepts in thyristor converters to obtain adjustable frequency and adjustable current waveforms is presented. The use of a dc filter choke and a current feedback loop to produce a regenerative current source is explained. The simplified inverter commutation circuit made possible by the current-source technique is also discussed. Finally, a brief review of the inherently rugged current-source converter's ability to provide wide range control of an ac induction motor is given.     

On the Torques and Losses of Voltage- and Current-Source Inverter Drives

The frequency controlled squirrel-cage induction motor is increasingly applied for variable-speed drives working under extraordinary operational or environmental conditions. Depending on the type of frequency inverter, negative effects like torque ripples and additional losses are different. The torques and losses are calculated for induction machines supplied by the two most frequently used inverter types: the PWM voltage-source inverter and the ASC current-source inverter. For comparison some measurements are presented from test stands with the ratings 45 kW and 25 kW.     

A hybrid solution for load-commutated-inverter-fed induction motor drives

A novel, hybrid solution employing a combination of a load-commutated inverter (LCI) and a voltage-source inverter (VSI) is proposed for induction motor drives. By avoiding the use of output capacitors and a forced dc-commutation circuit, this solution can eliminate all disadvantages related with these circuits in the conventional LCI-based induction motor drives. In addition, improved quality of output current waveforms and faster dynamic response can be achieved. The proposed hybrid scheme features the following tasks: 1) the safe commutation angle for the LCI, controlled by the VSI in the entire speed region of the induction motor and 2) a dc-link current control loop to ensure minimum VSI rating. Advantages of the proposed solution over the conventional LCI-based induction motor drives include the following: 1) sinusoidal motor phase current and voltage based on the instantaneous motor speed control; 2) fast dynamic response by the VSI operation; and 3) elimination of motor circuit resonance and motor torque pulsation. The feasibility of the proposed hybrid circuit for the high-power drive system is verified by computer simulation for a 500-hp induction motor. Experimental results to support the use of the proposed system are also included for a 1-hp induction motor laboratory setup.     

Performance and torque-ripple characterization in induction motoradjustable-speed drives using time-stepping coupled finite-elementstate-space techniques

In this paper, the time-stepping coupled finite-element state-space (TSCFE-SS) model developed in an earlier companion paper is applied here for assessments of effects of machine geometry and magnetic circuit design modifications, and effects of pulsewidth modulation (PWM) carrier frequency on performance characteristics of induction motor drives. Namely, this has been accomplished through analysis of developed torque profile ripples and harmonic spectra of mid-air-gap radial flux density waveforms of the case-study motor. Furthermore, consequent effects of design modifications pertaining to geometry and/or magnetic circuit modifications and PWM carrier frequency on ohmic and iron core losses are investigated. The investigation has been performed on a case-study motor, which is a Y-connected single-layer three-phase two-pole 1.2-hp 208-V squirrel-cage induction motor with 24 stator slots and a cage with 34 rotor bars     

A Current-Source Inverter in the Secondary Circuit of a Wound Rotor Induction Motor Provides Sub- and Supersynchronous Operation

A control system is described in which a current-source inverter is connected between the secondary winding of a three-phase slip-ring induction motor and the ac mains supply to give a variable speed sub- and supersynchronous operation. Control techniques developed in an earlier work were used to ensure fully stable operation over a very wide speed range with either driving or braking torque. Means are provided for power-factor control to compensate for the lagging power-factor effect of the inverter when operating into the large secondary electromotive forces (EMF) at high slip values. Experimental results are presented for a speed range from standstill to approaching twice synchronous speed for both driving and braking torque. There are no instabilities and the developed torque even at low frequencies near synchronous speed is steady with motor derating due only to the quasi-square wave current in the secondary circuit.     

Improved current source GTO inverter-fed induction motor driveswith PWM-controlled thyristor converter

An improved current-source GTO (gate turn-off) inverter system for driving an induction motor at high frequency was developed. This system is composed of an inverter using GTOs and a PWM (pulsewidth-modulated)-controlled thyristor rectifier. The energy rebound circuit in the inverter is used to turn off the thyristors in the rectifier and to apply PWM control techniques. This circuit plays an important role in the treatment of reactive power in a load. The capacitors connected to the AC input terminal to improve PWM control also function as a filter. Thus, the waveforms of the input voltage and current become almost sinusoidal. Principles and circuit operations of the rectifier section are described in detail. The current-source GTO inverter is used to drive a 5.5 kW induction motor. The experimental waveform and characteristics for the tested motor drives are given. It is shown that the harmonic components of the input voltage and current are eliminated or reduced by using the PWM control technique without spoiling the inherent characteristics of the current-source GTO inverter     

An Induction‐Motor Control Method for Minimization of Capacitor‐Voltage Fluctuations in a Modular Multilevel DSCC Inverter: Its Applications to a Quadratic‐Torque Load

This paper aims at minimizing capacitor‐voltage fluctuations inherent in a modular multilevel cascade inverter based on double‐star chopper cell (DSCC) or a modular multilevel DSCC inverter. The inverter can drive an induction motor loaded with a quadratic‐torque load. Both theoretical analysis and numerical calculation reveal that the voltage fluctuations can be minimized when the ratio of a magnetizing‐current component with respect to a torque‐current component in the motor current is set to unity, regardless of the motor mechanical speed. A three‐phase DSCC inverter is designed and constructed to drive a 380‐V, 15‐kW, 50‐Hz, four‐pole, induction motor loaded with a quadratic torque that is proportional to a square of the motor mechanical speed. Experimental results confirm the validity of the theoretical and numerical calculations.     

笼型异步电机直接转矩控制建模与Simulink仿真

研究了笼型异步电机直接转矩控制(DTC)理论,推导了相应数学模型,提出了控制方案,并仿真运行,结果表明了建立直接转矩控制系统的正确性。该控制系统的特点、结构简单、电机运行平稳、速度和转矩跟随性良好,动态性能优越,抗转矩变化能力显著。     

High Response Control of Stator Watts and Vars for Large Wound Rotor Induction Motor Adjustable Speed Drives

Extremely rapid control response (0.01 s) of stator watts and vars has been obtained on a 15 000-hp wound rotor induction machine with a cycloconverter controlled secondary by means of an orthogonal control scheme which linearizes the machine equations and combines both feedforward and feedback error signals. Leading and lagging power factor and positive or negative stator power flow can be smoothly and rapidly controlled over a speed range in excess of ±35 percent of the induction motor synchronous speed. This doubly fed drive may be termed a "Scherbiustat drive" because the wound rotor induction motor secondary power conversion equipment is the static equivalent of the Scherbius machine. This type of a drive does not employ a dc link in the motor secondary power conversion equipment. It should not be confused with a static Kraemer drive which employs a dc link in the frequency conversion process and was so named because of its similarity to the original Kraemer drive which uses a synchronous converter and a dc motor in the secondary power conversion process. Recently obtained field results have verified the original study results discussed herein.     

A Stator-Core Quality-Assessment Technique for Inverter-Fed Induction Machines

Localized heating caused by damage in the laminations or interlaminar insulation of the stator core increases the core losses and can lead to machine failure. Therefore, it is important to monitor the quality of the stator core for reliable and efficient operation of the machine. The methods currently available for core quality assessment are inconvenient since they require an outage and the machine to be disassembled or operated under no-load conditions. In this paper, a new method for monitoring the quality of the stator core for inverter-fed machines without motor disassembly or operation is proposed. The main concept is to use the inverter to apply a set of test signals to assess the quality of the core whenever the motor is at standstill. A set of high-frequency pulsating magnetic fields is produced using the inverter, and the power loss is observed as a function of field circumferential position to detect core problems. An experimental study on a 10-hp induction motor verifies that local interlaminar core faults can be detected with high sensitivity. The proposed technique is expected to provide a simple solution for frequent stator-core quality assessment without motor disassembly, motor operation, or additional hardware for reliable and efficient operation of inverter-fed induction machines.      

无轴承绕线型异步电机瞬态响应的有限元分析及实验研究

无轴承电机集磁性悬浮与转矩驱动于一体,具有无摩擦、无磨损、无损耗、免维修、寿命长等独特优点,从根本上改变了传统的支承与传动形式.该文介绍一种无轴承绕线型异步电机,通过瞬态有限元分析法(TFEM)计算其径向力和转矩的瞬态响应,结果表明该无轴承电机可以同时产生支承转子重量的稳定径向力与恒定转矩,且其控制电流相互独立,与鼠笼型转子无轴承异步电机相比,前者能够产生更有效的径向力,且转矩不受径向力绕组电流干扰;并在基于SVM-DTC的磁悬浮异步电机悬浮子系统独立控制实验平台上加以测试,结果表明该系统能够实现了无轴承绕线型异步电动稳定悬浮.     

High-performance direct torque control of an induction motor

A novel direct torque control method for an induction motor is presented which is quite different from field-oriented control. Improving the torque response of a large-capacity induction motor using two sets of three-phase inverters and an open-data induction motor is of special concern. Instantaneous voltage vectors applied by an inverter have redundancy characteristics which provide some flexibility for selecting the inverter switching modes. By using this switching freedom, control is achieved according to the following priorities: (1) high-speed torque control; (2) regulation of the primary flux; (3) decreasing the zero phase sequence current; and (4) minimization of the inverter switching frequency. Simulations and experiments have been carried out to verify the feasibility of this priority control, accompanied by comparisons with another control scheme. Torque frequency-response corner frequencies above 2000 Hz have been experimentally measured, and time constants of 4 ms have been achieved for rotor-speed step responses from -500 to 500 r/min. The peak transient torque during the step change is about 20 times the rate torque     

Improved Torque and Flux Performance of Type-2 Fuzzy-based Direct Torque Control Induction Motor Using Space Vector Pulse-width Modulation

The conventional proportional integral controlled direct torque control of an induction motor using the space vector pulse-width modulation technique may provide satisfactory dynamic response. However, the proportional integral controller (PIC) does not provide efficient dynamic performance in the induction motor drive during sudden changes in the load or speed. To improve dynamic performance of the induction motor drive, the PICs are replaced by type-2 fuzzy logic control. The type-2 fuzzy improves the starting transient performance as well as the steady-state response. In addition, the type-2 fuzzy direct torque control provides lesser current total harmonic distortion, flux distortion, and torque pulsation of the induction motor drive compared to conventional direct torque control. A MATLAB Simulink (The MathWorks, Natick, Massachusetts, USA) model for direct torque control with type-2 fuzzy logic control is developed to simulate the response of an induction motor drive with different operating conditions. The space vector pulse-width modulation technique is used to drive the inverter, as it produces lesser total harmonic distortion in inverter current and voltage waveforms for a given switching transition due to the single switching frequency for the movement of each state vector. A prototype type-2 fuzzy-based direct torque control induction motor with space vector pulse-width modulation is developed to validate the simulated response. The control signals for the inverter are generated by the DSPACE DS1104 (DSPACE GmbH, Germany) to drive a two-HP induction machine.