Slip-ring Induction Motor Research Articles

A new energy recovery double winding cage-rotor induction machine

This paper proposes a new double winding induction machine and its speed control methods. The machine consists of two stator windings and one cage rotor. One stator acts as a motor and the other as a generator. By controlling the voltage supplied to the secondary or the generator winding, the rotor speed can be adjusted. The machine has a similar speed control characteristic to that of a slip-ring induction motor equipped with the rotor energy recovery scheme. The construction, principle, equivalent circuit, and speed control schemes of the new machine are presented. The performance characteristics of the machine are analyzed using the equivalent circuit and verified by experimental results.

Selection criteria of induction machines for speed-sensorless drive applications

Induction motors, both three and single phase, are used extensively for adjustable-speed drives' applications. These machines are structurally very robust and are a primary source of motive power and speed control where DC machines cannot be used. For closed-loop control of these machines, sensorless speed estimation is usually preferred. Among the current estimation techniques available for speed-sensorless induction motor drives, speed measurement based on rotor-slot-related harmonic detection in machine line current happens to be a prominent one. While these harmonics can be strong in certain kinds of machines, some other machines may exhibit very weak rotor slot harmonics that can be obscured by noise. Skewing, slot shapes and types, structural unbalances, etc., also have a prominent effect on the detectability of these harmonics. This paper attempts to investigate this problem based on the interaction of pole pairs, number of rotor bars, and stator winding. Although the analysis and experimental results have been mainly provided for three-phase squirrel-cage induction motors, single-phase and slip-ring induction motors have also been addressed. Further, it has been shown that eccentricity-related fault detection could also be easily accommodated with this kind of speed detection technique at no or negligible extra cost when certain motors are selected.

Steady‐state and transient performance of a series excited synchronous motor

This study involves the development of a PSPICE 5.4 package oriented model for the dynamic behaviour of a series excited synchronised slip ring induction motor including the bridge rectifier for rotor circuit excitation. Furthermore, in this study is aimed at investigating the synchronising behaviour for various rotor connections of a series excited synchronous motor formed by connecting a bridge rectifier input in series with the stator winding of a slip‐ring induction motor whereas the rectifier output feeds the rotor winding. The steady state behaviour of such a scheme in comparison to a separately excited one is well known, but the behaviour during synchronisation requires further study. Presentation of results obtained by a PSPICE based simulation and comparison with experimental results acquired during this study points to a model to account for design variations and additional circuitry. It is convenient to use this model in a drive system. The advantage of this simulation method over the others is that it does not involve the utilisation of specialised programs and mathematical difficulties such as the solution of stiff differential equations. Furthermore, steady state behaviour of the scheme is explained by means of a simple analysis and illustrated by test results.

Torsional Oscillation Problems Associated with Shunt Induction Motor Driving Distributed Inertias

Shunt connection has been recently proposed by the authors to attain fast starting of slip-ring induction motors. Torsional torques, which might be experienced with this connection, is a reservation of this method and should be investigated. Therefore, this paper analyzes the expected torsional problems of the shafts of these motors when driving distributed inertias. The analysis aims at investigating the developed pulsating torque, the resulting torsional oscillations, the transmitted torques, and the twist angles at different shaft sections. Effects of motor parameters on the torsional oscillation problem are studied. For this purpose, a rigorous mathematical model is presented and simulated by a developed computer program.

Synchronous and slip-ring induction motors drive cement mills

In the process of selecting a suitable motor for a cement mill, there are several issues to consider that, for the purpose of analysis, can be grouped into two distinct categories. The first category is the technical analysis, which is based on engineering considerations that evaluate the merits of either a synchronous or a slip-ring induction motor installation. The second category is the economical analysis, which follows closely the results of the technical evaluation with emphasis on the original investment cost and the cost of operating and maintaining the equipment over its useful lifespan-estimated as 30 years. Among the technical issues, the first should be identifying the torque requirements and initial power system limitations. The choice of motor design will largely depend on these requirements. The other issues relate to electrical energy savings. With the cement mill being the largest single power consumer in the plant, the choice of motor and drive will directly influence these savings, which can be significant. Several cement mill installations in operation today are discussed.

Transient performance of series connected three phase slip-ring induction motors

The possibility of operating a three phase slip-ring induction motor as a series connected motor is explored. The merits and drawbacks of this mode of operation are discussed. For this purpose, the necessary mathematical models along with the corresponding computer simulation are developed and experimental verification of the theoretical results is also carried out. It has been found in general that operation of slip-ring induction motors as series connected motors is possible providing that the rotor winding is connected in a sequence opposite to that of the stator winding. This adds flexibility to the modes of operation of these motors. Moreover synchronous operation is also possible, providing that the series connected motor is driven initially at twice its synchronous speed and then connected to the supply.

Smooth starting of slip ring induction motors

The starting of three-phase wound rotor induction motors by stepped resistances added in the rotor circuit suffers from several problems. This paper suggests a novel method of motor starting by inserting a parallel combination of resistance, inductance and capacitance in the rotor circuit. The proposed method guarantees a smooth and high starting torque over most of the starting period besides a controlled starting current. A mathematical model is developed based on a frequency domain quasi-steady state equivalent circuit. An optimization technique is then applied to determine the values of the rotor added elements such that minimum starting time is achieved subject to current and torque constraints. Experimental verification has been carried out and the computed results are in good agreement with the measured values.

Controlling the speed of an induction motor by resonating the rotor circuit

Abstruct- The possibility of obtaining various torque-speed characteristics from a wound-rotor induction motor by operating such a motor close to its rotor resonance is described. It is shown that, by the use of reactive rotor networks, high starting and braking torques may be obtained. An induction motor operating with this proposed technique of speed control also reacts favorably to nonsinusoidal supply voltages and has an improved power factor. No auxiliary machines or power supplies are required and any slip ring induction motor of conventional design can be used. NOMENCLATURE Capacitance per phase externally connected (F). Air gap voltage (V). Rotor current (A). Constant, 27r/60. Rotor inductance per phase (augmented by externally connected inductance) (H). Rotor speed (dmin). Synchronous speed (dmin). Power transferred across the air gap (W). Rotor resistance per phase (0).

Evaluation and Testing of Turn Insulation of Large AC Motors

This paper reviews the results of tests performed on the stator windings of a 13.8 kV, 15000 hp variable speed slipring induction motor in order to assess its insulation condition and quantify the impulse strength of the motor winding. A surge voltage distribution test showed severe nonlinearity of turn voltage where up to 120% of the applied voltage was achieved across the first coil with 0.1-μs risetime. The effect of a surge capacitor was also demonstrated when a substantial reduction of the first coil voltage could be obtained at low risetime of 0.1 μs. Measurements of breakdown voltage (BDV) of coils at different locations gave indirect evidence that turn insulation degradation was most likely surge related. Finally, the paper contains a proposal of an alternative testing criteria to quantify impulse strength of turn insulation of large ac machines.

Microcomputer-based instrumentation and protection scheme for a slip-ring induction motor

The paper presents a microprocessor-based scheme for the on-line sensing of stator voltage, stator current, stator frequency and determining all the essential parameters of a slip-ring induction motor. The developed equipment also takes care of the various abnormal conditions that can occur in the motor while in operation. A prototype of the equipment was tested in real time in the laboratory and it gave a gratifying performance.

Energy Saving in Large Rating Fan Applications with Slip Power Recovery Systems

This paper deals with slip power recovery system for slip ring induction motor speed control, a method which is now a days extensively used for energy saving in large rating fan applications in cement industry. It also presents a methodology to derive the required characteristics of the fan (found many times not available) along with the energy saving formulation and system considerations.A series-parallel system, conceived with (semi-conductor) fuseless concept, is introduced for a 1850 kW smoke gas application and the experience is presented along with its major advantages and disadvantages over the conventional system.

Transient currents and torques in wound-rotor induction motors using the finite-element method

The paper describes a time-stepping finite-element technique for modelling transient torques and currents in a slip-ring induction motor. The method is based on a coupled fields and circuit equation approach that allows the winding inductances to be modified to account for certain three-dimensional effects. The numerical solution incorporates a moving rotor so that slotting and saturation effects can be modelled accurately. The method is verified by direct comparison with experimental results obtained from a motor subjected to a supply reconnection at speed. It is shown that the general method can be extended in a straightforward manner to accommodate cage induction motors.

Condition monitoring of slip-ring induction motors

A technique for real-time condition monitoring of slip-ring induction motors is presented. The proposed method utilizes on-line sensing and analysis of the rotor current to assess the integrity of the motor. The theoretical basis for this approach is presented; the results of an experimental investigation are given. The paper is concluded with a discussion of the efficacy of the proposed method in industrial power systems.

Microprocessor-Based PHESS Meter

The present investigation introduces engineers and technologists to a new microprocessor-based on-line performance recorder called the ``PHESS Meter'' for measurement and display of power factor, horse power, percentage efficiency, slip, and speed of a slip ring induction motor. The instrument continuously displays each set of data for a desirable time in a specific order and provides the under-voltage alarm sound and the overload protection disconnecting the system from the supply.

On the stability of a static slip energy recovery drive

It has been reported elsewhere that there are certain instability regions in the operation of the subsynchronous static slip energy recovery scheme for the speed control of a slip-ring induction motor. Eigen value analysis of the linearised model of an induction motor is used to study the stability behaviour of this scheme and the results show that the drive system does not become unstable. The effect of variation in system parameters has also been studied and it has been found that in spite of large variations in the parameters, the drive remains stable. These results are further verified by the transient response of the system to step changes in load torque at certain steady-state operating points which had been described as unstable by other authors. The transient response has been obtained by numerical integration of system equations. At all these points the steady state is reached within a reasonable time.

Analysis of slip-ring induction motors controlled by silicon-controlled rectifiers under unbalanced stator conditions

Abstract The steady-state performance of a slip-ring induction motor controlled by a silicon-controlled rectifier (SCR) is presented under unbalanced supply voltage and/or stator impedances. The method is based on the Fourier analysis of the applied chopped voltage waveform. The given analysis enables a direct and straightforward calculation of currents, average torque, pulsating torque patterns, motor power factor and efficiency to be predicted at any motor speed. Application of the method is illustrated and sample results are presented for comparison purposes.

Thyristor converter-modulated impedance under unbalanced system conditions

The paper presents the use of a digital simulation program to assess the concept of modulated impedance under unbalanced system conditions, and the relationship between apparant system impedance Z ac and thyristor converter rectifier firing delay angle α. Modulated impedance/reactance applications in power systems and machines include network stability enhancement tools, VAR shunt compensation, subsynchronous resonance damping, motor drive speed control, and starting of large slip-ring induction motors. The digital simulation results were validated by laboratory testing.

Characteristics of three-phase induction motors under unbalanced rotor impedances

Abstract An analysis is developed for three-phase induction motors with balanced or unbalanced rotor impedances. The validity of this analysis is verified by comparing its results, under balanced conditions and in the steady state, with those obtained using conventional methods. The method is also verified experimentally for a 7.3 kW, 220 V, four-pole, 50-cycle slip-ring induction motor with balanced or unbalanced rotor impedances. The analysis is then used to study the characteristics of three-phase induction motors under unbalanced rotor impedances.

Analysis of an induction motor with square wave currents in the rotor

A method of analysis for a slip ring induction motor with quasi square wave currents in the rotor and sinusoidal voltages in the stator is presented using the machine equations in terms of Park's vector. A close form solution to the stator flux linkage is obtained. The analysis shows that the stator currents are essentially sinusoidal superimposed by slip dependent harmonics. The torque developed is found to contain pulsations both at six times and twelve times the rotor frequency. At lower speeds where the rotor frequency is large, the higher order torque pulsations become insignificant. At very low rotor frequencies, the frequency range of these pulsations may cause dangerous vibrations.

Dynamic Braking of Induction Motors Using DC Injection

ABSTRACT Dynamic braking of 3-phase induction motors through dc injection in commonly used in applications such as cranes and hoists. The paper analyses the performance of a slip ring induction motor with dc injection and derives expressions for the optimum values of injection voltage and rotor resistance. Digital computer simulation of the sequence of operations like motoring and braking is also given.