Showing papers on "Induction motor published in 1968"

Polyphase Induction Motor Performance and Losses on Nonsinusoidal Voltage Sources

Abstract: Induction motors excited with static frequency converters almost invariably are subjected to nonsinusoidal voltage waveforms, and the presence of time harmonics in the applied voltage results in currents at the harmonic frequencies. These currents result in additional and sometimes rather large losses. A method for calculating these harmonic currents and losses is presented, and experimental data are included to substantiate it. The losses are separated into various components and it is shown that the largest loss is usually in the rotor bars as a result of deep bar effect. Harmonic losses are shown to be nearly independent of motor load and the fundamental magnetizing current is found to increase over that which would be present for the same rms fundamental voltage. These observations are explained on a theoretical basis. The encouraging correlation between test and calculated data confirms that the important elements which differ between motor performance on sinusoidal and nonsinusoidal waveforms have been identified and accounted for.

Induction-motor losses due to nonsinusoidal supply waveforms

Abstract: The paper presents an analytical and experimental study of the additional losses which occur in induction motors supplied with nonsinusoidal waveforms. Measurements on an inverted induction motor are used to show the importance of losses due to skew-leakage and end-leakage fluxes, and to verify the methods developed for calculating these components. Computed losses are compared with test results obtained on a variety of machines and supply waveforms, and the agreement is shown to be consistently good. The manner in which the losses are related to the machine design and to the supply harmonic content is described.

Induction Motor Speed Control by Stator Voltage Control

Abstract: This paper establishes the fundamental laws relating to the speed control of induction motors by simple voltage control and emphasizes the problems that may be caused by excessive input currents which cause stator overheating. Eight different thyristor voltage control circuits have been tested to determine a best control circuit for three-phase motors and test results are given. A practical speed control for a 4-hp motor is described and the way in which a special rotor design can minimize the problem of excessive stator losses is convincingly demonstrated.

The Analysis of Induction Machines Controlled by Series Connected Semiconductor Switches

Abstract: The widespread interest in applying semiconductor switching elements to induction motor speed control has created. a need for a fast and accurate means of predicting motor performance under the unconventional excitation inherent in these systems. In many cases this excitation takes the form of alternate periods of normal sine-wave excitation and periods of open-circuit operation. This paper presents an analytic means of determining steady-state performance under these conditions. Use of the techniques presented permits direct computer solution without iteration. Solution time is thus much shorter than can be achieved by iterative solution of the system differential equations and the necessarily long computation times required to allow the solution to reach steady state from arbitrary initial conditions. Computer results for a single-phase motor and for a capacitor run motor are presented. Results include average torque, motor losses, and efficiencies, and instantaneous torque, current, and voltage as a function of speed. Run time on an IBM 1620 Model II is approximately 100 seconds for each of the desired speeds. The method is thus well suited to evaluation of proposed motor designs and subsequent design optimization.

Unbalanced magnetic pull in a 6-pole induction motor

Abstract: A 6-pole 10kW induction motor has been specially constructed for a comprehensive series of steady-state and transient measurements of the magnitude of unbalanced magnetic pull resulting from an eccentric air gap. These measurements have shown that unbalanced magnetic pull is critically dependent on Saturation of the magnetic circuit, and that the magnitude in the cage-rotor motor is much less than in the wound-rotor motor. Comparisons between measurements and calculations show that most existing theories inaccurately predict the magnitude of transient unbalanced magnetic pull during starting, the effect of slotting and saturation, the effect of a cage rotor, and the action of load currents. A simple theory is developed which gives reasonable agreement with measurements for the wound-rotor motor over a range of supply voltages about the rated value.

Stabilizing phase controlled ac induction motors

Abstract: An adjustable speed capacitor run or polyphase AC induction motor energized by phase controlled voltage is stabilized by an AC phase control circuit whose timing is referenced to the zero crossing of the capacitor voltage in an RC circuit and supplies gating signals to a thyristor in series with the motor, characterized by a degenerative feedback circuit comprising an impedance element connected to modify the capacitor voltage in such manner that the cessation of thyristor conduction modulates the timing of the next gating signal.

On the Analysis of the Three-Phase Induction Motor with Voltage Control by Thyristor Switching

Abstract: The three-phase induction motor, at constant speed, can be represented by five loop voltage equations in terms of variable inductance coefficients and instantaneous winding currents. Voltage control by symmetrical triggering of inverse parallel connected pairs of thyristors in the stator winding branches causes the motor excitation voltages to be segments of sinusoids. Current conduction angle at a given speed depends on the triggering angle. The system currents reduce to a set of five first-order differential equations with periodically varying coefficients. No general solution of these is known but possible approaches are discussed. A numerical or analog approach seems desirable.

Comparison of Computer and Test Results of a Static AC Drive System

Abstract: The effectiveness of an analog computer (electronic differential analyzer) in studying the performance of a rectifier-inverter-induction motor drive system is demonstrated. Computer diagrams are not given. However, methods of simulating the system components sufficient to describe their dynamic and steady-state behavior are described and references cited. Computer recordings are compared with results obtained from a prototype of an actual drive system. In particular, computer and test results showing the six-phase rectifier output voltage, filter inductor current, filter capacitor voltage, inverter current, as well as the stator voltage and current of the induction motor are compared. The facility of the analog computer in predicting the behavior of a large involved drive system is demonstrated.

Starting switching means for a single-phase asynchronous motor

Abstract: The invention relates to starting switch means for a single phase asynchronous motor having a symmetrical controlled semiconductor valve as a starting switch. The valve and an ohmic resistor is in series with the starting winding of the motor. A PTC resistor is connected to the control electrode of the valve and is arranged parallel to the valve and the ohmic resistor. The starting switching means is enclosed, along with the electric motor, in the housing of a refrigeration unit.

Low-speed synchronous electric motor

Abstract: 1,196,213. Synchronous motors. A. S. KURAKIN, and F. M. JUFEROV. 30 April, 1968, No. 20363/68. Heading H2A. A low-speed synchronous motor having electromagnetic speed reduction comprises coaxially nested internal and external stators 3, 1 whose mutually facing surfaces have equal numbers of open slots, an A.C. winding 2 on the external stator 1, a motor frame 8 and end shields 5, and in the annular gap between the stators a non-wound hollow rotor 6 comprising a thinwalled metal cylinder 18 having ferromagnetic teeth and intervening non-magnetic portions acting as open slots 19. The speed reduction is effected by interaction of the harmonics of the fields of the toothed stators and rotor. As shown, the stator 3 is excited by a field winding 4, but in other embodiments it is excited by a permanent magnet. The rotor shown is a slotted ferromagnetic cylinder, but in Fig. 6 (not shown), is of A1. slotted to receive ferromagnetic strips (21). The motor is for use in tape recorders, time relays, and automatic control and electronic devices.

An experimental study of impulse voltage phenomena in a large AC motor

Abstract: High voltage ac motors with ratings from a few hundred kW may, during normal operation, be subjected to steep-fronted impulse voltages which can be hazardous to the turn-to-turn insulation. The rise time of these impulse voltages can be as short as 0.2 microsecond. These problems were discussed by R. M. Sexton in a recent paper1 where further references can be found.

Generalised theory of induction motors with asymmetrical primary windings, and its application to the analysis and performance prediction of shaded-pole motors

Abstract: An analysis is developed for induction motors having any number of asymmetrical primary windings and any symmetrical or asymmetrical power supply, with any symmetrical or asymmetrical impedances connected to the motor terminals, provided that symmetrical polyphase secondary windings are utilised. The basis of the analysis is the transformation of the asymmetrical primary windings to their equivalent tapped-quadrature windings, which has the advantage of facilitating the application of 2-phase rotating-field theory to the asymmetrical machine. All the parameters involved in the generalised analysis can be calculated from the design details of the motor, and its validity is demonstrated by the performance prediction of the shaded-pole type of single-phase motor.

Frequency-dependent dynamic, representation of induction-motor loads

Abstract: For comprehensive transient and dynamic stability studies, induction-motor loads have usually been represented by fixed shunt impedances at system nominal frequency. A method is proposed here which allows single induction motors or groups of induction motors to be considered as frequency-dependent and dynamic. If the necessary parameters of the motors are known and if computation effort is not a problem, accurate assessment can always be made for single machines or groups of machines but may involve lengthy calculations. Alternatively, an approximate, rapid method is presented which gives the change in active-power input to an induction motor or group of induction motors due to inertia and frequency-variation effects to within ±2% of the actual change for known power factor, inertia factor and full-load slip; and to within ±5% of the actual change when the assumed parameters are within the following limits: power factor within ±5% inertia factor within ±10% and full-load slip within ±20% (where the actual change in power has been assessed by the use of an equivalent circuit). The change in reactive power, as assessed, is within ±20% of actual change in reactive power. ±5% change in operating frequency has been considered. For simplicity, windage and friction losses have been taken as an integral part of the load supplied by the motor, and core losses have been assumed constant. These methods can be applied to power-system stability studies, as illustrated by an example.

Rerating Factors of Polyphase Induction Motors Under Unbalanced Line Voltage Conditions

Abstract: The load on the induction motors is to be reduced under unbalanced voltage conditions so as to limit the temperature rise to the normal value. The rerating factors are determined experimentally and compared with four methods of prediction. The first two methods are drawn from previously published work and have failed to correlate the experimental and predicted values. An attempt to incorporate the effect of winding details and heat conduction is made in the third and fourth methods, respectively. Based on the third method, generalized curves are presented which can be applied to any motor having short-chorded windings. This method has its own limitations. The fourth method is a new approach, the results of which agree very closely with the experimental data on two types of motors tested.

Cogging torques in induction machines

Abstract: Depending on the choice of design parameters, an induction machine can produce a cogging torque sufficiently high to give rise to locking at standstill. It is noted that the commonly used method of analysis of these torques, involving permeance waves, is strictly incorrect, and its use in conjunction with the ‘overlap’ method is inaccurate. The way in which cogging torques arise is discussed in terms of the energy associated with the magnetic field in the machine. A new method is described by which this torque variation can be determined from simple equations. Experiments on machines confirm the results of the analysis.

The Controlled Slip Static Inverter Drive

Abstract: The ability to use the versatile characteristics of the semiconductor inverter to achieve optimized control of a squirrel cage induction motor is demonstrated. The technique has been made possible during the past few years due to the development of fast, powerful, and efficient inverters. These, in turn, are the direct result of advances in semiconductor technology, which have produced thyristors with improved qualities and the advent of integrated electronics. If the slip of an induction motor is constrained and controlled to values below breakdown, high efficiency, high power factors, and moderate currents result in performance comparable to that of a dc machine. General expressions defining torque and involving the quantities of slip and excitation are easily derived. Excitation can be expressed in terms of volts per cycle, current, or flux. Torque can be controlled by adjusting slip or excitation or both in combination. The ability to control slip and escitation precisely and accurately depends on the inverter which is used. The pulse width modulated (PWM) inverter is an extremely effective motor controller accomplishing voltage and frequency adjustment in a single circuit. It is a fast, linear device; its response is virtually instantaneous. As a power amplifier it is comparable to the duel converter of dc systems, and its speed of response makes it applicable to virtually any feedback loop. In considering the mating of the motor and inverter, several principal factors are involved in the optimization of the system.

Characteristics of an Adjustable Speed Polyphase Induction Machine

Abstract: Variable frequency stepped-voltage wav eforms generated by an SCR bridge-inverter are used to drive a 3-phase induction motor. The system coefficient matrix associated with the volt- ampere differential equations of a polyphase induction machine is cyclic symmetric about its major diagonal axis. A basic property of this type of matrix is that it can be diagonalized by a transformation of variables by use of the well-known symmetrical component transformation. Here mutual cross-coupling terms give rise to speed- sensitive coefficients in the solution equations. Hence, the contribution presented in this paper is in the formulation of the solution of high-order complex systems in closed form. Solutions are obtained for phase currents and line and phase voltages on a per mode basis, where a mode is defined as one sixth of a cycle. Two cases arise as a result of both speed and slip conditions of the load. In one case conduction takes place over each entire mode and in the other case conduction ceases at some intermode time ?p. The latter case requires two sets of solution equations per mode. Torque equations are derived and typical response curves are plotted. Experimental testing of a prototype system corroborates derived analytical solutions.

Speed Control of Wound Rotor Motors with SCRs and Saturistors

Abstract: This paper describes means for stepless speed control of induction motors, using silicon controlled rectifiers with delayed firing to vary the primary voltage, and Saturistors' in the motor secondary circuits to limit the rise in current at low speeds.

Analysis of the Solid Iron Rotor Induction Motor for Solid-State Speed Controls

Abstract: This paper discusses the results of tests made on a 3-phase solid iron rotor motor and substantiates simple design equations that give a clear understanding of the motor performance. From this understanding, areas in which the motor could be exploited are suggested, and it is concluded that the solid rotor machine has characteristics that are particularly suitable for solid-state power controls.

Developments in Speed-Changing, Single-Phase Induction Motors

Abstract: Further development of single-phase, pole-changing motors based on the logic of pole-amplitude modulation are reported. The principles of preshaping and asymmetrical modulation have been extended to the single-phase motor, the former resulting in reduction of number of terminals from 12 to 6, while with the latter, the pole- amplitude modulation logic can be applied to any set of pole combinations. The versatility of this method is enhanced by extending it to the case of asymmetrical phase windings, resulting in 3-speed machines, which can be started as normal single-phase machines at any of the three speeds.

Methods and arrangements for testing induction motors

Abstract: Method apparatus Apparatus for testing induction machines by the back-to-back technique. The shafts of the motors are coupled together such that they will turn in the same direction through a motor or motor and gearbox combination. The motor or motor and gearbox combination enables the torques in the shafts of the motors being tested to be balanced.

A Single-Phase Induction Motor with One Distributed Winding

Abstract: A single-phase induction motor with one winding develops starting torque if the stator iron is not symmetrical about the winding axis. Four methods of producing the dissymmetry are described. The most significant design variables are rotor resistance, the location of the dissymmetry, and the difference of the magnetizing reactances perpendicular to and along the dissymmetry. Test and calculated data are shown. The motor can be used where low starting torque is required and, under certain conditions, it is superior to other motors.