William Shepherd

Slip Power Recovery in an Induction Motor by the Use of a Thyristor Inverter

The low-speed efficiency of an induction motor is improved by rectifying slip-frequency power, inverting this to line frequency, and injecting it back into the supply directly (line feed-back) or through auxiliary stator windings (stator feedback). Torque-speed curves then have the nature of a variable-speed drive. The low power factor and nonsinusoidal supply current of the line feedback connection are improved by use of the stator feed-back method but the improvement of efficiency is then much less. Line feedback with a two-phase induction motor eliminates the need for a variable control voltage source.

Operation of a Fixed Capacitor-Thyristor Controlled Reactor (FC-TCR) Power Factor Compensator

A fixed capacitor-thyristor controlled reactor (FC-TCR) type of power factor compensator with thyristor-controlled series R-L load is analysed using an approximate and also a more exact circuit. The variation of power and power factor before and after compensation is examined for both cases. It is shown that considerable power factor improvement can be achieved in the circuit for low values of load thyristor firing- angle. However, for higher values of firing-angle, any improvement is obtained at the expense of additional. power losses.

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

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.

Thyristor Control of Resistive and Series DC Motor Loads Using Integral-Cycle Switching

The use of integral-cycle conduction of power appears to be useful for resistive load control, especially in heating applications. Uniform control sensitivity may be obtained over the whole power range if the controller has a fixed ON plus OFF time with variable ON/OFF ratio. The incremental load power controllable is proportional to the number of ON plus OFF supply cycles constituting a control period. Rectifier integral-cycle pulses of current were obtained from a single-phase supply using a pair of inverse-parallel connected thyristors and a diode bridge. This particular form of pulsewidth modulation was used for speed control of a fractional horsepower series dc motor on both open loop and closed loop. Steady-state characteristics in terms of mean current and mean speed were identical with those obtained from conventional dc supply. For high-speed low-torque operation there was little speed ripple and smooth, quiet motor performance was obtained. To obtain low speeds, the ON/OFF mark space ratio of the current was less than unity and the intermittent conduction caused speed ripple with motor noise and vibration.

An Experimental Closed-Loop Variable Speed Drive Incorporating a Thyristor Driven Induction Motor

Closed-loop control of a 3-phase induction motor is investigated where the stator voltages are varied, at line frequency, by the controlled firing of silicon-controlled rectifiers (thyristors). Steady-state torque-speed curves are given and the transient response of the drive to step changes of reference signal is shown. The rapid transient response of the drive due to transistorized thyristor firing circuits is demonstrated. The transfer function of the thyristor power modulator cannot be specified independently of the transfer function of the motor.

Microprocessor-Based PWM Inverter Using Modified Regular Sampling Techniques

Pulsewidth modulation (PWM) techniques for inverter control of ac motors is becoming increasingly popular because of the number of practical advantages it offers over other techniques. Different PWM control strategies can be implemented more easily using microprocessor control. A modified microprocessor-based PWM technique incorporating linearization of the output voltage, with simultaneous reduction in the harmonic content is presented.

Analysis of Induction Motor Subjected to Nonsinusoidal Voltages Containing Subharmonics

Modulation techniques can be used in thyristor circuits to obtain frequency changing that is useful for ac motor speed control. Some of the modulation techniques result in three-phase applied voltage waveforms that contain subharmonics of the supply (carrier) frequency as well as higher harmonics. An analytical technique is developed for predicting the performance of an induction motor when subjected to this form of nonsinusoidal voltages. The technique involves the use of reference frames that rotate at subsynchronous speeds consistent with the known subharmonies of the voltages. A theoretical investigation of many applied voltage modulated waveforms shows that only one particular form of phase modulation is feasible for induction motor speed control.

Steady-State Analysis of the Series Resistance-Inductance Circuit Controlled by Asymmetrical Triggering of Thyristors

The load waveforms produced by asymmetrical triggering of an inverse parallel connected pair of thyristors are of two categories. If each of the two firing angles ?,? (3 are greater than load phase angle ?, the load current consists of asymmetrical discontinuous nonsinusoidal pulses. For the case when firing angle 3 is less than p and satisfies the relationship sx ? - ? (where x =? is the extinction angle corresponding to the other firing angle ?), complete half-sinusoids of load voltage appear. Both modes of triggering result in load waveforms with even harmonic content, which are not present with symmetrical triggering.

Steady-State Performance and Analysis of the Series Resistance--Capacitance Circuit with Control by Adjustable Thyristor Triggering

A pair of thyristors connected inverse parallel, with symmetrical triggering, are found to give smooth, stepless control of the current and load voltage in a series resistance-capacitance circuit. Large current pulses of adjustable amplitude are produced that appear to be suitable for electric welding. The application to power factor compensation of inductive loads is limited in usefulness due to poor waveform and high dissipation in the resistance-capacitance branch.

Analysis and performance of hybrid thyristor-controlled three-phase resistive loads

The authors describe a method of analysis for and the performance characteristics of hybrid thyristor power controllers for three-phase resistive loads, Different circuit configurations having full-wave or half-wave control are discussed. It is shown that of the various configurations considered, the three-phase, three-wire, star-connected system with full-wave control offers the best overall performance, having a sensitive range of power control over a wide range of firing angles. Line current distortion is small, so that the circuit is characterized by high distortion factor and input power factor. For some applications such hybrid power controllers appear to be more suitable than conventional thyristor power controllers.<<ETX>>