The authors summarize the state of knowledge of the effects of power system harmonics on equipment. The general mechanisms presented are thermal overloading, disruption, and dielectric stressing. Quantitative effects are presented or referenced whenever possible. However, many of the effects are can only be qualitatively described. The types of equipment considered are adjustable speed drives, capacitors, circuit breakers, fuses, conductors, electronic equipment, lighting, metering, protective relays, rotating machines, telephones, and transformers. >

Effects of harmonics on equipment

The control of a doubly-fed induction machine (DFM) with and without the use of a rotor position encoder is examined. First, a stator flux linkage oriented control scheme for power and speed control with a position encoder, shows the high performance control capabilities of this control arrangement and serves as foundation for the sensorless scheme. The power-control method is then applied to a sensorless method, based on a previous control arrangement. This method is then extended to derive a sensorless speed-control mechanism for the DFM. All control schemes are implemented and performed on an experimental test system, comprising two 80C167 microcontrollers and a 2.25 kW wound rotor induction machine.

Stator-flux-oriented control of a doubly-fed induction machine with and without position encoder

A new dual stator winding induction machine drive is described in this paper. The proposed induction machine consists of a standard squirrel-cage rotor and a stator with two separate windings wound for a dissimilar number of poles. Each stator winding is fed from an independent variable-frequency variable-voltage inverter. The proposed drive offers such advantages as speed sensorless operation, better reliability, and more flexibility to manipulate the resultant torque-speed curve of the motor. In the proposed drive, zero-speed operation is achieved by independently controlling the two sets of stator currents, hence, maintaining a minimum electrical frequency independent of the mechanical speed. This feature is especially important to minimize the negative impact of the stator resistance influence at low-speed operation and it greatly simplifies the implementation of speed sensorless control schemes. The drive is well suited for either constant volts per hertz or field-oriented (FO) operation. Circulating harmonic currents, common to most dual stator machines, are eliminated by the dissimilar pole number in each stator winding.

Dual stator winding induction machine drive

For pt.III see ibid., vol.29, no.1, p.143-151 (1993). An analytical technique is developed for predicting the instantaneous magnetic field distribution in the airgap-permanent magnet region of radial-field topologies of brushless DC motors equipped with a surface-mounted magnet rotor and operating under any specified load condition. It accounts implicitly for the stator winding current waveform and the effect of stator slot openings. The 2-D field analysis, in polar coordinates, combines the armature reaction field component, derived from an analytical determination of the relative temporal and spatial position of the two field components. The predicted open-circuit, armature-reaction, and load field distributions all show excellent agreement with results from corresponding finite element analyses. >

Instantaneous magnetic field distribution in permanent magnet brushless DC motors. IV. Magnetic field on load

The authors develop the concept of the brushless doubly-fed machine (BDFM), from consideration of the frequency and distribution of the EMFs induced in bars of a conventional doubly wound cage motor. These considerations lead to the development of a rigorous mathematical model which is suitable for the analysis of BDFM operating in the synchronous mode. Expressions for all impedances and electrical and mechanical quantities are derived from first principles and the concept of load angle is discussed.

Generalised theory of the brushless doubly-fed machine. Part 1: Analysis

AC adjustable-speed drives (ASDs) are finding increased opportunities for applications in which acoustic noise is a major consideration for acceptability. The major source of noise in an ASD is the excitation of the motor by nonsinusoidal currents. For load characteristics that simulate the performance required for fans and compressors, ASDs based on induction motors, permanent-magnet motors, and switched reluctance motors have been examined for correlation between current and acoustic noise. The effects that the specific types of noise produced by drives have on human subjects is examined with regard to temporary and permanent hearing-threshold shifts. >

Current harmonics and acoustic noise in AC adjustable speed drives

A mathematical model that has the necessary flexibility to investigate design alternatives of the brushless doubly fed machine (BDFM) is developed. The model development starts from a consideration of the basic stator coils and the basic nested loops that constitute the rotor cage. The simple time-domain interactions that occur between the basic machine components are developed into the performance equations of the BDFM by application of the appropriate connection matrices. These matrices represent the internal connection of stator coil groups and the final connections to two independent three-phase supplies of different frequency and voltage. The resulting model of electrical performance and the associated torque equations are suitable for solution as a set of state variable equations. This makes possible the simulation of the system interactions that occur between the machine, its power converter, and the utility supply in any combination of monitoring or generating, synchronous or asynchronous modes that is possible with the BDFM. >

Dynamic modeling of brushless doubly-fed machines

The more widespread adoption of adjustable-speed drives to promote process efficiency through energy conservation is currently hampered by the high cost of power electronic conversion units for use with AC motors. The examination of alternative methods of obtaining speed control in both synchronous and induction machines leads to the conclusion that a self-cascaded machine has considerable promise. The motor has six input terminals to the same stator windings: one three-phase set presenting a different pole number to the other three-phase set. The rotor is a specialized form of a robust cage construction without slip rings. When operated in a doubly-fed mode, this machine requires only a fraction of its input power to be processed by a power electronic converter. The resulting system has the potential to provide a low cost, robust, fault-tolerant, precise speed, and adjustable speed drive. >

The potential of brushless doubly-fed machines for adjustable speed drives

The authors use a digital simulation model to investigate several steady-state and dynamic behavioral modes of the brushless doubly fed motor (BDFM) drive, including operation as an induction machine, free acceleration, frequency separation between the different stator windings, and response to changes in operating conditions. For these modes, predictions are compared with test results obtained from an experimental laboratory machine. The drive system simulation is established, by the correlation of test and predictions, to be the logical first step in the development of design techniques for the BDFM. >

Performance simulation of brushless doubly-fed adjustable speed drives

A simulation model for the performance prediction of brushless DC drives is used to investigate the speed control obtained from modulation index and commutation advance variations for a motor of complex magnetic circuit geometry powered by a sine-coded PWM (pulsewidth modulation) inverter. The results of the study show the effects of motor model complexity on voltage and torque. Optimum performance conditions in the field-weakening mode are determined, and the value of motors using sinusoidal induced voltages is explored. >

Performance characteristics of brushless DC drives

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