A.K. Wallace

Multilevel inverter modulation schemes to eliminate common-mode voltages

It is well known that conventional two-level pulse-width modulated (PWM) inverters generate high frequency common-mode voltages with high dv/dt. Similarly, commonly used multilevel inverter modulation schemes generate common-mode voltages. Common-mode voltages may cause motor shaft voltages and bearing currents and conducted electromagnetic interference (EMI). Premature motor bearing failures and electronic equipment malfunctions have been reported to be directly related to bearing currents and EMI. In this paper, approaches to eliminating common-mode voltage when using multilevel PWM inverters are presented. It is shown that inverters, which have an odd number of levels, will generate zero common-mode voltage by switching among certain states. Therefore motor bearing currents will be eliminated and conducted EMI will be reduced. Both sine-triangle intersection modulation (SPWM) and space-vector modulation (SVM) schemes are discussed and detailed comparative simulation results between conventional and novel modulation schemes are provided.

Remedial strategies for brushless DC drive failures

The possibly of developing failure diagnostics and remedial operating strategies that together will enable adaptive fault tolerant operation is investigated for a brushless DC drive with a fan load. It is shown that the correlations between the predictions of a simulation program and test measurements provide confidence that the development of fault-tolerant drives can be undertaken with this tool. Methods by which a certain performance level can be re-attained, following some simple faults to a motor winding and an inverter transistor, are examined for their relative effectiveness.<<ETX>>

Two-axis model development of cage-rotor brushless doubly-fed machines

A two-axis model of a cage-rotor brushless doubly-fed machine (BDFM) which is suitable for machine and BDFM drive system dynamics studies is presented. The technique enables the two-axis model to be derived from a detailed model recently developed for investigation of design aspects of the machine. Unlike the conventional approaches to self-cascaded machines analysis, this method permits analysis of the machine characteristics in both dynamic and steady state conditions, and also facilitates machine and drive system design by directly correlating machine performance with machine structure and parameters. The approach allows the machine parameters to be calculated from machine geometry and developed into the model parameters. >

Experimental evaluation of a variable-speed, doubly-fed wind-power generation system

The authors evaluate the potential of a brushless doubly fed generation system for wind power applications. A 1.5 kW proof-of-concept laboratory prototype is used to investigate the feasibility of the proposed variable-speed generation principle. Experimental results show that the prototype system can achieve high efficiency over a range of speeds. The system efficiencies achieved at the power levels considered compare favorably with conventional squirrel cage induction machines. It is experimentally demonstrated that the brushless double fed system achieves variable-speed operation with a power converter of reduced rating. For the prototype under consideration, a 2:1 speed range can be covered with a converter rated at approximately 25% of system capacity. While reactive power control can be realized with the proposed system, the magnitude is limited by the desired low converter rating. The prototype system is also shown to exhibit excellent output current waveforms.<<ETX>>

An evaluation of mitigation techniques for bearing currents, EMI and over-voltages in ASD applications

With the increased application of adjustable speed drives (ASDs) for the efficient speed control of AC motors, there has been a growing number of costly motor-drive related process failures. The popular ASDs consist of a pulse-width modulated (PWM) inverter switching insulated gate bipolar transistors (IGBTs) at 2 to 20 kHz, with rates of voltage rise (dv/dt) of 6000 V//spl mu/s. The high dv/dt has adverse effects on the motor insulation due to motor terminal overvoltages (exacerbated by longer cable lengths) and contributes to electromagnetic interference (EMI) and damaging bearing and leakage currents caused by common-mode voltages. In response to these problems, a variety of mitigation techniques have emerged. However, oftentimes the industrial user searching the literature for an immediate solution faces difficulties determining which mitigation technique is best for his/her application. Using laboratory measurements this paper evaluates and compares various suggested mitigation techniques for bearing currents, EMI and motor terminal overvoltages in terms of their effectiveness and losses and provides practical application assessments for users.

Dynamic simulation of brushless doubly-fed machines

Dynamic and steady-state models for the simulation of the performance of experimental brushless doubly-fed machines (BDFM) are presented. The dynamic simulation results are obtained using a two-axis representation which has been developed from a detailed machine design model. In turn, it is shown that several forms of steady-state equivalent circuit can be developed from the two-axis model for different specific modes of operation. Test data in dynamic conditions are compared with the predictions given by the two-axis model. It is concluded that these simplified models will provide adequate representation of full performance for control, stability, and scoping studies. >

Investigation of appropriate pole number combinations for brushless doubly-fed machines applied to pump drives

Brushless doubly-fed machines have the capability of precise (synchronous) speed control over a wide speed range through the use of a bi-directional power converter due to the magnetic coupling of two different pole numbered stator windings through a specifically configured rotor. The application of a brushless doubly-fed machine, with different pole-pair combinations is investigated for use as a limited speed range pump drive by using a steady state equivalent model developed in a companion paper. The proposed brushless doubly-fed machine designs when compared to an equivalent induction motor drive show the advantages of operation at close to unity power factor and reduced power converter rating. These results from the simulation demonstrate the substantial economic advantage of using the brushless doubly-fed machine in limited speed-range operations. >

Synchronous drive performance of brushless doubly-fed motors

Recent studies of the steady-state performance of the brushless doubly fed machine for adjustable speed drive applications are presented. The equivalent circuit and basic performance equations are presented, followed by a proven technique for the solution of the least restrained form of the equations. Model predictions and experimental results are given. It is shown that the machine acts like both a synchronous motor, with power factor adjustable by control winding excitation, and a slip-ring induction machine with slip power recovery and a low power converter rating. The model is suitable for development of control schemes and performance index optimization.<<ETX>>

Current harmonics and acoustic noise in AC adjustable speed drives

The major source of noise in an ASD (adjustable-speed drive) 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 has on human subjects is examined with regard to temporary and permanent hearing-threshold shifts.<<ETX>>

General pole number model of the brushless doubly-fed machine

The brushless doubly-fed machine is receiving attention as a contender for several niche applications. In some cases the speed ranges of these applications appear to be more readily met by judicious selection of the pole-pair numbers of the stator phase winding. In order to compare different proposed pole-pair configurations and to help in the development of specific designs, a general analytical model, in both dynamic and steady state forms, is presented. This model supersedes previous analyses which are restricted to the 3/1 pole-pair combination or early laboratory machines. The new model is also completely valid for both positive and negative control voltage sequences. The increased modelling flexibility enables sound estimations of the drive power convertor rating, the reduction of which is the key to the economic advantage or these machines. An example of the correlation or the performance predictions and test results encourages use of this new model.<<ETX>>