Allan B. Plunkett

Development of a Modulation Strategy for a PWM Inverter Drive

When induction motors are driven by electronic inverters, the applied voltage waveforms are quite nonsinusoidal. The fundamental component of the current is controlled by the load. The harmonic currents are limited principally by the motor leakage inductance and are independent of load. These harmonic currents can lead to increased motor heating and to increased peak currents. In addition, the modulation technique can give rise to problems in the control. An analysis method was developed to predict the performance of an induction motor in response to inverter waveforms. The correctness of the method was verified by comparison to measurements made on an operating system. This tool was then used to aid in the development of a modulation strategy, to accurately evaluate the motor heating problem and to analyze a new method of transitioning from pulsewidth modulation to square wave operation.

Direct Flux and Torque Regulation in a PWM Inverter-Induction Motor Drive

A dc motor drive is controlled by varying the armature current and field current. The field is a measure of flux, and the armature current times field current is a measure of torque. Various approximate means of estimating the flux and torque levels in an induction motor exist. Most of these methods are sensitive to motor parameter value changes and do not work well near zero speed. Also, the harmonics in the motor voltage and current due to the nonsinusoidal inverter waveform cause errors in the estimated torque. A practical method has been developed to measure the flux level in an induction motor in the actual operating environment. Using the flux signals and stator current, the actual electromagnetic torque can be obtained. This torque signal responds correctly to motor saturation and inverter voltage waveform harmonics. The motor can be designed to operate without the customarily required flux margin, since the flux level is accurately controlled. The control strategy for use with these feedback signals does not require the use of a tachometer.

Load-Commutated Inverter/Synchronous Motor Drive Without a Shaft Position Sensor

Synchronous motor drives are beginning to enjoy renewed popularity for certain applications such as flywheel energy storage. One of the reasons is that a synchronous motor drive can be made self-commutating so that the inverter becomes equivalent to a conventional phase-control bridge converter without large commutating components. The drive to be controlled consists of an inductor alternator coupled to a flywheel that will be used for temporary energy storage. The flywheel speed will vary over a two-to-one range; thus full performance can be achieved using a load-commutated inverter. A simple auxiliary commutating circuit is provided only for initial starting. The drive must operate both from a fixed dc voltage source (battery) and from a variable dc voltage source (dc motor), as it is intended for use in a battery-powered vehicle for load leveling. The load-Commutated inverter consists of two bridges connected in parallel so as to provide two-way power flow by reversing the dc current. The control system described does not use shaft position sensing, but senses the motor terminal voltage and the ac line current to determine the motor Nare internal operating conditions. This control method maintains the silicon-controlled rectifier (SCR) turn-off time as a constant percentage of the period of the generated back electromotive force (EMF) as speed and current are varied during operation. The inverter turn-off time is thus maintained by feedback control rather than the use of special function generators. A 20-kVA laboratory development drive system has been built and successfully tested using this control strategy.

Synchronous Control of a Static AC Induction Motor Drive

Conventional methods of controlling an induction motor utilize regulation of stator current and motor slip frequency in order to maintain system stability. This control strategy requires a shaft speed feedback and fast-response current regulation. An alternative method of controlling an induction motor is presented which achieves the necessary system stabilization by controlling only the motor frequency. The control inherently regulates the motor torque angle by properly adjusting the phase of the converter firing signals. By synchronizing the inverter firing pulses to the motor back electromotive force (EMF) possible adverse inverter operating modes are avoided. The concept of synchronous control eliminates the preprogrammed functional relations previously required and allows the control to adapt to any desired motor flux level. Any desired outer regulating loop can be incorporated to form a fast-response wide-range ac drive system.

A Novel Approach to Induction Motor Transfer Functions

The scope of static ac drives is increasing rapidly, and with this increase has evolved the need to devise control strategies for a variety of new applications. Purely analytical approaches to design of control systems for ac drives are hindered by the highly coupled nature of the ac induction motor equations which, in the past, necessitated lengthy manual derivations of transfer functions. This paper presents an alternative approach using a state variable formulation. The equations are arranged in a form such that the entire derivation procedure can be relegated to a digital computer. Transfer function poles, zeros, and gain for any practical input-output pair of variables can be rapidly obtained.

Inverter-Induction Motor Drive for Transit Cars

The advent of large power semiconductors has made it possible to apply inverters and ac motors to traction applications. Either synchronous or induction motors and several types of power converters can be considered. The induction motor and the pulsewidth modulated (PWM) inverter are selected as favorable for application to a transit car drive. A general method of sizing the PWM inverter and induction motor in terms of the car performance requirements is out-lined. This method results in a minimum size inverter and allows optimization of system weight and cost. A discussion of wheel size effects and the optimization of regenerated energy is included.

Transistorized PWM Inverter-Induction Motor Drive System

The development of a transistorized pulsewidth modulated (PWM) inverter-induction motor traction drive system is described. A vehicle performance analysis was performed to establish the vehicle tractive effort-speed requirements. These requirements were then converted into a set of inverter and motor specifications. The inverter was a transistorized three-phase bridge using General Electric power Darlington transistors. The description of the design and development of this inverter is the principal object. The high-speed induction motor is a design which is optimized for use with an inverter power source. The primary feedback control is a torque angle control with voltage and torque outer loop controls. A current-controlled PWM technique is used to control the motor voltage. The drive has a constant torque output with PWM operation to base motor speed and a constant horsepower output with square wave operation to maximum speed. The drive system was dynamometer tested, and the results are presented.

System Design Method for a Load Commutated Inverter-Synchronous Motor Drive

In a thyristorized load commutated inverter synchronous motor drive system, the inverter relies on the back EMF of the synchronous motor to provide the reverse voltage for commutation. The level of the back EMF is a function of both the synchronous motors field current and rotational speed. The inverter operating frequency is determined by the motor speed and number of pole pairs. For a fixed thyristor firing angle, an increase inmthe level of armature current decreases the available thyristor turn-off time due to overlap of the incoming and outgoing thyristor. This overlap time is a function of the commutating reactance of the synchronous motor. These factors, field current, motor speed, inverter frequency, and armature current, together with a thyristor turnoff time specification, set a maximum limit on the value of motor commutating reactance. The motor designer and inverter designer must perform a tradeoff study of these factors to arrive at a satisfactory drive system. The results are presented of an analysis that allows the synchronous motor reactance to be specified based on the other given parameters of the inverter-synchronous motor drive system. Test data are provided from a 20-kVA 500-1000 Hz load commutated inverter/ inductor type synchronous motor drive system. The effects of motor speed and field excitation on the operating condition of the drive are discussed.

New Methods of Induction Motor Torque Regulation

Two methods of induction motor torque regulation are described utilizing speed and current feedback into the frequency and amplitude channels of a pulsewidth modulated inverter. Transfer characteristics illustrating the steady-state behavior of the two configurations are presented. Transient characteristics are discussed in detail by examining the linearized system transfer functions. An analytical approach to design of a closed loop controller for a practical application is outlined. Experimental results are presented showing correlation with predicted results.

Steady-State Performance of Voltage Source Inverter/Synchronous Machine Drive Systems

The steady-state performance, including harmonic losses and torques, of synchronous machines excited with voltage source inverters is investigated. A frequency-domain solution is presented that uses the Fourier series of the inverters line-to- neutral voltage and the d-q equivalent circuit for the synchronous machine. The method for calculating the Fourier series of the inverter line-to-neutral voltage only requires the dc link voltage and the switch times for the inverter switching elements. It is shown that a single-frequency harmonic stator voltage causes a harmonic stator current of two frequencies due to the nonsymmetry of a salient pole or nonsalient pole synchronous machine. The analysis has been embodied in a library of Fortran computer programs. Program output includes the Fourier series of inverter output voltage, inverter link current, feedback diode current, Fourier series of stator current, peak stator current, average and peak electromagnetic torque, and I2R losses in the machine windings. Also available are plots of the instantaneous voltages, currents, and electromagnetic torque. Experimental waveforms of a laboratory voltage source inverter/synchronous machine drive system are included for comparison purposes.