Michael Møller Bech

Analysis of power and power spectral density in PWM inverters with randomized switching frequency

A novel method for accurate calculation of power spectra of the pulsewidth modulation (PWM) voltage-source inverter with randomized switching frequency is presented. The case of a limited pool of switching frequencies, convenient and adequate for technical purposes, is considered, and a mathematical background for this type of random PWM (RPWM) is provided. It is shown that the limited-pool RPWM gives rise to not only a continuous spectrum, but, also, under certain circumstances, pure power spectral components (harmonics), typical for the deterministic PWM. Criteria for the existence of harmonics are given, and formulae for both the power and the power spectral density are derived and verified experimentally.

A methodology for true comparison of analytical and measured frequency domain spectra in random PWM converters

The analysis of random PWM techniques has matured into a state where analytical expressions have been derived to aid in understanding the frequency domain characteristics. Derivations of the formulas require a lot of algebra, and the expressions must be verified by laboratory measurements. It is shown, however, that factors originating from digital signal processing techniques make the comparison difficult if proper measures against misinterpretation are not taken. A methodology to overcome the problems is presented. Novel expressions for the random lead-lag pulse position technique and the random switching frequency technique are also presented, as well as their verifications by laboratory measurements on a full-bridge DC/DC converter.

Random modulation techniques with fixed switching frequency for three-phase power converters

This paper proposes a new family of random modulation techniques for three-phase power converters which operate with a fixed switching frequency. The techniques are based on adjusting the duration of the zero-vectors or adjusting the three PWM pulses in a switching period. Three methods are selected for experimental tests because they preserve the fixed switching frequency known from standard PWM techniques. The new methods are also compared with a classic random modulation and with fixed switching frequency modulation. Voltage, current and acoustic noise spectra are used for comparison and it is concluded that two of the techniques are especially useful at lower fundamental frequencies. The techniques can substitute classic random modulation techniques with variable switching frequency in some applications.

Reduction of the electromagnetic interference conducted to mains in inverter-fed AC drives using random pulse width modulation

Results of an experimental investigation of the impact of random pulse width modulation (RPWM) on the electromagnetic interference generated in inverter-fed drive systems are presented. A novel voltage space-vector RPWM technique with a limited pool of switching frequencies was employed. Experiments with an induction motor supplied from a voltage-source inverter, performed in a certified shielded chamber, were focused on the electromagnetic noise conducted to mains. In comparison with the deterministic modulation case, significant reduction of the noise, especially with regard to the quasi-peak values, is shown, demonstrating an important advantage of random modulation.

Optimization of switching frequencies in the limited-pool random space vector PWM strategy for inverter-fed drives

A space vector PWM technique for inverter fed AC drives which, in contrast with known random modulation methods, is characterized by a very small pool of randomly used switching frequencies, is presented. The limited size of the pool facilitates design of digitally implemented subsystems of the drive, whose settings depend on the length of sampling interval. A method for optimal selection of pool frequencies is described and validated by experimental evidence, including EMI measurements.

An integral space-vector PWM technique for DSP-controlled voltage-source inverters

A novel PWM technique for voltage-source inverters, based on the idea of space vectors of switching integrals is presented. In contrast to most existing PWM methods, no explicit timing is required, smooth transition from the PWM mode to the square-wave mode is ensured, and minimum allowable widths of pulses and notches are guaranteed. The control algorithm is conceptually simple and easily implementable in a DSP. Four parameters of the modulation strategy are adjustable, allowing fine tuning of the modulator to requirements imposed by the application. Voltage spectra have a low harmonic content, most of the power being dispersed as the continuous power density. A prototype modulator is described and illustrated by the results of laboratory experiments.

Accurate torque control of saturated interior permanent magnet synchronous motors in the field-weakening region

This paper focuses on accurate torque control of saturated interior permanent magnet (IPM) synchronous motors operating in the field-weakening region, where the maximum torque per ampere strategy cannot be used. First, a simple-but yet accurate-torque calculation method that takes saturation effects into account is devised: the torque developed at an arbitrary (i/sub d/, i/sub q/) may be calculated using only six parameters, which makes the method well suited for DSP implementation. Next, a field-weakening strategy is suggested, which incorporates the new torque calculation method. In this way the torque is controlled accurately deep into the field-weakening region, where voltage and current limitations impose constraints on the allowable (i/sub d/,i/sub q/) excitation. Experimental results on a 15 kW low-voltage IPM motor are included to validate the practical usefulness of the work.

Field-oriented control of an induction motor using random pulsewidth modulation

The principle of field-oriented control (FOC) is a well-established means to obtain a high-performance drive, but the de facto standard FOC does not solve an important secondary problem for inverter-fed motors-the acoustic noise emitted from the motor is tonal due to the constant switching frequency operation. This may be very annoying, even if the exposure time is short. From this background, this paper proposes a classic FOC synchronized to a current-controlled pulsewidth modulation inverter operating with a random switching frequency. This implies that the sampling frequency of both the current controller and the flux estimator is nonuniform, but nevertheless, it is shown that the good dynamic response inherited from the FOC can be maintained while lowering the subjective acoustic annoyance to a great extent due to the random switching frequency operation. Besides laboratory test results documenting the dynamic and steady-state characteristics of the proposed system, design considerations are presented.

Optimization of switching frequencies in the limited-pool random space vector PWM technique for inverter-fed drives

A space vector PWM technique for inverter fed AC drives which, in contrast with known random modulation methods, is characterized by a very small pool of randomly used switching frequencies, is presented. The limited size of the pool facilitates design of digitally implemented linear current control schemes, in which the controller settings depend on the length of sampling interval. A method for optimal selection of pool frequencies is described and validated by experimental evidence, including EMI measurements.

A new hybrid random pulse width modulator for industrial drives

A new hybrid pulse width modulator (HPWM) is suggested. It provides (a) extended linearity (up to modulation index of 1.154), (b) reduced switching losses, and (c) less annoying acoustic noise by changing the current spectrum. The spectrum is modified by changing randomly the ratio of the durations of the application of the two zero-state vectors, V/sub 0/ and V/sub 7/. Such a way to modify the current spectrum is effective at modulation indices up to m/spl les/0.8. A new method that provides additional capability of controlling the spectrum for m>0.8, and thus the emitted acoustic noise from the drive, is proposed and investigated. The method extends the HPWM with an additional algorithm which randomly selects the PWM carrier frequency as either f/sub carr/ or 2/spl middot/f/sub carr/. The extra losses, which normally result in an increase of the average switching frequency, are avoided by dropping one of the zero-vector states for m>0.8. This corresponds to using a discontinuous modulation function. The transition between the different operation modes is seamless and the method is easy to implement due to the HPWM structure. Simulation results and experiments validate the usefulness of the method proposed.