Research into the Causes of Undesired Synchronisation Between the Motor Speed and the PWM Modulation Frequency in a PM BLDC Drive

Badania przyczyn niepożądanej synchronizacji prędkości obrotowej silnika z częstotliwością modulacji PWM w układzie napędowym PM BLDC

Andrzej TUTAJ; Tomasz DRABEK; Pawel PIATEK; Pawel KOCWA; Jerzy BARANOWSK

Abstract

In a drive system comprising a high- speed PM BLDC motor and an inverter implementing motor phases commutation and phase voltages PWM modulation, a spontaneous synchronisation between the PWM modulation frequency and the motor rotational speed may occur. The drive behaves like an unintended phase locked loop. The effect distorts mechanical characteristic of the drive by introducing flat segments where the rotational speed remains constant despite changing load torque. It makes the speed - torque relationship discontinuous, nonlinear, and non unique. The phenomenon is closely related to the problem of phase current and motor torque ripples. The paper presents experimental results recorded with a laboratory test bench for drives for electric power tools where the phenomenon of the parasitic speed locking has been observed. A root cause of this effect is discussed, and the mechanism of unwanted synchronisation is investigated. Several mitigation techniques are proposed and verified with computer simulations.

Streszczenie

W układzie napędowym składającym się z wysokoobrotowego silnika PM BLDC i falownika realizującego komutację faz silnika i modulację napięcia fazowego PWM może wystąpić spontaniczna synchronizacja częstotliwości modulacji PWM z prędkością obrotową silnika. Efekt ten zniekształca charakterystykę mechaniczną napędu poprzez wprowadzenie płaskich segmentów, w których prędkość obrotowa pozostaje stała pomimo zmieniającego się momentu obciążenia. W pracy przedstawiono wyniki doświadczeń zarejestrowane na laboratoryjnym stanowisku badawczym dla napędów elektronarzędzi, na którym zaobserwowano zjawisko pasożytniczej synchronizacji prędkości. Przeanalizowano i omówiono przyczyny tego zjawiska. Zaproponowano kilka technik łagodzenia jego skutków i zweryfikowano je za pomocą symulacji komputerowych.

Keywords

PM BLDC, parasitic phase locked loop, speed-frequency synchronisation

Słowa kluczowe

PM BLDC, pasożytnicza pętla synchronizacji, synchronizacja prędkość

Rys. / Fig.

Bibliografia / Bilbiography

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