In converter-fed locomotives employing d.c. machines the single-phase 50 Hz (or 60 Hz) voltage is rectified thus leading to a.c. components of 100 Hz (or 120 Hz). Hence the frequency of the pulsating torques is also 100 Hz (or 120 Hz) and integer multiples of 100 Hz (or 120 Hz) also arise. In the case of an inverter-fed variable-frequency induction machine drive, during starting, the initial value of supply frequency, f1, is 1 Hz and this is continuously increased to approximately 100 Hz (or 120 Hz). Since the output voltage of the inverter is nonsinusoidal and contains backward 5th, forward 7th, backward 11th, forward 13th etc. time-harmonics, the torque of the machine will have pulsating components with the following frequencies: 6f1, 12f1, 18f1 ... etc. In the case of voltage source inverters the amplitudes of the pulsating torques can be reduced by appropriate impulse control to a few percent of their original amplitudes. For current-source inverters their amplitudes are several times 10 % of their original amplitudes. Whilst the frequency of the pulsating torque is continuously increasing from the smallest value of 6f1 = 6 Hz to 600 Hz (or 720 Hz), it is likely that resonance will occur in the mechanical system.
Oscillatory behaviour of induction machines employed in railway traction
Schwingungsverhalten im Bezug auf selbstangefachte Drehschwingungen bei umrichtergespeisten Bahnmotoren
1985
5 Seiten, 7 Bilder, 9 Quellen
Aufsatz (Konferenz)
Englisch
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