line commutated rectifier applications

In lecture 30, it was mentioned that the chapter on line commutated rectifiers may be eliminated as they don’t provide power factor correction (PFC) and thus not much used. I talked to Prof. Maksimovic about that statement and he clarified that there are indeed applications where such rectifiers are well suited for relative simplicity, low cost, or power levels. I would like to take this opportunity to mention one such application.

The application I am familiar with is where the ac voltage is not from the power line, but from an alternator (for instance, the generator in a small wind turbine system). The alternator source has no PFC requirement, per se, as it is isolated from the power grid. In such systems, the simple (line commutated type) rectifier is used to convert the variable frequency ac output of the wind turbine generator, i.e., alternator, to (variable) DC, which is then boosted and then inverted to power line AC (for connection to the power grid). The line commutated rectified DC is generally boosted using a simple conventional boost circuit. That boost circuit does provide some power factor correction action. As a result, the alternator current no longer looks like current spikes (when the rectifier diodes conduct), but actually with a nice current waveform (although, not quite sinusoidal). In my experience, the generator current lags the generator voltage by no more than, say, 15degrees, in the worst case (which is generally under light loads and also depends on the generator winding inductance and resistance). With such a design, one does has to be careful about the alternator current waveform at various operating powers. Even though there is no requirement for PFC, it is important that the alternator current waveform be not too much distorted or too much out of phase with the alternator voltage, as it results in torque ripple and acoustic effects. High current harmonic content may also lead to excessive loss in the windings.