History
The doubly fed machine has its origins in wound rotor induction motors with multiphase winding sets on the rotor and stator, respectively, with the rotor winding set connected to resistors via slip rings for starting. However, the slip power was lost in the resistors. Thus means to increase the efficiency in variable speed operation by recovering the slip power were developed. In Krämer (or Kraemer) drives the rotor was connected to an AC and DC machine set that fed a DC machine connected to the shaft of the slip ring machine. Thus the slip power was returned as mechanical power and the drive could be controlled by the excitation currents of the DC machines. The drawback of the Krämer drive is that the machines need to be overdimensioned in order to cope with the extra circulating power. This drawback was corrected in the Scherbius drive where the slip power is fed back to the AC grid by motor generator sets.
The rotating machinery used for the rotor supply was heavy and expensive. Improvement in this respect was the static Scherbius drive where the rotor was connected to a rectifier-inverter set constructed first by mercury arc based devices and later on with semiconductor diodes and thyristors. In the schemes using a rectifier the power flow was possible only out of the rotor because of the uncontrolled rectifier. Moreover, only sub-synchronous operation as a motor was possible.
Another concept using static frequency converter had a cycloconverter connected between the rotor and the AC grid. The cycloconverter can feed power in both directions and thus the machine can be run both sub- and oversynchronous speeds. Large cycloconverter controlled doubly fed machines have been used to feed 16 2/3 Hz railway grid and run the turbines in pumped storage plants.
Today the frequency changer used in applications up to few tens of megawatts consists of two back to back connected IGBT inverters.
Several brushless concepts have also been developed in order to get rid of the slip rings that require maintenance.
Read more about this topic: Doubly Fed Electric Machine
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