In high power industrial applications such as steel rolling mills, drives must have excellent dynamic performances in a wide speed range. Field-weakening control for constant power operation must be applied above base speed. Also armature reaction is highly recommended because the armature reaction leads to a motor power factor drop and, if left unremedied, to the need of a heavier motor and a largersize converter.
Okuyama et al (1978) proved that a cycloconverter-fed synchronous drive is well adapted to these requirements. Armature reaction compensation can in this case be performed by a compensation winding in quadrature with the main field winding. This solution needs a special purpose synchronous motor with two field windings in quadrature, which is thus expensive.
As a three-phase winding is magnetically equivalent to a two-phase winding, armature reaction compensation is also possible in a double fed slip-ring asynchronous motor drive. If both stator and rotor are fed by a cycloconverter, the resulting drive has four degrees of freedom (amplitude and frequency of the stator and rotor voltages) that can be used to control the speed, stator flux, power factor and slip independently even during transient operation, if an appropriate control i.e. vector control is used. Figure 1 shows a general scheme of the proposed drive, suitable for high power applications.
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