Control strategy of a direct association “6 SCRs rectifier-GTO chopper” feeding a low voltage power DC motor from a high voltage 3 phase AC mains

This paper deals with an original way to control in a single quadrant, a low voltage (for example 300 V) DC motor fed from a high voltage (for example 5000 V) 3 phase mains. The traditional solutions either use a half controlled (SCRs/diode) bridge, or cascade a diode bridge and a GTO buck chopper. For a high inertial load, the electromotive force at start up is almost zero, and the voltage to be applied to the rotor is very low. The high level of harmonics of the voltage applied to the rotor, and in the current, causes oscillations of the driving torque at the frequency of these harmonics and, therefore, vibrations of the motor. First, because of the high reduction of the voltage which is necessary at start up, and second because of the minimum working time necessary to the GTO chopper, it is interesting to put a SCR full bridge rectifier ahead of the chopper in order to reduce the voltage before hand. In this way the minimum on state time of the GTO is respected and the frequency remains superior to the first harmonic bridge output voltage frequency. Me compare the performances of this association “SCRs bridge-GTO chopper” with that of a half bridge and that of a full bridge with a free-wheeling diode. We show that it is possible to obtain a control law minimizing both the reactive power and the first harmonics of the input alternative currents