DC motor-driven elevators have traditionally employed Ward-Leonard motor-generator sets to convert AC power from the utility grid to the DC power required by the motor. More recently, phase-controlled thyristor bridge rectifiers have been used to convert three-phase AC from the utility grid directly to DC power. Principal disadvantages of such systems include the poor power factor of the converter requiring large ratings of input feeders and transformer, and the severe distortion of the AC power system created by the thyristor converter.
A more recent approach to achieving unity power factor and low distortion at the utility AC interface is to employ a boost converter front end to draw sinusoidal current at unity power factor from the AC mains while converting the 3-phase AC voltage to a regulated DC voltage. A PWM rectifier front end converter (boost converter) converts the 3-phase AC mains voltage to a controlled DC bus voltage, which is maintained by a DC bus capacitor bank. The power to the DC motor armature is switched via an output H-bridge, DC-PWM (pulse width modulation) converter. Additional filters are required at the input and output of the converter to limit current and voltage ripple. While fully regenerative and providing unity input power factor, the output H-bridge converter uses only one switch element pair (in each modulation period) to deliver positive current, and the other pair to deliver negative current, so each pair of switches must be rated to carry the maximum rated current of the drive. The poor utilization of the power semiconductors and the need for the DC bus capacitors result in high component cost.
A controlled rectifier fashioned by implementing only two phases of the output of a three phase AC/AC matrix converter is described in Holmes et al, "Implementation of a Controlled Rectifier Using AC-AC Matrix Converter Theory". Therein, the mathematical algorithms are too complex for real time processing in an elevator motor controller.