Cycloconveter

Hi I have a good knowledge of the Mine hoist you are describing, and have designed several similar ones. I am not sure quite what you are looking for from the questions that you have posed. I will confirm that the 7171kW is correct. This is a very big beast with a flat out speed of ~60rpm.

The hoist has three sets of anti-parallel connected thyristor convertors that are arranged as a three phase voltage generator. Each convertor produces a DC output that varies in a sinusoidal fashion with its anti-parallel connected partner providing the negative half of the sinewave. The outputs of the three convertors are star connected together and each shifted by 120 degrees to produce the three phase waveform. To get the required voltage output, each phase is in fact the output of two convertors connected in series.

The control system and algorithm that determines the actual timing of the firing pulses is very complex! The current that the CT's in each convertors AC lines measure is converted to DC that flows in a burden resistor and a diode. The voltage across the burden resistor is "read" by the control regulator and depending on which anti-parallel convertor is active is interpreted as either a positive or negative current value. The voltage across the diode is "read" to determine if the convertor bridge output has dropped to a safe value to permit convertor bridge changeover. The three values of current are then transformed via software into values of Id(magnetising flux current) and Iq(torque current). It is these values that are used as feedback in two software regulators (one for the magnetising flux control and the other for the torque producing element). The outputs from these regulators are then transformed back into three phase current references that are used as inputs to the individual phase convertors.

The low frequency is generated by the software by adding the frequency ref (speed) to the slip angle (required to produce the torque) and clocking the sinewave generator (lookup table) with it to produce a threephase sinewave.

I'm not sure if this fully answers your questions but the topic you have chosen is a complex one and may require much discussion to make things clearer.

Regards Bob