Variable voltage drive controller including adaptive earth leakage detection threshold

A variable voltage drive controller, system and method which includes earth leakage sensing. The system includes an inverter 110 having inverter control inputs 142 , power outputs 114 coupled to an AC load 116, and power inputs 120/118 coupled to DC supply rails 102/104. There is a Core Balance Current Transformer CBCT or zero sequence current transformer ZSCT 122 positioned to detect zero sequence currents flowing between the power outputs 114 and the AC load 124/116. In operation, the Core Balance Current Transformer 122 provides an inverter output monitoring signal 136 indicative of the zero sequence currents (ZSC). The system includes a variable voltage drive controller 148 that has a memory module 134 storing an inverter profile instruction set that includes instructions for producing a discrete RMS voltage level at the power outputs. The inverter profile instruction set also includes a zero sequence (current) threshold value (ZTV) representing a zero sequence current threshold level assigned to each discrete RMS voltage level. There is also a processor module 132 coupled to the memory module 134. The processor module has a least one zero sequence current processing input coupled to the Core Balance Current Transformer, and inverter control outputs are coupled to the inverter control inputs. In operation, the processor module provides control signals at the inverter control outputs to thereby vary the RMS voltage level at the power outputs of the inverter (figure 3). For a selected discrete RMS voltage level, the processor module generates an earth fault signal only when a zero sequence current signal resulting from processing the inverter output monitoring signal, exceeds the zero sequence current threshold value assigned to the selected discrete RMS voltage level (340 figure 3). The variable voltage controller may be used to control the speed of motors in automotive applications.