Among the signals generated by an inverter is the pulse-width modulated (PWM) voltage, which controls motor torque. The bandwidth required to measure a PWM signal depends on the duty cycle, which means it is easier to consider a square wave.
However, to capture the amplitude of a square wave correctly (up to 99.95 percent), engineers need a frequency response of at least 400 times the switching frequency. So, if the switching frequency is 20kHz, a bandwidth of 8MHz is necessary.
There are similar requirements for achieving the precise measurement of current and electrical power, although the effect on motor current when fed from an inverter is triangular, as it cannot respond fast enough to follow the square.
To minimise the error of bandwidth limitation in line with power meter accuracy, a bandwidth of at least seven times the switching frequency is necessary, according to the characteristics of a triangular waveform, while including the complete signal needs a bandwidth of up to 25 times the switching frequency. Again, using the example of 20kHz switching frequency, the bandwidth required to include all signal components is 500kHz.
Today’s electric motors are already working at efficiencies of 95 percent and better, so improving efficiency is a challenging but critical ambition for manufacturers.
Read the full article in DPA's November 2024 issue