Basic Operation of AC Induction Motors (6) – FINISH

Voltage and Current Waveforms
Today’s AC variable speed drive systems (up to 600 Volts and about 1500 HP) are dominated by PWM configurations. The current waveforms seen today (Figure 14) are much closer to the ideal sinusoid, thanks mostly to higher switching rates of transistors. The availability of low switching loss devices has allowed this to occur.

One of the negative aspects of the newer devices is that the low switching loss is typically accompanied by a very short transition time. This short transition between on and off states implies a high dV/dt output of the inverter. The high dV/dt results in capacitively coupled current flow according to Equation 4.

In addition to the capacitively coupled current, the high dV/dt also results in a higher peak voltage (ringup) due to cable-to-load mismatch. Finally, the high dV/dt also results in an instantaneous high voltage across the first windings within the ac motor. A companion paper, “AC Induction Motor Insulation Issues in High dV/dt Environments,” addresses this in greater detail.

Conclusions
AC induction motors are likely to continue to be increasing sources of variable speed rotating power. Their successful use in variable speed applications is a function of the collective understanding of the various parties involved in the specification, design, application, and integration of the system.

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