Q6Electric Drives

The inverter is the key power-electronic stage in a variable-frequency induction motor drive that converts the DC-link voltage (itself typically obtained by rectifying the fixed-frequency AC mains supply) into a controllable, variable-frequency and variable-magnitude three-phase AC output waveform, which is then supplied directly to the induction motor's stator terminals. Since an induction motor's synchronous (and hence approximate operating) speed is directly proportional to the frequency of its stator supply, the inverter's ability to generate any desired output frequency (typically using PWM switching techniques, as discussed elsewhere in this paper) is precisely what allows the drive to continuously control the motor's speed across a wide range, simply by commanding the inverter to output the appropriate frequency (and correspondingly adjusted voltage magnitude, to maintain approximately constant flux as required by V/f control, or independently controlled according to vector/DTC control principles) for the desired motor speed.