Q7Electric Drives
Question
Q.7. How does V/f control improve the performance of an induction motor?
Answer
V/f (volts-per-hertz) control maintains a constant ratio of stator voltage to frequency as the supply frequency is varied to control induction motor speed, keeping the air-gap flux approximately constant across the operating speed range, thereby avoiding both magnetic saturation (at low frequency, if voltage were not also reduced) and reduced torque capability (at high frequency, if voltage were not increased).
In an induction motor, the air-gap flux is approximately proportional to the ratio of stator voltage to supply frequency (φ ∝ V/f), so simply varying the supply frequency alone (without also adjusting voltage) to control motor speed would cause the flux to change substantially — at low frequencies, flux would rise excessively (since V is held at its rated value while f decreases), causing magnetic core saturation, increased magnetizing current, and increased core losses; at high frequencies, flux would drop excessively, reducing the maximum torque the motor can develop for a given current. V/f control avoids both these problems by proportionally varying the stator voltage together with the frequency, maintaining a nearly constant V/f ratio (and hence a nearly constant air-gap flux) across the motor's controlled speed range below the motor's rated (base) frequency, ensuring the motor operates with adequate flux (avoiding saturation) and consistent torque-producing capability at every commanded speed, which is precisely why V/f control (also called scalar control) is the simplest and most widely used method for variable-speed induction motor drive control in applications such as pumps and fans that do not demand the highest dynamic performance.