Power Electronics Circuits
Which of the following correctly combines what can happen when an induction motor driven by an inverter is decelerated quickly by lowering the frequency with the countermeasure?
- AThe rotor runs faster than the synchronous speed and acts as a generator, and the returning energy raises the DC voltage, so the energy is dissipated in a braking resistor or regenerated to the supply
- BDuring deceleration the motor current becomes zero and the motor is effectively disconnected from the supply, so nothing in particular happens and no countermeasure is needed
- CDuring deceleration the DC voltage of the inverter suddenly falls to zero, so the smoothing capacitor is removed to prevent voltage fluctuations from occurring at all
Show answer and explanation
Answer: A. The rotor runs faster than the synchronous speed and acts as a generator, and the returning energy raises the DC voltage, so the energy is dissipated in a braking resistor or regenerated to the supply
Lowering the frequency quickly lowers the synchronous speed, and the rotor, by inertia, turns faster than it, so the slip becomes negative and the induction motor acts as a generator. The generated energy returns to the DC side through the diodes of the inverter and charges the smoothing capacitor, raising the DC voltage. Too high a voltage destroys the devices, so a braking resistor is switched in to dissipate the energy as heat, or a regenerative converter returns the power to the supply. Trains and electric vehicles save energy with regenerative braking, which returns this energy to the overhead line or the battery.
- A. Correct. The deceleration energy returns to the DC side, so it has to be dealt with.
- B. Current keeps flowing during deceleration, and energy returns from the motor to the inverter.
- C. The DC voltage actually rises. The smoothing capacitor cannot be removed.