Power Electronics Circuits
In a PWM inverter driving a motor, which of the following correctly describes the effect of raising the carrier frequency?
- AThe current ripple becomes smaller and audible noise is easier to reduce, but switching loss increases roughly in proportion to the frequency
- BFiner switching actually reduces the switching loss, but the motor current ripple becomes larger and the noise also increases
- CThe frequency of the fundamental of the inverter output rises in proportion to the carrier frequency, so the motor turns correspondingly faster
Show answer and explanation
Answer: A. The current ripple becomes smaller and audible noise is easier to reduce, but switching loss increases roughly in proportion to the frequency
Raising the carrier frequency moves the harmonic components produced by PWM to higher frequencies, where the inductance of the motor windings smooths them more easily, so the current ripple becomes smaller. Raising the carrier above the range audible to the human ear (up to about 20 kHz) also makes the motor quieter. On the other hand, the switches change state more often, so the switching loss increases roughly in proportion to the frequency. The motor speed is set by the frequency of the reference, not by the carrier frequency.
- A. Correct. It is a choice between current quality and switching loss.
- B. More switching events increase the switching loss. The current ripple becomes smaller.
- C. The fundamental frequency of the output is set by the reference. Changing the carrier frequency does not change the speed.