What are the notches or gear changes heard on some AC drive units?
On some AC, PWM inverter drive systems (particularly GTO thyristor types) it is beneficial to keep the inverter switching frequency synchronous with applied induction motor frequency. As the motor speeds up so the switching frequency must increase as a multiple of the motor frequency. To prevent the switching frequency exceeding a practical upper limit, at predetermined speeds the switching frequency is dropped to a lower value, producing a new PWM mode. This mode changing is often referred to as “gear changing”, from analogy with a manual gearbox and an ICE, making a distinct sound pattern that characterises the nature of the traction drive.
