A new VFD whines like a mosquito. The customer calls and says the motor is defective. You bump the carrier from 2 kHz to 8 kHz. Quiet now, but the drive is running hot and the motor insulation fails three months later. Carrier frequency is a tradeoff, not a dial you crank for silence.
What the carrier actually is
A VFD doesn’t output a sine wave. It outputs a stream of pulses whose width changes. The pulse rate is the carrier frequency. At 2 to 4 kHz, that’s right in the middle of human hearing, so the motor sings. At 8 kHz and up, you can’t hear it.
Cranking the carrier has a cost.
Higher carrier means a hotter drive
Every switching event loses energy. Double the carrier, double the switching losses. The IGBTs and heat sink run hotter. In a hot cabinet in summer, a high-carrier setting trips the drive on over temperature.
If the motor is far away and nobody stands next to it, leave the carrier at 2 to 4 kHz. That’s where the drive is happiest. If you really need 8 kHz, upsizing the drive or adding a fan is the price.
Motor insulation does not like fast edges
The VFD’s pulse rise time is steep, and that creates overvoltage between winding turns. Longer cable makes the overvoltage worse. Higher carrier means more of these pulses per second, which accelerates insulation breakdown. A standard motor on high carrier will lose megohms by month six.
Cable over 30 meters needs an output reactor or du/dt filter. Over 100 meters, use a sine wave filter. Skipping this buys a cheap drive and a new motor.
When you actually raise the carrier
Not every application needs it. Raise it when the motor sits next to an office and the noise is a complaint. Raise it when running at very low speed and the low-frequency whine is loud. After you’ve installed an output filter, a higher carrier is safer.
Leave it low on big fans and pumps far from people. Long cable runs actually want a lower carrier, not higher, to cut reflected wave stress.
Bearing current
High-frequency common-mode voltage finds its way through the motor bearings and pits the races. Shielded cable with both ends grounded helps, or a shaft grounding brush. This is a known failure mode on older motors; don’t ignore it because the drive runs fine.
