VSD Carrier Frequency
VSD switching frequency refers to the rate at which the DC bus voltage is switched on and off during the pulse width modulation (PWM) process. The on and off switching of the DC voltage is done by Insulated Gate Bipolar Transistors (IGBTs). The PWM process utilises the switching of the IGBTs to create the variable voltage and variable frequency output from the VSD. The switching frequency, sometimes called the “carrier frequency,” is defined using the unit of Hertz (Hz) and is typically in the kHz (Hz*1000) range, typically ranging from 4 to 16khz, or 4000 to 16000 switches on/off per second.
The carrier frequency of the VSD can be used to help reduce motor noise, avoid resonance of the mechanical system, and reduce leakage current to earth and interference generated by the VSD. If the carrier frequency is too low, the output current will have a high harmonic wave and could cause motor power loss and rising temperatures. If the carrier frequency is too high, the frequency inverter will be impacted by power loss, rising temperature and interference. The higher the carrier frequency, the larger the leakage current will be, however, reducing the carrier frequency may result in additional motor noise (installation of a reactor is also an effective method to remove the leakage current).
To determine what switching frequency would work best for your application, it is important to look at the advantages and disadvantages as the switching frequency is increased. Please take note of the following:
- The longer the cable between the VSD and the motor, the higher the harmonic leakage current of the output will be, which will lead to adverse impacts on the VSD and the peripheral devices (for safety purposes, ensure that the VSD and the motor is reliably grounded). Refer to the following table for the suggested carrier frequency setting based on cable length:
*Please Note: If the cable between the VSD and motor is 50m+, it is recommended to install an output choke/reactor, from 150m+ a SineWave Filter is recommended (instead of the Output Choke/Reactor).
- The factory setting of carrier frequency varies based on the VSD power rating (kW). Note that if the set carrier frequency is higher than the factory setting, it will lead to an increase in temperature rise of the VSD’s heatsink. In this case, de-rate the VSD (select a VSD of kW rating larger than normal), otherwise the VSD may overheat (will generate an alarm).
- Adjusting the Carrier Frequency (d6-00) could have various impacts as listed in the following table:
* In the event of a shrill motor noise (audible noise) when running a motor using a VSD – Adjusting the Random PWM Depth (d6-05) can be used to soften the noise and reduce the electromagnetic interference to other equipment (the higher the value the lower the noise, but will increase the temperature of the VSD). If this parameter is set to 0, random PWM is invalid (when adjusting this parameter, lower values are more preferable).
