In the classic combination of a standard VFD without a sine filter, and one motor and with short, shielded motor-leads, operation is generally unproblematic. In this situation, the motor is usually designed specifically for AC drive speed control, and includes special measures, such as improved motor winding insulation and ceramic bearings.
If motors (small fan motors) are operated using these VFDs and have not been adapted for VFD operation, or if groups of motors are operated in parallel using VFDs without sine filters, motor winding failures and problems in the motor bearings can result.
External sine filters for retrofitting –
Integrated sine filter in ZIEHL-ABEGG Frequency control
Most manufacturers of standard VFDs do not provide sine filters in their devices. The VFDs are generally intended for switch cabinet installation and the manufacturers assume that the user will install an external sine filter in the switch cabinet if necessary.
An external sine filter is not all-pole and merely filters the voltages between phases. The all-pole effective sinusoidal filters as used in the ZIEHL-ABEGG Frequency control VFDs filter the voltage not only between phases but also between each phase and earth.
The integrated sine filter ensures the sinusoidal shape of the output voltage and the output current. It's even possible to refer to the Frequency control VFD as an "electronic transformer".