You should not use a VFD (Variable Frequency Drive) in applications where the motor must always run at a constant full speed or where speed control gives no real benefit. For example, simple fixed-speed equipment like exhaust fans, constant-load conveyors, or standard water pumps may not need a VFD — it only adds cost and complexity without improving performance. Also, in systems that require very high starting torque or have frequent start-stop cycles, a VFD might struggle or wear out faster.
VFDs are also not ideal for single-phase motors, non-inverter-rated motors, or applications sensitive to harmonics and electrical noise. If the motor runs in extreme environments — like high temperatures, heavy dust, or strong vibration — it can shorten the VFD’s life unless special protection is used. In short, avoid using a VFD when speed control isn’t needed, when the motor type is incompatible, or when the environment is too harsh. In such cases, a soft starter or direct-on-line (DOL) starter may be a better and simpler choice.