At higher frequencies, they lead to eddy currents and skin-effect losses that produce no usable work, but do produce heat, which must be dissipated. Second, harmonics cause motor currents that produce additive and subtractive torques, which are neither useful nor efficient in their production. While torque effects have seemed minor, motor-heating problems have remained a concern. The first solution was to derate motor capacity and control ambient environment. However, as drive inverters for variable-torque centrifugal applications developed, so did the motors suited to these applications.
Harmonics also can travel up to the feeder power system supplying the drives.
Inverters did not allow input current to flow as a regular sine wave. Instead, current to the rectifier came in bursts, and harmonics moved around the entire connected power-distribution system. These harmonics were confined by isolating the power system for mechanical loads from the rest of the facility. Placing inductance in the input circuit to a VFD also can control harmonic proliferation on power systems and is used routinely. Isolation transformers are an even better approach, but cost more and require more space.