This work presents and discusses different problems associated with the operation of ASDs based on active front end PWM rectifiers in power distribution systems. Problems generated with high frequency current harmonics, interaction with sensitive digital protection schemes, and the circulation of leakage grounding currents, are described. Practical problems generated by the interaction with existing ASDs based on diode rectifiers are also analyzed. Actual current and voltage waveforms associated with the operation of active front end based frequency changers in industrial power distribution systems illustrate the practical problems discussed in this paper.