Third harmonic problem

Elevator and Escalator problem

Since the beginning of 21st century variable frequency drive (VFD) has become the preferred choice of controlling elevators and escalators. It replaced the erstwhile legacy of a motor generator (MG set) system which had been used practically through the entire 20th century. It used an AC motor to drive a DC generator for feeding power to a DC motor in a configuration called the Ward-Leonard system. The VFD drives have considerably reduced the lift room footprint, simplified commissioning, and improved energy efficiency.