The power-electronic drive unit (variable-frequency drive, VFD) that converts fixed-frequency mains supply to variable voltage and frequency for precise speed and torque control of the elevator traction motor.
Full Definition
The elevator motor driver (variable-frequency drive, VFD; also: frequency inverter, VVVF drive) is the central power-electronic device in modern traction elevator systems. It rectifies the three-phase AC supply to a DC bus and then inverts it via IGBT switches to produce a variable-voltage, variable-frequency (VVVF) output to the permanent magnet synchronous motor (PMSM) or induction motor. Key functional features: (1) closed-loop vector (field-oriented) control using encoder feedback for precise speed and torque regulation — motor speed held within ± 0.01 m/s of reference; (2) pre-torque function — motor torque pre-loaded before brake release using load-cell input, eliminating rollback; (3) regenerative braking — braking energy returned to the grid via an active front end (AFE) rectifier, saving 20–40 % energy compared to resistive braking; (4) safety stop STO (Safe Torque Off) function per IEN 61800-5-2 SIL 2, providing immediate drive inhibit without mechanical brake wear; (5) automatic rescue operation (ARO) — drives car to nearest floor on battery/UPS supply during power failure. Drive sizing: rated output current ≥ 150 % of motor full-load current; derating applied for ambient temperatures above 40 °C and altitudes above 1000 m ASL.
