A microcomputer-based speed controller for lift drives

A microcomputer-based speed controller for squirrel-cage induction motors for lift drives is presented, designed as a replacement for the classical analog versions. The appropriate speed control is achieved by phase-cut stator voltage control and by dynamic braking with DC current injection. Both actions are implemented by standard analog phase-control integrated circuits and 8-bit D/A (digital-to-analog) converters. A speed feedback signal is generated by a noisy tachogenerator, so that a low-pass antialiasing filter, followed by a true-averager circuit, is involved for proper signal conditioning. The true averager provides high rejection of the mains frequency and its higher harmonics. The built-in self-test routines and associated circuitry provide part-by-part testing of the hardware. This together with simple diagnosis, supports faster installation and more efficient troubleshooting. Complete firmware is organized in the shape of interrupt-driven routines, which are reentered at every mains zero crossing in the positive direction. A reference-speed profile with the shape of parabola is used to achieve a constant acceleration-time gradient. A discrete proportional integral (PI) control algorithm with lag-lead frequency compensation gives acceptable control accuracy, stability and robustness.