Analysis of High Performance Controlled Permanent Magnet Brushless DC Motor Drives without Position Sensor

Abstract

Permanent Magnet Brushless DC (PMBLDC) motor is a new generation of converter fed machines (CFMs) becoming more and more popular. PMBLDC motors are extensively used as industrial motors due to its fast dynamic response, high power density, large torque to inertia ratio, high efficiency with increased reliability, less noise, long life, silent operation, compact fonn, low maintenance and better controllability. For innumerous applications, PMBLDC motors are used as replacement for AC motors. PMBLDC motor is a trapezoidal shaped back EMF permanent magnet synchronous motor with solid state commutation system. The absence of a commutator and brushes or slip rings in the PMBLDC motors reduces maintenance needs and raises mechanical reliability. In solid state commutation system, rotor position information must be needed for proper commutation sequence with proper control algorithms. Motor performance degrades due to improper control action. In this study, an adaptive PT speed controller based field oriented vector controlled current fed delta modulated PMBLDC motor drive is designed. An adaptive P1 controller is proposed based on motor speed error. Scalar control of this PMBLDC motor is also performed at direct axis current component is equal to 1.0 and at direct axis current component equal to quadrature axis current component. The performance of these control drives is compared under different conditions. In this thesis, a novel approach to enhance the torque handling capacity of a PMBLDC motor is invented. The novelty is that, torque handling capacity of a PMBLDC motor can be increased up to 25% from the conventional 120 conduction square wave current fed drive by only changing the pattern of reference current of a field oriented vector controlled PMBLDC motor drive. Toque handling capacity has to be enhanced without exceeding the maximuni current rating of the PMBLDC motor. The performances of trapezoidal, square and sinusoidal current fed field oriented vector controlled PMBLDC motor drives are compared on the basis of response time, load torque handling capacity, dynamic speed and load torque changing condition, settling time of the system and torque pulsation. In conventional PMBLDC motor, for proper commutation of the phase currents rotor position information must be obtained from the position sensors. But there are some vital disadvantages of position sensor including high cost, installation difficulty of mechanics, and poor reliability. Moreover, misalignments in position sensors, running in extreme ambient conditions, or electromagnetic interference introduce error in the position information. Instead of rotor position or speed sensor, two novel algorithms are proposed in this thesis to estimate the rotor position and speed to perform the operation of position sensorless field oriented vector control of PMBLDC motor. For algorithm I, both rotor position and speed of the motor are determined from the estimated flux linkage. For algorithm 2, only rotor position of the PMBLDC motor is determined from the estimated flux and the speed of the motor is detected by the estimated developed electromagnetic torque and power equation. The performance of flux estimation algorithm, rotor position estimator, speed estimator is depicted by the comparative study between the actual and estimated flux, rotor position and speed respectively.

The performance of the proposed algorithms for sensorless operation is also justified through the sensorless trapezoidal current fed field oriented PMBLDC motor drive. In this drive, both the advantages of position sensorless operation and enhanced torque handling capacity are incorporated due to establish a novel high performance control drive for PMBLDC motor. A novel position sensorless two phase conduction direct torque controlled PMBLDC motor drive is also proposed in this study. Without considering flux control, two-phase conduction direct torque control (DTC) of a PMBLDC motor on the basis of electromagnetic torque, rotor position and speed estimation is pictured in the proposed drive system. The performance of the proposed position sensorless direct torque controlled PMBLDC motor drive is compared with the performance of conventional position sensored two phase conduction direct torque controlled PMBLDC motor drive in terms of starting, dynamic speed and load torque changing characteristics.