Comparative Control Loop Stability & Eigen Analysis of Transfer Functions in 7.5 Hp BLDC Sensorless SMO
In this paper, the effect of 3-level inverter’s MOSFET switching at 33 different PWM frequencies, has been compared from 15 kHz to 2 MHZ, in relation to control loop stability, transfer function’s coefficients variation, system response and speed characteristics. The position of rotor in 8 pole, wye-winding 7.5 Hp i.e., 5.5 kW with service factor of 1.15 (≈8.6 Hp), 72VDC, 85.6A, 14.27 Nm torque, with per phase insulation temperature of 311 ◦ F, at an efficiency of 89.2% is estimated without sensor using sliding mode observer implemented with field oriented control algorithm. The decoupled flux and torque components are controlled by discrete PI with anti-windup/reset and IIR filters. The coefficients of IIR applied in the speed and SMO control block, and other parameters like gain, ADC are mentioned along with the resulting without SMO or with SMO closed loop transfer functions at each of these 33 frequencies along with the change in the coefficients of PI (errors, Ki, Kp), of Iq, Id, field weakening, speed controllers are mentioned. The 132 values, 33 values of e, Ki, Kp for each of these four controllers at these 33 PWM frequencies are tabulated, which can be used to design prototypes with higher power densities. The relative control loop stability with SMO is analyzed and tabulated with gain, phase margins along with eigen values of the characteristic equation.