Permanent magnet synchronous motors (PMSMs) are widely used in industries, home appliances and electric vehicles due to their high energy density, high torque capability at low speed, and higher efficiency compared to induction machines. To improve the system reliability and to reduce the overall system cost, sensorless vector control techniques are used for the drive control. Back-emf based sensorless vector control of PMSM drives provides a simple, robust and low-cost solution for applications requiring only medium and high speed control. However, during the low-speed operation, back-emf is nearly zero and hence for starting and speed ramp-up, open-loop I-f control is commonly used. For a variety of applications like compressors, pumps, fans and heating ventilation and air conditioning (HVAC) systems, and also for critical applications like electric ship propulsion, and emergency heat and smoke exhaust, a high precision dynamic control at low speed is not necessary. Instead, a simple and reliable control in medium and high speed range together with quick starting performance would be sufficient. Hence, back-emf based sensorless vector control together with I-f control for starting is an appropriate choice for such applications. Once the motor reaches a sufficient speed with I-f starting, the control is transferred to sensorless vector control. A direct transition from I-f to sensorless control results in speed and current oscillations, depending on the error in sensorless estimated position. To solve this issue, in the existing methods, an additional transition interval for aligning the reference frames of the I-f and sensorless control is used to ensure a smooth transition. However, the additional transition interval limits the usability of these methods for applications requiring a quick startup. Another issue with I-f starting is that the system stability highly depends on the frequency profile used for ramp-up in the I-f control. Since the slope of the frequency ramp is usually fixed arbitrarily, loss of synchronism or pole slipping can occur, restricting the method for critical applications. The conventional solution of fixing the frequency ramp slope to a minimum value for pole slipping prevention deteriorates the dynamic performance during starting.

In this work, two new methods, one for achieving a quick and smooth changeover and the other for dynamic frequency slope control are proposed. In the proposed changeover method, a quick and seamless transition is achieved by accurately estimating the rotor position from the sensed back-emf, at zero stator currents. The inverter pulses are disabled for a short duration to achieve the zero stator current interval. The proposed changeover method is also extended to perform on-the-fly start for power failure ride through during short time power supply interruption. In the second work, a torque controller is proposed to dynamically vary the frequency slope depending on the moment of inertia and load torque present in the system, achieving fast and reliable starting. The proposed methods are experimentally verified on a 25kW PMSM drive.