A comparative analysis of speed control methods for induction motor FED by Neutral point clamped inverter

Abstract

Many industrial applications need variable speed operation of the motors. Earlier DC motors were used for the applications which needs variable speed operations because they could easily achieve the required speed and torque without the need of any sophisticated electronics. However, they have several disadvantages like they are expensive, having sparking problems and their maintenance is di_cult. Therefore, there is the evolution of AC variable speed drive technology by the desire to imitate the excellent performance of the DC motors such as fast dynamic response and speed accuracy. Induction motors are emerged as the best choice for many industrial applications as they are rugged, inexpensive and maintenance free AC motors.

In the induction motor drive, there is a need of inverter topology in order to control the speed of the motor. Today, many industries employ Medium Voltage (MV), high power drives. 3-Level inverter emerged as the suitable inverter topology to be used with MV drives in comparison to conventional 2-Level inverters. In 3-Level inverters, Neutral Point Clamped (NPC) inverters are best suitable to be used with MV, high power drives.

In this thesis, Space Vector Modulation (SVM) based on vector angle technique is used for NPC inverters. In this technique, in order to precisely locate the reference vector, one more vector is introduced and the angle ranges of both the vectors are used to _nd the position of the reference vector. The SVM technique also helps in balancing the DC link voltage, which is the inherent problem for NPC inverters by using the redundancies in the small vector states. The SVM for NPC inverter is experimentally validated using dSPACE for three di_erent types of loads.

In this thesis, four di_erent speed control methods for induction motor fed by NPC inverter are presented. The methods discussed are scalar control, Indirect Field Oriented control (IFOC), sensorless vector control and Direct Torque Control (DTC). All these methods are simulated in MATLAB/Simulink and the comparative analysis among them is discussed on the basis of the results obtained.