Induction motors fed from voltage source inverters are commonly used in a number of applications. An open-loop volts-per-hertz (V/f) control is a simple scheme which is used to control the speed of the motor drive. Such a scheme is typically used to control motors which drive fans and pumps for applications such as irrigation systems, HVAC (heating ventilation and cooling systems), and various other industrial processes.
During the testing of a 100 kW induction motor drive at IISc Bangalore, sustained low-frequency oscillations were observed in the motor currents at low speeds. The oscillations were so severe that these even caused the inverter to trip under certain conditions. Many a time, practicing engineers misconstrue such oscillations to be on account of shaft misalignment and other issues related to mechanical factors in the installation of the motor. However, an investigation identified the main cause of the problem as inverter dead-time or blanking time which is related to the electrical subsystem. Dead-time is introduced between the gating pulses to complementary switches of an inverter leg to prevent what is called as a shoot-through fault or a short circuit. It was found that this inverter dead-time, which is a system non-ideality, could lead to stability issues in the motor drive.
In this talk, I will talk about this problem, how it was analyzed and how a solution was attempted to solve the same.