Speed estimation during the starting transient of induction motors

Abstract

Typically, the rotor speed of electric motors was measured directly by the use of electromechanical sensors. Even though these devices are very precise, they are also fragile and expensive to install. Currently, some alternatives are based on speed estimations from the measurement of stator currents. Some of these, very accurate, are used in variable speed drives. In industrial power applications, many large induction motors (IMs) are directly driven with special starters. For these cases, new speed estimation strategies must be developed. This article presents a self-sensing method for speed estimation during the starting transient of both wound rotor and squirrel cage IMs. The speed estimation is based on the simultaneous tracking of multiple harmonic components of the rotor on the spectrogram of the stator currents in sequence networks. The proposal is validated with experimental results obtained in the laboratory with a squirrel cage IM. It is concluded that the estimation is not sensitive to measurement noise and tracking errors caused by other harmonic components that do not depend on the rotor position.