Accurate Low Speed and Torque Control for Induction Motor with Secondary Current Feedback Using MI Sensor Installed in Shaft Hole

摘要

Highly accurate speed and torque controls are strongly required for induction motors (IM, for short) especially at a low speed to maintain an accurately constant torque through a long time operation typically for rolling spread of steel sheet production. Conventional control methods such as the vector control generate some control errors due to increase of the temperature inside of the IM during a long time operation, which decreases speed and torque with decreasing of the secondary current I2. I2 has been roughly estimated from a detected primary current I1 using a simply linealized equivalent circuit in conventional control methods. On the contrary, the authors have already established a new accurate control system with a feedback of I2 directly detected using a sensitive and stable micro magnetic sensor (MI sensor) [1] installed in a drilled shaft hole where a slip frequency small AC magnetic field proportional to the unbalanced end ring circle current (I2) is stably detected [2]. Torque control errors for a 2.2.kW, 4-pole three phase IM stationarity operated through 83min. were 0.39% for 5.50N-m using the I2 feedback control system as illustrated in Figure 1 which is about 1/4 of torque control error without I2 feedback. However, high accuracy of the I2 feedback control system is based on a condition of clear detection of I2 waveform at relatively higher speed region over around 500rpm where the disturbing signal for I2 coming from I1 is relatively small due to the magnetic shielding effect of the shaft steel. Therefore, a separation method of I1 and I2 waveforms at low speed region such as around 40rpm should be established to carry out of the accurate control at low speed operation.