Abstract
We often use induction motor in the hard environment including vibration and high ambient temperature, or in maintenance free operation, because induction motor has durable and simple structure. However, when we use it as servo actuator or accurate speed control motor, we have to equip sensor such as encoder and tachogenerator with the motor control system. And generally those sensor's abilities against bad environment are less than the induction motor itself. So if we can remove these sensors from the system, it'll have more environmental resistance, and the cost will also be reduced. Actually, estimation of the states such as speed or generating torque from the current or input voltage has realized in sensorless vector control for special purpose. However, that needs complex calculations. In our study, we intended to estimate the rotational speed from the motor current instead of speed sensor, easily and rapidly in comparison to former methods. For it, we use the special test current to estimate the rotational speed of induction motor easily without extra speed sensor. To generate this special current, we added sinusoidal voltage to only one terminal of three-phase motor, while delivering normal balanced three-phase voltage to all of three terminals. Then, the phase difference of the current through another two terminals excited by this added input gives us rotational speed of motor. Further, most induction motors are driven by PWM inverter for variable speed operation. So, we attempt the application to PWM operation.
