Abstract
This paper presents the characteristic behavior of direct vector control of two induction motors with sensorless speed feedback having the same rating parameters, paralleled combination, and supplied from a single current-controlled pulse-width-modulated voltage-source inverter drive. Natural observer design technique is known for its simple construction, which estimates the speed and rotor fluxes. Load torque is estimated by load torque adaptation and the average rotor flux was maintained constant by rotor flux feedback control. The technique’s convergence rate is very fast and is robust to noise and parameter uncertainty. The gain matrix is absent in the natural observer. The rotor speed is estimated from the load torque, stator current, and rotor flux. Under symmetrical load conditions, the difference in speed between two induction motors is reduced by considering the motor parameters as average and difference. Rotor flux is maintained constant by the rotor flux control scheme with feedback, and the estimation of rotor angle is carried out by the direct vector control technique. Both balanced and unbalanced load conditions are investigated for the proposed AC motor drive system. Experimental results presented in this paper show good agreement with the theoretical formulations.
Highlights
Controlling of AC motor drives in closed-loop strategy requires the speed transducers, such as tacho generators, resolvers, or digital encoders, to obtain the speed information that leads to the expensive system
In speed sensorless vector control, the speed can be estimated based on the information of line voltages and currents, and its accuracy is mostly meant for a single-inverter-driven single induction motor
The natural observer with load torque adaption technique was presented, in particular to estimate the speed and load torque of motors connected in parallel and fed by a single-stage inverter drive
Summary
Controlling of AC motor drives in closed-loop strategy requires the speed transducers, such as tacho generators, resolvers, or digital encoders, to obtain the speed information that leads to the expensive system. For estimating the rotor speed by natural observer, the fourth-order induction motor model in stator flux oriented reference frame is used, where stator currents and rotor fluxes are considered as state variable parameters. If the current flowing in each motor is unbalanced, and if there is a slight difference between the wheel diameters or the machine parameters, this causes unbalanced load conditions where is will not be equal to is2 In this situation, the average current and torque can be expressed as follows:. When the stator current and the rotor flux are estimated using the observer, eq (36) is represented as (37)
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