Mihai Comanescu

Decoupled Current Control of Sensorless Induction-Motor Drives by Integral Sliding Mode

This paper discusses the problems of current decoupling control and controller tuning associated with sensorless vector-controlled induction-motor (IM) drives. In field-oriented control, the d-q synchronous-frame currents should be regulated to have independent dynamics such that the torque production of the IM resembles that of a separately excited dc motor. However, these currents are not naturally decoupled, and decoupling compensators should be used. Current loop tuning is an additional problem, since controller gains obtained by theoretical methods or simulation, quite often, do not work well on the real system. This paper proposes a new approach for current control that uses integral-sliding-mode (ISM) controllers to achieve decoupling. The synchronous-frame control voltages are synthesized as the sum of two controller outputs: a traditional one (PI) that acts on an ideal plant model and an ISM controller. The ISM controller decouples the d-q currents and compensates the parameter variations in the current loops of the machine. Simulations and experimental tests on a 0.25-hp three-phase induction machine show satisfactory results.

Instantaneous voltage measurement in PWM voltage source inverters

The use of pulse width modulated (PWM) inverters is nearly universal in industrial drives, making accurate voltage measurements problematic due to the switching nature of the applied phase voltage. Often, the applied phase voltage is reconstructed by using a combination of the DC link voltage and the commanded PWM duty cycle. Another approach is to apply a low-pass filter to the phase voltage to remove the high frequency PWM switching component, but leaving the fundamental component of interest. In this paper, a strategy for detecting the instantaneous phase voltage is presented. The approach consists of integrating the switched phase voltage over either a full, or half PWM cycle. The integrated signal is then converted to voltage by dividing by the integrating period. Implementation details of the proposed approach are outlined in the paper, and experimental results are used to compare the proposed technique with other methods of voltage measurement in terms of measurement accuracy and transient performance. The results will demonstrate the opportunity for improvements to any inverter-driven motor control system that relies on accurate terminal voltage measurement to estimate internal machine states.

Sliding Mode MRAS Speed Estimators for Sensorless Control of Induction Machine under Improper Rotor Time Constant

The paper discusses the speed estimation accuracy of two Sliding Mode Model Reference Adaptive System observers under improper rotor time constant. The SM MRAS observers are used for speed estimation in a sensorless direct field oriented (DFO) induction machine (IM) drive. In a sensorless DFO drive, the rotor fluxes are estimated based on the measured voltages/currents and are used for field orientation. Speed estimation by the rotor flux MRAS method is attractive since the reference quantities (rotor fluxes) are already available. Classic MRAS or SM MRAS speed estimators are viable alternatives and they work well under ideal conditions. The paper analyzes the speed estimation error of the SM MRAS observers under real conditions (improper rotor time constant and nonideal integration) and compares it with the Classic MRAS method.

Sliding-mode observers with compound manifolds for the induction motor

The paper discusses the problem of speed and flux estimation for the induction motor (IM) drive and presents a series of Sliding Mode Observers (SMOs) that are constructed using compound manifolds. Observers with compound manifolds have not been widely investigated because they cannot be designed following a standard procedure; however, it is shown that they have unique and useful properties. The paper introduces the candidate manifolds associated with IM estimation and shows various approaches for designing SMOs with compound manifolds. Generally, using the IM model, it is typical to design observers with simple manifolds. However, since the state transition matrix of the IM model depends on the motor speed, the problem is more complicated – the speed must either be known or it must be estimated. In the paper, several sensored and sensorless observer designs are presented. The paper presents the theoretical developments and shows simulations and experimental results.

Instantaneous voltage measurement technique for PWM voltage source inverters

The pulse width modulated (PWM) voltage source inverter (VSI) is almost universally used in industrial motor drives. Although measurement of the VSI output voltage is required in many applications, accurate measurement is problematic due to the high frequency inverter switching. Many commonly used inverter output voltage measurement methods, such as indirect and direct voltage measurement, produce inaccurate results under a variety of operating conditions. In this paper, the common sources of voltage measurement error are identified and an instantaneous voltage measurement technique is presented that significantly reduces inaccuracies introduced by common voltage measurement strategies. Implementation details of the proposed strategy are outlined and experimental results are used to compare the proposed technique with other methods of inverter output voltage measurement. The results demonstrate the opportunity for improvements to any inverter-driven motor control system that requires precise terminal voltage measurements to carry out the intended control objective.

Speed and rotor position estimators for PMSM with sliding mode EMF observer front

The paper discusses the problem of speed and rotor position estimation for the Permanent Magnet Synchronous Motor (PMSM) and presents a family of estimation methods. Estimation is done sequentially: first, the EMFs are obtained using a sliding mode observer; then, the speed is estimated. The estimators are based on the model of the PMSM in the stationary reference frame. The paper presents an SM observer for the EMFs (the front observer) and various observers for the speed (the back observers). It is shown that the speed estimation process offers many mathematical options - it can be adaptive, sliding–mode based, algebraic, or it can use a double–manifold sliding mode structure. The properties of the various speed observers and their feasibility and accuracy are discussed. The method can be applied in a sensorless PMSM drive where the speed and rotor position angle are needed for control implementation. The paper presents the design of the observers along with simulation results and experimental tests.

Sensorless rotor position estimation of PMSM by full-order and sliding mode EMF observers with speed estimate

The paper discusses the problem of rotor position estimation for the nonsalient PMSM and presents a Full-Order (FO) observer and a Sliding Mode (SM) observer; both are constructed using a speed estimate. The observers are developed using the PMSM model in the stationary reference frame and use the motor voltages, currents and speed as inputs. The PMSM is treated as a time-varying plant. Convergence is analysed assuming that the speed signal is inaccurate - this corresponds to the situation when a speed estimate is used to obtain a sensorless observer. Under improper speed, the FO observer gives errors; however, the SM observer can be forced to converge by increasing the gains. The SM structure proposed allows design of a variety of sensorless observers.

A sliding mode speed and rotor position estimator for PMSM

The paper discusses the problem of state estimation for the non-salient permanent magnet synchronous motor (PMSM) and presents a method for estimation of speed and rotor position based on a sliding mode (SM) EMF observer. The observer is developed based on the full order PMSM model in the stationary reference frame. The motor speed is a required input in the proposed observer and a speed estimate obtained from the output rotor position is used. The SM observer is analysed using Lyapunov’s theory of non-linear stability and is validated by simulations and experimental tests. The proposed method is applicable in a sensorless PMSM drive; the estimator uses only the measured motor voltages and currents and produces estimates of the speed and rotor position. The influence of the feedback gains of the SM observer is discussed. The method is relatively simple to implement, involves a reduced number of design gains, does not require correction of the rotor position and has excellent accuracy for a wide speed range and at low speed.

The influence of decoupling voltages on the performance of synchronous current controllers in induction motor drives

The paper discusses the design of the current controllers for the induction motor (IM) drive where field oriented control is done in the rotor flux synchronous reference frame. The most common current control method is to use PI controllers alone – this is easy to implement and works well in most cases. A better scheme is to use decoupling compensation voltages in addition to the PIs, however, this is more difficult to implement and the decoupling voltages are often omitted. The paper compares the two current control schemes. The influence of the feed-forward decoupling voltages on the performance of the current loops is investigated in torque control mode and speed control mode. The theoretical findings are validated by simulations and experimental results.

A novel speed estimator for induction motor based on Integral Sliding Mode current control

The paper presents a new speed estimation method for the induction motor. The speed estimator is made possible by a special current control scheme that is based on Integral Sliding Mode (ISM). The traditional method for synchronous d-q current control is to use PI controllers along with a decoupling compensator. Synthesis of the compensation voltages requires knowledge of the speed and motor parameters. In the ISM scheme, the PIs are complemented with sliding mode (SM) controllers. This structure offers comparable performance without knowledge of the speed signal. The SM controllers play the same role as the compensation voltages used in the traditional method. Based on this observation, the speed is estimated using the filtered (average values) of the SM outputs. The paper describes the ISM current control method and presents the structure of the speed estimator. Simulations results are shown for a 1/4 hp three-phase machine.