Analysis and implementation of a new method to retain the original speed and torque of synchronous reluctance motor during sustained voltage dip

Abstract

This study presents the operation of a synchronous reluctance motor drive under a sustained voltage dip condition while being connected to a weak grid. In the presence of an input voltage dip, motor may not be able to produce sufficient torque and maintain its original speed. A new technique is proposed in this study to maintain the desired reference torque and speed during a sustained voltage dip. The proposed technique is realised by evaluating mathematically the motor's allowable voltage dip margin corresponding to an operating point pertaining to a particular torque and speed in the current locus diagram of the motor. With prior knowledge of the voltage dip margin for every operating point in the current locus diagram, it would be possible to retain the same reference torque and speed by moving to a new operating point, which can be with lesser or greater i d as compared to the initial operating point, in case a voltage dip greater than the tolerable margin occurs. To start with, voltage dip margin has been derived theoretically and the operation of the motor is investigated by deliberately introducing a voltage dip. The proposed methodology has been verified through simulations and experiments in this study.

Impact of inductance variation on the operation of a synchronous reluctance motor connected to a weak grid

“…Equation 11gives the expression for obtaining the normalised voltage of a SynRM at a particular speed and torque, which is defined as the voltage required by the SynRM for delivering a particular torque at any speed and any current for a definite current angle δ. Substituting the value of K from (6) in (11), (12) is obtained v n…”

“…The value of v n will always be less than or equal to 1, since motor cannot operate beyond rated voltage to satisfy a particular torque at any speed. It is evident from (12) and (13) that the voltage dip margin that can be tolerated, depends upon the motor inductance. Without considering the variations in L d and L q , the voltage dip margin estimated will be erroneous.…”

“…(i) The torque and speed delivered by the machine are maintained as constants while analysing the voltage dip margin for a changed inductance value. (ii) Since the torque and speed are constants, from (12) it can be observed that with varying L d and L q , the only quantity that could be varying is the current angle δ. So δ is assumed to be changing.…”

“…Here ΔL d1 , ΔL q1 and Δδ 1 refer to percentage changes in L d1 , L q1 and δ 1 , respectively, from their original designed values because of some manufacturing defect. Substituting (16), (17) and (18) in (12), the normalised voltage expression, considering T n and ω n as constants is…”

“…Since the voltage dip margin depends on the inductance as seen from (28), the change in the voltage dip margin can be understood better by considering the actual parameters of the motor. Utilising the motor parameters in Table 1 and assuming the motor operates at T n = 0.52356, ω n = 0.667, tan δ = 1 with initial inductances at L d1 = 0.418H, L q1 = 0.061H, K 1 = 3.26113, and utilising (10), (12), and (13), the voltage dip margin, major axis and minor axis for the voltage ellipse can be obtained under decreasing inductance value (considering the constant d-axis current control of the SynRM), as shown in Table 2. Table 2 The diagrammatic representation of the trajectory change in the current locus diagram for varying inductance under the same torque and speed operation is shown in Fig.…”