Switched reluctance motor, the electrical machine utilizes reluctance torque, is gaining acceptance in variable speed applications worldwide. In electrical machines related research, one of the most challenging tasks is the complexity of the implementing for the efficiency control, mentioned in[1,3,13,14,41,48,50,56,57,59]. Only few have considered the efficiency improvement among the SRM drives studies issued on that by a simple control strategy for ease of practical applications. For the efficiency research, most efforts have been devoted to the SRM structure design and it can be clearly realized from [19,48,49,53], which is focused on the modified type and special design for motor phase number, flux path, and air gap parameters to achieve the efficiency improvement. The circuit topologies for SRM drives are also taken to be the research topics for the efficiency research, such as dual-decay converter, C-dump inverter, and modified (n+1) switch converter, which have been described in [56,57], respectively. Besides of that, the current profile via switching angle control is also applied to involve the efficiency related research opinions [18]. To integrate the considerations
Chapter 1 Introduction
originate from a motor model research, a method that being proposed is the optimizing efficiency control which makes use of the parameter information based on the equivalent magnetic inductance, hereafter can be described as inductance, without losing the conventional performance requirement in a simple way. The optimizing efficiency control is a novel current command decision method for adding the efficiency regulation capability for the SRM drives[24,25,75,76]. In practice, the proposed optimizing efficiency strategy needs some information of parameters to form a control scheme with considerations of actual operation states. Thus, followings will also arrange the related research topics for parameters that may be needed or helpful to this efficiency improving approach. .
Inductance:
For improving the performance of SRM drives, several approaches had been proposed and focused on the inductance measurement related issues.
The frequency of inducing signal would be useful to enhance the measurement precision, addressed in [6,12,33,34,36,37,60]. The position estimation using the information of inductance profile to excite the stator winding at appropriate instant is applied referred to [58,44]. Besides of that, the inductance information is also taken in SRM sensorless drives applying resonant method, linked control strategy design, and flux estimation, respectively, in [58,60,33,34].
Mutual inductance:
A method that provides dynamically analytical information reflecting accurate working properties of SRM is needed [4,10,11,24,26,27,29,54,55].
However, most analytical schemes are either based on the simplified model or sophisticated mathematical computations, which means elimination of some parameters that are not measurable during operation or complex analytical functions may be involved. Broadly speaking, the SRM adjacent phases have overlapping current conduction, and hence there exists mutual flux linkages between these adjacent phases that result in mutual inductance between the windings. Some researchers have been devoted to the related studies for the influence of the mutual inductance in SRM drives [30,54].
Thus, the practical SRM drives system applied in the applications, such as high-speed and high-performance drives, to contain identification scheme of all affection parameters, including the mutual inductance, has been the trend in drive technology as long as it allows an easy utilization relationship with
Chapter 1 Introduction
been utilized to try to achieve both the improvement of parameters estimation and drives performance [26,29,30,54,55].
Resistance:
In sensorless drive, the parameter of phase resistance, hereafter can be called resistance, plays a crucial role to the speed estimation [2,61]. The resistance of switched reluctance motor may change with temperature obviously, and it has been the goal to find the value under operation described in [61,65]. Especially in low-speed operation, the influence of the resistance may lead to a larger portion to the estimation of speed[31,65].
Generally speaking, direct measurement, one-point probe measurement, two-point probe measurement, linear four-point measurement, and nonlinear measurement can be applied in resistance measurement, however, it may not be accepted in practice for consideration of convenience of usage and cost related issues.
Speed:
Designs of Sensorless’ SRM drives have eliminated the encoder in systems. Most sensorless methods monitor the current and voltage in each winding of the motor in order to estimate the inductance from which position can be inferred by the use of look-up tables. In order to reduce cost and increase reliability and minimize sensitivity to line voltage and temperature variations, new sensorless techniques are under development, such as Mavrik Motors (U.S.) staggered tooth design motor system. Other techniques shape an induced voltage in an inactive phase of the motor adjacent to an energized phase in order to estimate the shaft position of the motor’s rotor. Related researches on the parameters identification and estimation for SRM have moved toward on-line estimation from off-line operation and computation described in [25,31]. Furthermore, there are more researches focusing on the combination of parameters estimation applied in sensorless drives discussed in [31,45].
Torque:
Many methods proposed for estimation of torque address fundamental issues about requirement of reflecting its accurate properties [38,41,52].
However, most analytical schemes are based on the simplified linear torque model and sophisticated mathematical computations may involve, or require large numerical tables for looking up [22,23,39,40,42]. Furthermore, the linear model contradicts the fact of high proportion magnetic saturation
Chapter 1 Introduction
operation for SRM, which means that the neglect of nonlinear effect probably yield poor computation results. Likewise, during on-line operation, the model structures and parameters of SRM may differ from the standstill ones due to the related effects of saturation and losses, especially at high current, which makes the previous schemes practically difficult to implement and thus can only be accurately described by a nonlinear model [52,64,45]. Besides, some parameters of SRM are highly nonlinear functions of phase current, rotor position, and rotor speed. They are not measurable during operation, and are hard to be expressed with analytical functions [40,44]. Hence, a estimation model is needed herein to construct a torque model to improve the torque evaluation capability. Moreover, it is also expected that being with the information acquired and computed from the torque estimation model, it can be applied for further speed estimation based on the motor electromagnetic derivation and it is applicable for speed sensorless applications.