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Design Approach to a Wound Rotor Induction Motor Towards Optimization

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Design Approach to a Wound Rotor Induction Motor Towards Optimization ISSN (Online) : 2454 -7190 ISSN (Print) 0973-8975 J.Mech.Cont.& Math. Sci., Vol.-13, No.-3, July-August (2018) Pages 159-172 Design approach to a wound rotor induction motor towards optimization 1Pritish Kumar Ghosh, 2Pradip Kumar Sadhu, 3Amarnath Sanyal, *4DebabrataRoy, 5Biswajit Dutta 1,2Electrical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, 826004, India 3Ex Power Engineering, Jadavpur University, Kolkata,W.B.- 700032,India 4Electrical Engineering,Techno International Batanagar, South 24Parganas, W.B.- 700141,India 5ElectricalEngineering,Seacom Engineering College,Howrah,W.B.-711302,India [email protected],[email protected],3ansanyal@yahoo. co.in,[email protected],[email protected], *Corresponding author: Debabrata Roy, Abstract About 88% of the driving power is produced by 3-phase and single-phase induction motors. In most part it is by squirrel-cage motors, only a small fraction by the slip-ring or phase-wound type. It is because the cage-type motors are relatively inexpensive. But they suffer from low p.f. operation and low starting torque which cannot be manipulated by inserting resistance in the rotor circuit. Also, this type of induction motors is not easily speed-adjustable. Though a little more expensive, the slip-ring type induction motors do not have these disadvantages. Therefore, they are used as speed-adjustable drives and for drives where heavy duty starting is involved. The design of any kind of power equipment should be made cost-optimally in the present day competitive market. A new approach to reaching optimal solution has been shown in this paper by the method of sequential searching with respect to the chosen design variables. Also, another design has been made following a hybrid of analytical and synthetic approach. The design variables have been chosen from designers’ experience. In contrary to the popular belief that there is no need for going in for complexity of optimal design, the quasi-optimal solution may be obtained by the designer from his accumulated experience, we find that the idea is wrong. The optimal design approach saves a lot of money. 159 J.Mech.Cont.& Math. Sci., Vol.-13, No.-3, July-August (2018) Pages 159-172 Keywords: Analytic design, synthetic design, hybrid design, optimal design, sequential searching, development of electrical engineering, Electrical applications. I. Introduction There are two types of induction motors in use- the squirrel cage and the phase-wound or slip-ring type. The squirrel cage type is cheaper, more robust and rugged and for these reasons it is used in almost all industrial applications. But the squirrel-cage type has torque limitations, they run at poor lagging power factor and are not easily speed-adjustable whereas torque, power factor etc. can be adjusted by various means in phase-wound types. Also the cage-type motors have poor starting torque and are unsuitable for heavy-duty starting. Hence, slip-ring types are used as adjustable drives and for high torque applications [I]. II. Constructional features and field of application In a wound-rotor induction motor, the rotor windings are connected to external resistances through slip rings.Adjustment of resistance allows control of the speed/torque characteristic of the motor. A low inrush current can start wound-rotor induction motors, by inserting high resistance into the rotor circuit; the resistance decreases as the motor accelerates. By employing slip-power recovery methods both speed and power factor can be adjusted. The rotor of a slip ring motor has more winding turns as compared to the squirrel- cage rotor. The induced voltage is higher and the current is lower, compared to a squirrel cage rotor. During the starting, a typical rotor has three terminals for the 3- phases connected to the slip rings. Each pole is wired in series with a variable power resistor. The rotor poles are switched to short circuit as soon as the motor reaches full speed.At the stator, the resistor reduces the field strength during start-up. Due to this, the inrush current reduces,higher starting torque provides another advantage over squirrel cage induction motor. In several forms of adjustable-speed drive, a wound-rotor motor is used. In certain types of variable-speed drives, wide speed range with high-energy efficiency is allowed, if the slip-frequency power is recovered from the rotor circuit and fed back to the supply.To supply external power to the rotor circuit in a doubly fed electric machine, the slip rings are used which in turn allows wide range speed control. Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives [I,V,XVII]. The schematic view of a slip-ring type motor is shown in fig. 1. 160 J.Mech.Cont.& Math. Sci., Vol.-13, No.-3, July-August (2018) Pages 159-172 III. Optimization While computer programs are used for design calculations [XIII,XVII], the aim is always to reach an optimal solution through iterative procedure. Optimal solution is the best out of all feasible solutions with respect to a chosen objective function. The objective function may be maximum starting torque or maximum efficiency or minimum cost, subject to given constraints [IV,XV]. The cost of production of a slip-ring type motor depends on several design variables e.g. air-gap flux-density, current density in stator and rotor conductors, length/pole pitch ratio etc. The optimal solution subject to given constraints is sought by using mathematical programming techniques [IX]. Many designers have conducted extensive research on design optimization of induction motors. S.S. Sivaraju et al advanced a paper on novel design of induction motor enhancing the performance variables [XIX]. H.B. Ertran et al proceeded a new approach to optimal design of induction motors [VII]. R.N. Hasana emphasized on energy-saving through design optimization [VIII]. A large no. of authors used soft-computing techniques to reach the optimal design. R. Kannan et al [X] used PSO for optimization and V.P. Sakthivel et al applied the same [XVI] for economic design. C. Thanraj et al did the same with improved PSO [XX]. T. Tudorache et al used finite element methods [XXI], M. Cunkas et al, as well as A. Krishnamoorthy et al used genetic algorithm [III,XII] to reach the optimal solution. K. Ranjith Kumar et al worked on performance enhancement of slip-ring type induction motors [XIV]. F. Kentli advanced a paper on survey of design optimization through decades [XI] which gives a comprehensive study in this area. In this paper an attempt has been made to figure out the cost-optimal design under given constraints using classical techniques. Soft computing techniques have not been used as there is no ambiguity or uncertainty in the problem. However, a single mathematical expression for the objective function is very complex and is based on approximations. So the optimization has been sought through the design subroutine for slip-ring type motors, placed as a part of the multiple loop operation 161 J.Mech.Cont.& Math. Sci., Vol.-13, No.-3, July-August (2018) Pages 159-172 Figure1.Schematic view of a slip ring type induction motor IV. Bio gas through bio-methanation The over-all cost of production has been taken as the objective function. The key design variables [VI,XVII] are: ac = amp.conductor/m; Bav = average flux-density in the gap, height; width ratio of the slots, length: pole pitch ratio. The current density in the conductor is not separately treated as a variable as it depends on the choice of ac. In addition, there are some decision variables e.g. the choice for the conductor materials: copper or Aluminium, particularly for rotor, the choice of core materials etc. V. Theconstrains There is generally a no. of inequality constraints in a design problem that may be imposed by the customer or the authorities [IX,XV]. Limits are imposed on maximum temperature rise at full load, minimum efficiency and minimum power factor. Also there may be specifications demanding a certain minimum value of maximum torque/full load torque and starting torque/full load torque, maximum starting current/full load current etc. the design must conform to these constraints and at the same time be fabricated for minimum possible cost. Therefore, the design is a constrained non-linear optimization problem. VI. Design procedure Analytical method gives only a feasible solution, not the best out of them. In the synthetic approach, it may be possible to bring forth some changes so as to conform to the specs. However, it has got limitations and is unable to reach the optimal solution. To reach the optimal solution, some logically sound iterative procedure is to be followed, which in multiple steps converges to the optimal for 162 J.Mech.Cont.& Math. Sci., Vol.-13, No.-3, July-August (2018) Pages 159-172 a given objective function. Every design variable has its bounds. If all of them are varied within their respective bounds at regular interval and an exhaustive search is made for the minimality of the cost function, it will be reached. However, this procedure takes longer computation time and larger memory. Therefore, mathematically more effective methods like gradient search etc. are normally used [IV,VII,XV]. By running the program, it has been found that the hypersurface of the cost function is concave. Taking advantage of this finding, we have made a sequential gradient search on the chosen design variables and we have been able to reach the minimal in much smaller no. of steps. Thus, the method has been found to be very effective in searching for the optimal. VII. Case-study The case-study has been made on a 15 kW, 415 V, 3-phase, 50 Hz., delta- connected slip-ring type induction motor.
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