On the Influence of Stator Slot shape on the Energy Conservation Associated with the Submersible Induction Motors

Problem statement: A good design of Submersible Induction Motor is required to maintain maximum efficiency level. This can be achieved in one way by optimum design of magnetic circuit. The magnetic circuit of an Induction Motor is formed by the combination of Stator core, Rotor core and air gap. In applications like Submersible Motor Pumps, the air gap cannot be minimized beyond 0.5 mm, because of the machining tolerance required, due to large core length. Now a day, the efficiency of such motor is improved by die-cast copper rotor. But the cost of such motor is more and sometimes the rotor core properties are getting changed due to high temperature during diecasting process. Approach: A new stator slot shape is proposed based on the results obtained from the iterations with Rotational Machine Expert (RMxprt) software and the performance is compared with that of the existing stator slot shape, for a 3-phase, 5 hp, 380 V, 2-pole motor The motor with both the stator slot shapes have been analyzed with the Rotational Machine Expert (RMxprt) software. Results: The magnetizing current of new design is 3.25 A, compared to the existing design of 4.67 A. The overall efficiency of the Submersible Induction Motor pump set is 5% more than that of the existing one. The discharge of the pump set has increased by about 80 liters per minute. Conclusion: The proposed stator slot dimension is suitable for more efficiency, less magnetizing current and a good power factor. This design modification does not require much cost and needs no complex manufacturing process.


INTRODUCTION
Submersible Induction Motor with the pump sets are extensively used for agriculture and domestic purposes, owing to their simplicity, ruggedness and low cost. In India, during this decade, most of the Submersible Induction Motors are being manufactured with smaller frame size, due to increasing material cost, increasing labor cost and to provide economical model. But, this is at the expense of efficiency, large magnetizing current and poor operating power factor. In order to maximize the efficiency and consequently minimize the electrical energy consumption of a 3-Phase Submersible Induction Motor, many optimization techniques have been used and suggested in the published literature. The corresponding objective function is generally employed for an economic manufacturing of motors .Such a research effort was undertaken before the energy crisis in 1973. Since then, trend of optimum design has changed; in fact efficiency and consumed electrical energy appear to be more important than in the past.
Many works have concentrated upon this type of design ( Mujal-Rosas andBoix-Aragones, 2007, Hakiki Khalid et al., 2007 ) A combination of efficiency and cost objective functions is possible and efficiency-cost objective function can partly benefit from the advantages of the efficiency and cost optimum design (Cistelecan et al., 2009).
An attempt has been made to use two optimization techniques simultaneously for different aims as described in (Frauman et al., 2007), but again for efficiency optimization of the motor. (Subramanian et al., 2010), two optimization techniques (Powell and boundary search around active constraints methods) have been compared. Two other techniques, simplex and Hooke-Jeeves methods have been compared Application of genetic algorithms for the design optimization of electric machines has been tested in Other aspects in the design of Induction Motor have been introduced in (Mahdi S et al., 2009, Lubis et al., 2009. A further contribution is the optimum design of a three-phase Induction Motor, using three objective functions, namely Efficiency (E), Cost (C) and a combined objective function of Efficiency-Cost (EC). A design package has been developed specifically for a 3 -phase squirrel-cage type Induction Motor. A unique search routine of Hooke -Jeeves is then used for the three optimization processes. In many investigations, it is found that the solutions or suggestions given by optimization techniques are for Induction Motor only; but these suggestions are not suitable for manufacturing a Submersible Induction Motor, because of the limitations in D/L ratio, the technological complexity in design and manufacturing tools and cost.
The aim of this study is to present the results of performance comparison of a 3-phase, 5 hp, 380 V, 2-Pole Submersible Induction Motor with two different stator slot shapes. A 5 hp Submersible Induction Motor, having specifications shown in Table 1, has been chosen for comparison with two different stator slot shapes. The basic specifications of these designs are similar and the same constraints are imposed upon the design process. Design package software namely RMxprt from ANSOFT Corporation has been used specifically to determine the optimum stator slot shape, which consumes less magnetizing current. Simulated Steady-state performance characteristics of the motor have been plotted for comparison. The motor has been manufactured with two different stator slot shapes and have been tested by applying pump load and the performance characteristics have been plotted for comparison. The proposed stator slot shape is advantageous in respect of magnetizing current, efficiency, power factor and slip.

For analysis in RMxprt, a 3-phase Submersible
Induction Motor model has been selected with the specifications as shown in Table 1.
With all other motor specifications (Stator dimensions, Rotor dimensions and winding details) remaining the same, the motor has been simulated with two different stator slot shapes. One of these types is an existing type and the other being the proposed shape based on the results obtained from the iterations with Rotational Machine Expert (RMxprt) software, with the objective of minimizing the magnetizing current and to improve the efficiency.

Stator slot configurations:
Existing slot: Table 2 shows the dimensions of the existing slot and the shape is shown in Fig. 1.
Proposed slot: Table 3 shows the dimensions of the proposed slot and the corresponding shape is shown in Fig. 2. M-47 steel has been chosen as the stamping material and the number of stator slots being 24, the stator stamping is formed in RMxprt with both the existing slot and the proposed slot. Figure 3 shows the RMxprt view of stator stampings for both the slots.
Keeping the shape and the number of rotor slots same for both the type of stator slots, the Induction Motor assembly has been formed. The assemblies of the respective motor models are shown in Fig. 4.

RESULTS
With the fixed operating conditions, the machine has been simulated in RMxprt and the result is tabulated as shown in Table 4. The performance curves have been obtained for both the stator slot types Following inferences have been made from Table 4: • The motor with proposed slot consumes less magnetizing current and found power factor is improved • The stator phase current of motor with proposed slot is less when compared to that in a motor with existing slot • The efficiency of the motor with the proposed slot is significantly improved • The operating slip of the motor is reduced in good amount • The stator resistance of the proposed slot shape is being less, due to the less over hang portion of the winding The modification in the geometry of the slot has resulted in higher impedance of the stator core, in comparison with the existing slot, resulting in decreased magnetizing current; these facts can be visualized from Fig. 5-6. The simulation results of the motors with both the stator slot shapes have been presented in the Fig. 7-8. The load current and the efficiency of the proposed stator slot shape have been found improved.
The design variables are specified along with its bounds in Table 2-6 bits are encoded for each continuous.
Here all the constraints cannot be directly described in terms of design variables; hence, they are implicit and their calculation requires analyzing the motor.
GA is ideally suited for unconstrained optimization problems (Subramanian et al., 2010)). As the present problem is a constrained optimization one, it is necessary to transform it into an unconstrained problem to solve it using GA (Sendil and Nagarajan, 2009).

DISCUSSION
The proposed stator slot consumes less magnetizing current as 3.26 A, compared to the existing one with the current of 4.67 A. This results improved power factor, pump out put with more discharge and efficiency. The slip of the motor is also reduced. The losses in the proposed motor are considerably reduced. The volume of the material and copper consumption is also reduced.

CONCLUSION
This study has compared 3-phase Submersible Induction Motors with a two different stator slot shapes of the same rating. A 5 hp motor has been designed as an illustrative example. On the basis of the results obtained, one may conclude the following: • The new design has resulted in a lower magnetizing current, higher efficiency, good operating power factor and less drop in speed • The overall efficiency of the Submersible Induction Motor pump set is 5% more than that of the existing one • The discharge of the pump set has increased by about 80 liters per minute • The cost involved in the proposed design is same as that of the existing one; the technological complexity involved in manufacturing is less Mr.V.Jeyaprakash, Design Engineer, TEXMO Industries, Coimbatore, for providing consultancy and the necessary help in using RMExprt software for the simulations.