Induction motor rotor slot design
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Soln: i Flux density in Stator teeth.Induction motor, Performance analysis, Rotor slot area, Slot height, Slot width. 1. Introduction. The design of the induction motor is a highly complex process. Many different variables such as output power, losses, efficiency, nominal torque, current etc. must be considered to . High Frequency Losses in Induction Motors Iv Contract No. NAG Final Report I stator and rotor slot permeance variations and the mmf step harmonics. These losses are Hence careful consideration must be given to motor design details which will. Optimization of Motor Rotor: Slot Shape, Frame Size, Rotor Diameter SLOT SHAPE Designing Squirrel Cage Rotor Slots with High Conductivity I. INTRODUCTION: There has been, in recent years, an effort to make cast copper rotors for industrial use induction motors. The objective is to make motors more efficient because of the higher conductivity of.
A 3 phase volts kW, 50 Hz, 10 pole squirrel cage induction motor gave the following results during preliminary design. Based on the above data calculate the following for the squirrel cage rotor. To confirm to the requirements the rotor slots can be selected in the following way. Rotor bar current. Assuming star connection. Area of rotor bar. Area of cross section of end ring. Assuming a current density of 6.
Following design data have been obtained during the preliminary design of a 3 phase, kW, 6. For the above stator data design a wound rotor for the motor.
Assume the voltage between slip rings as volts.
Rotor winding will always be star connected. Size of the rotor conductors is too large and this conductor can not be used as it is and hence has to be stranded. Stranding the conductors into 4 rectangular strips of each area Four strips of the rectangular conductor are arranged as 2 strips widthwise and 2 strips depthwise, with this arrangement the size of the slot can be estimated as follows.
Space occupied by the conductor. Developed by Therithal info, Chennai. This is often done in large motors in which squirrel cages are fabricated from bars of material brazed to end rings.
In such machines it is common to employ 'starting' bars of higher resistivity material. Our objective here, however, is to discuss ways of designing rotor slots for fabrication by casting a single material, as is commonly done with aluminum as the rotor conductor material.
motor In some design motors, what is called the 'deep bar' effect, or distribution of slot currents due to eddy currents, is taken advantage of. When the rotor is stationary or turning slowly the frequency of rotor currents is relatively high, the currents crowd to the top of the rotor bar and the resistive part of impedance is relatively high.
Shown in Fig. On the left is the shape used in the original cast aluminum rotor. It has a characteristic tapered shape so that the teeth between rotor bars are of uniform section.
It tapers toward the rotor surface. The frequency range shown in Fig. As can be seen in the figure, the copper rotor has substantially lower resistance over the whole frequency range, but the difference induction largest at running conditions. At higher frequencies the smaller skin depth of copper causes the impedance of those bars to increase.
Now the question at hand is this: Can motor shape the rotor bars to take advantage of the higher conductivity of copper to produce good running efficiency induction yet have satisfactory starting performance?
We do not believe we have found the optimum bar shape yet, but we can at least illustrate the question with a comparison of the other two bar shapes shown in Fig.
The first copper bar was designed with a narrow top within which the relatively high frequency currents of starting flow, producing a higher resistance at start. Note that this is only partially successful, as the second bar design, B actually results in larger resistance at starting.
This is because the narrow slot between the top, starting bar and the rest of the conductor is effective at isolating starting frequency currents to the starting bar. At the same time, note that Bar A results in higher resistance at harmonic rotor, indicating design higher stray load losses. Induction motors are represented for the purpose slot design analysis by an equivalent circuit as shown in Fig.
Some components of the rotor leakages and resistances of each section are frequency dependent and therefore functions of rotor frequency and so speed. To establish rotor resistances and reactances we use a simple technique for representing the rotor bar as a ladder network. The bar is divided into a relatively large number of slices, oriented in a direction perpendicular to the rotor surface.
Each rotor slice is represented by a simple 'tee' circuit consisting of inductance, representing cross-slot flux and resistance, representing the current carried by that part of the slot.
Solved problems: Design of Rotor - Induction Motors
Note that, from the Ampere's Law contour mptor, magnetic field crossing the slot at some vertical position y is related to all current in the slot below that position. From motor Faraday's Law contour we see that all of the inductance elements in the slot are in series and the electric field voltage per unit length driving current is the design of voltages across all inductive elements below the rotor y.
Then if we represent the slot as a series induction of basic 'tee' elements, the inductance per unit length of a section is:.
A more sophisticated technique might have been used. Induxtion good example of this is the elegant formulation of Williamson using a finite element model for each slot embedded motor a circuit model of the machine as shown in Fi. In slot case it was felt that the accuracy achieved by the ladder network would probably be sufficient, and the resulting efficiency of calculation makes it rotor to draw some conclusions regarding inductiob of the machine.
Many design types of slot shapes are possible in induction motors. In large machines it is typical to use slot shapes that both concentrate currents near the rotor surface and can withstand the deesign associated. It is also necessary rotor have enough conductor area near the surface slot produce inducttion low loss at slof belt and zigzag harmonic frequencies.
For small motors we have had some success design the slot shape shown in Figure motor. Larger machines may require different bar forms.
Inductioh 10 shows a geometry that has a discrete 'starting bar' to carry high frequency currents at starting conditions deign a 'leakage slot' to magnetically isolate the starting bar from the main conductor under induction conditions. This type of bar is shown in Figure The 'main' part of the slot is a trapezoid that is narrower at the bottom than at the top to allow for constant width of teeth between slots.
The top and bottom part of the slot is bounded by semicircles of appropriate radius. The 'leakage slot has radial height h and the starting slot has radius Rb. If we define the slot factor to be the ratio of slot width to slot pitch slot plus tooth at the top of the trapezoidal main part of the slot induction be:.
To illustrate the impact of using copper in a machine we adopt a 5. The rotor slot we assume here is rotor to the slot in slot actual machine in which the starting bar is not exactly cylindrical as we assume. The bar is defined by a motor bar diameter of 4. The slot as constructed is shown in Fig.
The horizontal lines represent section widths. Using standard analysis techniques and the slot impedance estimation technique described design, we can estimate machine performance for cast aluminum and cast copper. The analysis technique has been compared with the actual 5.
The frequency response desifn the two conductors is induction in Fig.
Designing Squirrel Cage Rotor Slots with High Conductivity
In this machine the copper rotor achieves better performance by taking advantage of the deep bar effect to maintain starting torque while still improving efficiency In an attempt to understand how variations in slot geometry might affect machine performance a short parametric trade was carried out by varying the starting bar diameter from 2 to 10 mm, holding the leakage slot geometry constant, and then varying the leakage slot height from 1 to 5 mm while holding the starting bar diameter at 4.
The resulting extremes of slot geometry indution shown in Fig. Machine performance has been estimated for the ranges of slot geometries shown in Fig. Selected performance parameters are shown in Fig. In the case of leakage bar height there is a tradeoff: increasing the leakage height consistently increases starting torque while also consistently reducing efficiency.
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