Calles de rodaje

There are various rotor slot shapes available and these depend upon the starting and rated load specifications, voltage/frequency (V/f) supply operation and the torque range. The rotor slot shape can be semi-closed, rectangular deep bar, rounded trapezoidal slots with rectangular teeth or completely closed rotor slots. Fig. 2.1 shows typical rotor bar shapes that are used in IM as illustrated in (Boldea & Nasar, 2009).

The choice of the shape will therefore depend on the size of the motor and its application. Semi- closed slots can be used for high efficiency induction machines with low power at constant V/f. The round semi-closed slots can be used for variable V/f. As for the rounded trapezoidal slots with rectangular teeth are typically used in small induction machines for medium starting torque. Completely closed rotor slots are used mainly in low power applications for noise reduction and reduction of torque oscillations. For very high starting torque and high rated slip, the rectangular deep bar rotor slots are used (Boldea & Nasar, 2009).

19

(a) (b) (c) (d) (e) (f)

Figure 2.1:Typical Rotor Bar Types (a) Rectangular, (b) Double cage, (c) Conventional (semi closed), (d) Trapezoidal, (e) Conventional (fully closed) and (f) Round

Fig. 2.1 Rotor Bar Shapes (a) Bar 1, (b) Bar 2, (c) Bar 3, (d) Bar 4, (e) Bar 5 and (f) Bar 6 The rotor bar shape has direct influence on the performance of squirrel cage induction machines. By varying the geometric parameters of the rotor, different torque-speed curves can be achieved, the design of rotor bars particularly have a significant impact on the starting behaviour of an induction machine, for example, choosing a particular rotor bar design can lead to increased starting torque, reduced starting current which will reduce the efficiency as a result of a greater slip (Gyftakis, et al., 2010). A particular rotor bar shape can be selected to suit a given application, for example use of large rotor bars result in reduced rotor resistance which in turn yields reduced starting torque.

According to (Gyftakis, et al., 2010), rotor bar variables (rotor bar depth, rotor bar width, rotor bar bottom shape and rotor slot opening width) can be optimized in order to better the performance of SCIMs. These authors applied FEM software to model and analyse the behaviour of this induction machine by optimising the motor’s starting behaviour, output power and efficiency. From the results, it is evident that finding an optimal rotor slot can yield an improved efficiency, starting torque and output mechanical power.

A study by (Galindo, et al., 2002) shows how rotor slots can be redesigned in order to improve the performance of squirrel cage induction motors and this can be achieved using computing tools based on numerical analysis. In this study the authors modified the geometrical parameters of the rotor slot until an optimal performance of the motor is achieved. Galindo and his colleagues

20

made use of 2D Finite Element Method (FEM) to modify rotor slot geometry in order to identify the design that gave the best starting performance of the motor.

Five years later (Virgiliu, et al., 2007) did a similar study on how changing rotor slot and rotor bar shapes of an induction machine would help improve performance of SCIMs. The parameters that were studied included the starting torque, breakdown torque, rated efficiency, power factor and rotor heating. Based on simulation results comparison, it was realised that step-holed rotor bar shape increased efficiency and starting torque without significantly reducing the breakdown torque. Increase of the starting torque was as a result of increased rotor resistance.

In recent years, FEM has been combined with other methods in order to study the effect that rotor bar shape has on the performance of three phase SCIMs. As reported by (Zhang, et al., 2013), the equivalent circuit method combined with FEM were used to evaluate performance of three phase SCIM with different rotor slot geometry. The target performance indexes that were studied were the torque-speed curve as well as efficiency. Various types of candidate initial rotor shapes were adopted; based on the developed algorithm, an optimal design was selected. In the new design, the rotor slot was smaller than the initial one, owing to the increased rotor resistance. The rotor slot opening was also wider and shallower compared to the initial one, as a result of reduced slot leakage reactance. It is therefore evident that a variety of torque-speed curves depending on different applications can be achieved by changing the geometry of the rotor.

In the work by(Appiah, et al., 2013) the equivalent circuit method combined with finite element method were employed for performance evaluation and an optimization algorithm to design optimal rotor slot geometry of a three phase squirrel cage induction machine to achieve specific torque-speed curve with high efficiency. Various types of candidate initial rotor shapes were adopted; based on the developed algorithm, an optimal design was selected. In the new design, the rotor slot was smaller than the initial one, owing to the increased rotor resistance. The rotor

21

slot opening was also wider and shallower compared to the initial one, as a result of reduced slot leakage reactance. It is therefore evident that a variety of torque-speed curves depending on different applications can be achieved by changing the geometry of the rotor.

(Brojboiu, 2001) Studied the effect of rotor bar shapes on the starting performance of squirrel cage induction motors. The study was conducted using Matlab and the design of the machine was done with CAD program. The rotor bar shapes that were adopted were, the rectangular bar, trapezoidal bar, T-inverse bar and a special bar. From the results, the special bar presented better starting characteristics as a result of its good rotor reactance variation compared to other rotor bars. In terms of the mechanical characteristics, the T-inverse bar showed a better starting torque compared to the other bars and this was because of the intense current skin effect at the bottom of the bar which is much thicker than the top of the bar.

Another factor that has an impact on the performance of squirrel cage induction machines in relation to rotor bars is how deeply buried the rotor bars are into the rotor. Squirrel cage rotor bars that are much nearer to the surface of the rotor will result in reduced leakage reactance whereas the rotor bars that are buried deeper in to the rotor will result into leakage reactance increase. With the leakage reactance increased, the starting torque, starting current and break- down torque will be minimised (Chapman, 2012).On the other hand, if the cross-sectional area of the rotor bar is reduced while the bar is kept closer to the rotor surface, the rotor resistance will increase which will result in increased starting torque and reduced starting current. The full load slip and efficiency will also increase (Hamdi, 1994).