Size of the slot: Normally different types of slots are employed for carrying stator windings of induction motors. Generally full pitched double layer windings are employed for stator windings. For double layer windings the conductor per slot will be even. These conductors are suitably arranged along the depth and width of the winding.
Stator slots should not be too wide, leading to thin tooth width, which makes the tooth mechanically weak and maximum flux density may exceed the permissible limit. Hence slot width should be so selected such that the flux density in tooth is between 1. Further the slots should not be too deep also other wise the leakage reactance increases.
As a guideline the ratio of slot depth to slot width may assumed as 3 to 5. Slot insulation details along the conductors are shown in Fig. This slot insulation is called the slot liner, thickness of which may be taken as 0. Suitable thickness of insulation called coil separator separates the two layers of coils.
Thickness of coil separator is 0. Wedge of suitable thickness 3. Lip of the slot is taken 1. Figure 13 shows the coils placed in slots. Fig The flux density in the stator tooth is limited to 1. Depth of stator core below the slots: There will be certain solid portion below the slots in the stator which is called the depth of the stator core.
This depth of the stator core can be calculated by assuming suitable value for the flux density Bc in the stator core. Generally the flux density in the stator core may be assumed varying between 1. Depth of the stator core can be calculated as follows.
Problems Ex. Obtain the following design information for the stator of a 30 kW, V, 3 , 6 pole, 50 Hz delta connected, squirrel cage induction motor, i Main dimension of the stator, ii No. Assume suitable values for the missing design data. Using data of this machine determine the core dimensions, number of slots and number of stator conductors for a 11kW, volts,6 pole, 50 Hz motor.
The winding factor is 0. July Soln. During the preliminary design of a kW, volts, 3 phase, 8 pole 50 Hz slip ring induction motor the following design data have been obtained. Standard size of the conductor selected satisfying the requirements is 2. Thus sectional area of the conductor Six conductors in each layer are arranged as 2 conductors depth wise and 3 conductors width wise. With this arrangement the width and depth of the slot can be estimated as follows.
One is the squirrel cage rotor and the other is the slip ring rotor. Most of the induction motor are squirrel cage type. These are having the advantage of rugged and simple in construction and comparatively cheaper.
However they have the disadvantage of lower starting torque. In this type, the rotor consists of bars of copper or aluminum accommodated in rotor slots. In case slip ring induction motors the rotor complex in construction and costlier with the advantage that they have the better starting torque.
This type of rotor consists of star connected distributed three phase windings. Between stator and rotor is the air gap which is a very critical part. The performance parameters of the motor like magnetizing current, power factor, over load capacity, cooling and noise are affected by length of the air gap. Hence length of the air gap is selected considering the advantages and disadvantages of larger air gap length.
Hence in designing the length of the air gap following empirical formula is employed. Cogging and Crawling are the two phenomena which are observed due to wrong combination of number of rotor and stator slots. In addition, induction motor may develop unpredictable hooks and cusps in torque speed characteristics or the motor may run with lot of noise. Let us discuss Cogging and Crawling phenomena in induction motors. Crawling: The rotating magnetic field produced in the air gap of the will be usually nonsinusoidal and generally contains odd harmonics of the order 3rd, 5th and 7th.
The third harmonic flux will produce the three times the magnetic poles compared to that of the fundamental. Similarly the 5th and 7th harmonics will produce the poles five and seven times the fundamental respectively.
The presence of harmonics in the flux wave affects the torque speed characteristics. The Fig. The motor with presence of 7th harmonics is to have a tendency to run the motor at one seventh of its normal speed. The 7th harmonics will produce a dip in torque speed characteristics at one seventh of its normal speed as shown in torque speed characteristics.
Cogging: In some cases where in the number of rotor slots are not proper in relation to number of stator slots the machine refuses to run and remains stationary. Under such conditions there will be a locking tendency between the rotor and stator. Such a phenomenon is called cogging. Hence in order to avoid such bad effects a proper number of rotor slots are to be selected in relation to number of stator slots. In addition rotor slots will be skewed by one slot pitch to minimize the tendency of cogging, torque defects like synchronous hooks and cusps and noisy operation while running.
Effect of skewing will slightly increase the rotor resistance and increases the starting torque. However this will increase the leakage reactance and hence reduces the starting current and power factor. Fig 16 Torque speed characteristics Selection of number of rotor slots: The number of rotor slots may be selected using the following guide lines. Cross sectional area of Rotor bar: Sectional area of the rotor conductor can be calculated by rotor bar current and assumed value of current density for rotor bars.
As cooling conditions are better for the rotor than the stator higher current density can be assumed. Higher current density will lead to reduced sectional area and hence increased resistance, rotor cu losses and reduced efficiency. With increased rotor resistance starting torque will increase. Once the cross sectional area is known the size of the conductor may be selected form standard table given in data hand book. Shape and Size of the Rotor slots: Generally semiclosed slots or closed slots with very small or narrow openings are employed for the rotor slots.
In case of fully closed slots the rotor bars are force fit into the slots from the sides of the rotor. The rotors with closed slots are giving better performance to the motor in the following way. From the above it can be concluded that semiclosed slots are more suitable and hence are employed in rotors. Copper loss in rotor bars: Knowing the length of the rotor bars and resistance of the rotor bars cu losses in the rotor bars can be calculated.
End Ring Current: All the rotor bars are short circuited by connecting them to the end rings at both the end rings. The rotating magnetic filed produced will induce an emf in the rotor bars which will be sinusoidal over one pole pitch.
As the rotor is a short circuited body, there will be current flow because of this emf induced. The distribution of current and end rings are as shown in Fig. Thus the maximum end ring current may be taken as the sum of the average current in half of the number of bars under one pole.
Copper loss in End Rings: Mean diameter of the end ring Dme is assumed as 4 to 6 cms less than that of the rotor. At one end of the rotor there are three slip rings mounted on the shaft. Three ends of the winding are connected to the slip rings. External resistances can be connected to these slip rings at starting, which will be inserted in series with the windings which will help in increasing the torque at starting.
Such type of induction motors are employed where high starting torque is required. Number of rotor slots: As mentioned earlier the number of rotor slots should never be equal to number of stator slots. Generally for wound rotor motors a suitable value is assumed for number of rotor slots per pole per phase, and then total number of rotor slots are calculated.
So selected number of slots should be such that tooth width must satisfy the flux density limitation. Semiclosed slots are used for rotor slots. Number of rotor Turns: Number of rotor turns are decided based on the safety consideration of the personal working with the induction motors. The volatge between the slip rings on open circuit must be limited to safety values. In general the voltage between the slip rings for low and medium voltage machines must be limited to volts. For motors with higher voltage ratings and large size motors this voltage must be limited to volts.
Based on the assumed voltage between the slip rings comparing the induced voltage ratio in stator and rotor the number of turns on rotor winding can be calculated.
Then the standard conductor size can be selected similar to that of stator conductor. Size of Rotor slot: Mostly Semi closed rectangular slots employed for the rotors. Based on conductor size, number conductors per slot and arrangement of conductors similar to that of stator, dimension of rotor slots can be estimated. Size of the slot must be such that the ratio of depth to width of slot must be between 3 and 4. This flux density has to be limited to 1. If not the width of the tooth has to be increased and width of the slot has to be reduced such that the above flux density limitation is satisfied.
The flux density in rotor can be calculated by as shown below. Flux density in the rotor core can be assumed to be between 1. Then depth of the core can be found as follows. During the stator design of a 3 phase, 30 kW, volts, 6 pole, 50Hz,squirrel cage induction motor following data has been obtained. Based on the above design data design a suitable rotor.
A 3 phase kW, 3. Assuming air gap flux density to be 0. 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.
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