HIGH TORQUE ROTOR'S AND THEIR CLASSIFICATION

👷HIGH TORQUE ROTORS 👷

⚡in case of slip ring induction motor and external resistance can be added in the rotor circuit during starting this gives high starting torque and lower starting line current at an improved power factor.  this resistance is then gradually cut from the rotor circuit which would otherwise result in degrees of full  load speed  poor speed regulation more total losses and hence reduced efficiency.  with lower rotor resistance it gives constant speed low sleep less losses and high efficiency.  this is the major advantage of the slip ring induction motor that it gives high rotor resistance at starting and low rotor resistance at normal operating speed.

⚡in case of squirrel cage induction motor there is no provisions made for adding external resistance.  with the resistance is designed in such a way that it gives better running performance than It has high starting current and consequently Low starting torque.  this is the major disadvantage of squirrel cage induction motor although it is having the other qualities of low cost righteousnes and maintenance free operation.  thus the designers have found different ways of improving the starting performance of the motor without affecting the running performance of the motor.

⚡in squirrel cage Induction Motor High starting torque can be obtained by the use of dip bar or double cage rotors.  both these types of rotor make use of skin effect in which distribution of current is not uniform for the alternating current has the 10 tendency to concentrate near the surface of the conductor.  due to this effect effective area of cross section of the conductor is reduced and hence the resistance of the conductor is increased when carrying alternating current.

⚡the solid conductor can be considered to be made up of a large number of Trans is carrying a small part of current.  the inductance of each strand will vary according to the position.  restaurants in proximity ad of the centre are surrounded by a greater magnetic flux and has a greater inductance than near the surface.  due to high reactance at the centre the alternating current flows near the surface of the conductor.  the skin effect depends upon the nature of the material diameter of the wire shape of the wire and the frequency.

⚡Thus the current in the rotor during starting is having the frequency of the supply.  while under running condition the frequency of rotor current reduces to slip frequency.  this variation in frequency change the rotor resistance as it depends on the skin effect.  during starting it gives high resistance whereas it gives low resistance during running condition which is desirable.  does the variation in rotor resistance can be achieved by debba or double cage construction of rotor of induction motor.  both these types of Constructions made use of skin effect phenomenon.

👷DEEP BAR ROTOR CONSTRUCTION👷

⚡there is no constructional difference between stator of the motor and that of the ordinary induction motor.  the rotor consists of a deep Bass short circuited by 2 and rings one on each side.  The Deep and narrow rotor bar of rectangular cross section is shown in the figure 4.13.1(a). The Other rotor bar shapes are shown in the figure 4.13.1(b). the magnetic leakage flux lines are shown by dotted lines now considered that the bar consists of many number of layers of different apps the top and bottom layers are also shown in the figure 4.1 3.1.

⚡the leakage inductance of the bottom strip is greater than that of the toxic as more flux linked with bottom strip compared to the top strip.  all the strips are in parallel electrically.  the bottom strip has greater leakage inductance than the top strip.  during starting the rotor frequency is equal to the state of frequency and hence leakage reactance of bottom state is largest and current in it is least.  the top strip has low leakage reactance and current in it is large.  does the current in Lori active stocks will be greater than that in Hai reactance lower strip and the current will be forced to was the top of the slot and face of current in copper strip will lead that the of the current in the lower one.  thus there is non uniform distribution of current which is shown in the figure 4.1 3.2 due to this non uniform distribution of current and due to the skin effect effective area of cross section decreases.  Hans rotor resistance increases result in high starting torque.

⚡as leakage reactance is proportional to frequency the non uniform distribution of current depends upon the rotor frequency.  in the figure 4.1 3.2 shows a curve indicating alternating current effective resistance to direct current resistance with the change in frequency for the copper bar of 2.5 CM Deep.  the skin effect is maximum when rotor is at standstill.

⚡with the increase in rotor Speed the rotor frequency decreases and the skin effect also decreases.  the reactance of different steps at this low frequency become almost equal and the current density of the conductor cross-section becomes uniform so it is alternating current resistance is equal to direct current resistance.  10 with the dip bar Rota has a low starting current with high starting torque without affecting running performance of the motor.  the net reactance of the dip bar Rota at a standstill is higher than in the normal bar design The breakdown or pull out  torque in deep bar rotor is lower. The torque-slip  characteristics of deep motor and normal induction motor is shown in figure 4.1 3.3.

⚡the equivalent circuit of induction motor is applicable to the rotor also where as proper value of resistance and reactance must be determined for satisfactory running performance.  during starting their values  should correspond to their effective values at lower rotor frequency.

👷DOUBLE CAGE ROTOR CONSTRUCTION

⚡this is another way of obtaining improved starting performance without affecting its running performance.  do it is more expensive it gives better performance than Deep bar rotor construction.

⚡the stator of double cage rotor induction motor is same as that of ordinary induction motor where as its  Rotor  consists of two cages for two layers of bar short-circuited bye and drinks since the upper  cage is having smaller cross sectional area than the lower cage the upper cage is having higher resistance than that of the lower cage.  with equal cross sectional area of two cases the upper cage is made up of high resistance material like brass aluminium bronze etc.  and the lower cage is made up of low resistance material like Copper.  the upper cage and lower kg are separated by a narrow slit aur constriction.

⚡the slot leakage flux pattern is also shown in the figure 4.1 3.4 for The Double cage Rotor.  similar to Deep bar rotor construction The rotor bars in the upper case has less leakage flux linkage and therefore has lower reactance.  the dimensions of their construction control the self leakage flux linking upper and lower bars.  if their construction would have been observed that the main flux would written by means of iron path between the two slots and thereby missing the bus in the lower cages which will not contribute to production of torque in that case.  hence it is see it can be seen that the upper case has higher resistance and lower reactance whereas the lower cages lower  resistance and high reactance.

⚡during starting the rotor frequency is same as data frequency of the supply frequency.  the division of rotor current in upper and lowercase is inversely proportional to their leakage impedances.  at the time of starting the leakage reactance of lower case is very high and consequently its leakage impedance is several times greater than that of upper cage whose leakage reactance is small.  most rotor current flows in the upper cage having lower leakage impedance.  the per cage having high resistance sharing the rotor current results in lower starting current at improve power factor giving high starting torque.

⚡when rotor speeds up the rotor frequency decreases which decreases the leakage reactance of lower cage.  at normal operating Speed the reactance  difference between the two cages is negligible small.  Hans dead division of rotor current in this case is mainly decided by the resistance of the two cages.  as resistance of upper kg is very high most of the current flows through the lowercase giving excellent operating characteristics under the running condition.  it can be noted that the starting current is confined mainly with upper cage so that there is frequent starting of motor then it would cause overheating and burning of upper cage.  the torque slip characteristics of double cage induction motor are shown in the figure 4.1 3.5.  another type of double cage rotor construction is also possible which is shown in the figure 4.1 3.6.  the slot leakage flux pattern for this type of construction is also shown in the figure.  the approximate equivalent circuit of double cage rotor induction motor is shown in the figure 4.1 3.7. Though  the two cages are somewhat coupled magnetically they can be treated as independent for simplicity and it use approximately same results.  the two cages are assumed to be parallel while drawing the equivalent circuit.

⚡the secondary upper case resistance and secondary lowercase resistance R the current in the upper and lower cases respectively referred to the stator secondary upper cage resistance and secondary lower cage resistance  of uppercase and lowercase referred to the stator whereas secondary uppercase reactance and secondary lowercase reactance are the leakage reactance of the two cases referred to the stator of the motor. 

👷⚡COMPARISON OF SINGLE CAGE AND DOUBLE CAGE ROTOR⚡👷