71 What is the normal value of slip of an I M operating at full load?
3 - 5%
72 Why is not possible for the rotor speed of an I M to be equal to the speed of its rotating magnetic field?
The machine will not be able to develop any mechanical torque to run as a motor.
73 State the condition at which the torque developed in a 3 phase induction motor is maximum.
When R2=SX2
74. What are the advantages of slip-ring I M over cage I M?
(i) Rotor circuit is accessible for external connection.
(ii) By adding external resistance to the rotor circuit the starting current is reduced with the added advantage of improving starting torque.
(iii) Additional speed control methods can be employed with the accessibility in the rotor circuit.
75. What are the losses occurring in an I M and on what factors do they depend?
Magnetic losses Wi Electrical losses Wcu Mechanical losses Wm
For I M operating in normal condition (with constant voltage and frequency) magnetic and mechanical losses remain constant whereas electrical losses vary in square proportion to the current.
76. What care should be taken at the time of construction to reduce eddy current losses in I M?
Make the resistance of the core body as large as possible.This is achieved by laminating the stator core,stacked and revetted at right angles to the path of eddy current.The laminations are insulated from each other by thin coat of varnish.
77. Why is there not appreciable magnetic losses in the rotor core of Induction motors?
Although the rotor core is also subjected to magnetic flux reversals and since the frequency of flux reversals in the rotor, fr = Sfs, is very small, the iron loss incurred in the rotor core is negligibly small.
78. What is meant by synchronous watt?
With the power input to the motor Pi, after the losses in the stator winding. Wcui and stator core, Wi, are met with, the remaining power is transferred to the rotor by the rotating magnetic field as power input to the rotor Pir
Pir = Pi –waui - Wi
The power input to the rotor Pir is transferred from the stator to the rotor by rotating magnetic field which rotates at synchronous speed Ns. Torque Td is developed in the rotor as
a result of Pir and the equation for Pir can alternatively be expressed as
Pir (2 Ns160). Td W Or Td = Pir Syn. W
79. How does the shaft torque differ from the torque developed in 3-phase Induction motor?
The mechanical power developed Pd causes the rotor to rotate at a speed Nr due to the torque Td developed in the rotor . Therefore, equation for Pr can be written as
Pd 2 NrTd 1 60
The remaining power, after the mechanical losses Wm are met with, available
in the shaft as mechanical power output Po
Po = Pd – Wm
The mechanical power output Po, which is less than Pd is available in the shaft running at a speed of Nr and with a shaft torque T. Therefore the shaft torque (T) is slightly less than the torque developed Td,
Pd 2 NrT / 60
Wm = Pd – Po [2 Nr(Td – T)] / 60
80. Name the tests to be conducted for predetermining the performance of 3-phase induction machine.
(a) No load test
(b) Blocked rotor test
3 - 5%
72 Why is not possible for the rotor speed of an I M to be equal to the speed of its rotating magnetic field?
The machine will not be able to develop any mechanical torque to run as a motor.
73 State the condition at which the torque developed in a 3 phase induction motor is maximum.
When R2=SX2
74. What are the advantages of slip-ring I M over cage I M?
(i) Rotor circuit is accessible for external connection.
(ii) By adding external resistance to the rotor circuit the starting current is reduced with the added advantage of improving starting torque.
(iii) Additional speed control methods can be employed with the accessibility in the rotor circuit.
75. What are the losses occurring in an I M and on what factors do they depend?
Magnetic losses Wi Electrical losses Wcu Mechanical losses Wm
For I M operating in normal condition (with constant voltage and frequency) magnetic and mechanical losses remain constant whereas electrical losses vary in square proportion to the current.
76. What care should be taken at the time of construction to reduce eddy current losses in I M?
Make the resistance of the core body as large as possible.This is achieved by laminating the stator core,stacked and revetted at right angles to the path of eddy current.The laminations are insulated from each other by thin coat of varnish.
77. Why is there not appreciable magnetic losses in the rotor core of Induction motors?
Although the rotor core is also subjected to magnetic flux reversals and since the frequency of flux reversals in the rotor, fr = Sfs, is very small, the iron loss incurred in the rotor core is negligibly small.
78. What is meant by synchronous watt?
With the power input to the motor Pi, after the losses in the stator winding. Wcui and stator core, Wi, are met with, the remaining power is transferred to the rotor by the rotating magnetic field as power input to the rotor Pir
Pir = Pi –waui - Wi
The power input to the rotor Pir is transferred from the stator to the rotor by rotating magnetic field which rotates at synchronous speed Ns. Torque Td is developed in the rotor as
a result of Pir and the equation for Pir can alternatively be expressed as
Pir (2 Ns160). Td W Or Td = Pir Syn. W
79. How does the shaft torque differ from the torque developed in 3-phase Induction motor?
The mechanical power developed Pd causes the rotor to rotate at a speed Nr due to the torque Td developed in the rotor . Therefore, equation for Pr can be written as
Pd 2 NrTd 1 60
The remaining power, after the mechanical losses Wm are met with, available
in the shaft as mechanical power output Po
Po = Pd – Wm
The mechanical power output Po, which is less than Pd is available in the shaft running at a speed of Nr and with a shaft torque T. Therefore the shaft torque (T) is slightly less than the torque developed Td,
Pd 2 NrT / 60
Wm = Pd – Po [2 Nr(Td – T)] / 60
80. Name the tests to be conducted for predetermining the performance of 3-phase induction machine.
(a) No load test
(b) Blocked rotor test
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