Obtain an expression for the condition of maximum torque of an induction motor. Sketch the torque- slip curves for several values of rotor circuit resistance and indicate the condition for maximum torque to be obtained at starting. If the motor has a rotor resistance of 0.02 2 and a standstill reactance of 0.1 92, what must be the value of the total resistance of a starter for the rotor circuit for maximum torque to be exerted at starting ? [0.08 $2] (City and Guilds, London) 1000 Ō

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h has
rotor e.m.f.
Speed of the induction motor and the frequency of its
[970 r.p.m.; 1.5 Hz]
4.
A 3-phase, 50-Hz induction motor with its rotor star-connected gives 500 V (r.m.s.) at standstill
between the slip-rings on open-circuit. Calculate the current and power factor at standstill when the
rotor winding is joined to a star-connected external circuit, each phase of which has a resistance of
10 2 and an inductance of 0.04 H. The resistance per phase of the rotor winding is 0.2 2 and its
inductance is 0.04 H.
Also, calculate the current and power factor when the slip-rings are short-circuited and the motor is
running with a slip of 5 per cent. Assume the flux to remain constant.
[10.67 A; 0.376; 21.95 A; 0.303]
5.
Obtain an expression for the condition of maximum torque of an induction motor. Sketch the torque-
slip curves for several values of rotor circuit resistance and indicate the condition for maximum
torque to be obtained at starting.
If the motor has a rotor resistance of 0.02 Q2 and a standstill reactance of 0.1 2, what must be the
value of the total resistance of a starter for the rotor circuit for maximum torque to be exerted at
starting ?
[0.08 $2] (City and Guilds, London)
6.
The rotor of a 6-pole, 50-Hz induction motor is rotated by some means at 1000 r.p.m. Compute
(i) rotor voltage (ii) rotor frequency (iii) rotor slip and (iv) torque developed. Can the rotor rotate at
this speed by itself ?
[(i) 0 (ii) 0 (iii) 0 (iv) 0; No] (Elect. Engg. Grad I.E.T.E. June 1985)
7.
The rotor resistances per phase of a 4-pole, 50-Hz, 3-phase induction motor are 0.024 ohm and 0.12
ohm respectively. Find the speed at maximum torque. Also find the value of the additional rotor
resistance per phase required to develop 80% of maximum torque at starting.
Ps W
0
Transcribed Image Text:h has rotor e.m.f. Speed of the induction motor and the frequency of its [970 r.p.m.; 1.5 Hz] 4. A 3-phase, 50-Hz induction motor with its rotor star-connected gives 500 V (r.m.s.) at standstill between the slip-rings on open-circuit. Calculate the current and power factor at standstill when the rotor winding is joined to a star-connected external circuit, each phase of which has a resistance of 10 2 and an inductance of 0.04 H. The resistance per phase of the rotor winding is 0.2 2 and its inductance is 0.04 H. Also, calculate the current and power factor when the slip-rings are short-circuited and the motor is running with a slip of 5 per cent. Assume the flux to remain constant. [10.67 A; 0.376; 21.95 A; 0.303] 5. Obtain an expression for the condition of maximum torque of an induction motor. Sketch the torque- slip curves for several values of rotor circuit resistance and indicate the condition for maximum torque to be obtained at starting. If the motor has a rotor resistance of 0.02 Q2 and a standstill reactance of 0.1 2, what must be the value of the total resistance of a starter for the rotor circuit for maximum torque to be exerted at starting ? [0.08 $2] (City and Guilds, London) 6. The rotor of a 6-pole, 50-Hz induction motor is rotated by some means at 1000 r.p.m. Compute (i) rotor voltage (ii) rotor frequency (iii) rotor slip and (iv) torque developed. Can the rotor rotate at this speed by itself ? [(i) 0 (ii) 0 (iii) 0 (iv) 0; No] (Elect. Engg. Grad I.E.T.E. June 1985) 7. The rotor resistances per phase of a 4-pole, 50-Hz, 3-phase induction motor are 0.024 ohm and 0.12 ohm respectively. Find the speed at maximum torque. Also find the value of the additional rotor resistance per phase required to develop 80% of maximum torque at starting. Ps W 0
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