Induction motor:Condition for Maximum Torque Under Running Conditions.

Condition for Maximum Torque Under Running Conditions

The torque of a rotor under running conditions is

The condition for maximum torque may be obtained by differentiating the above expression with respect to slip s and then putting it equal to zero. However, it is simpler to put Y = 1/T and then differentiate it.

Hence, torque under running condition is maximum at that value of the slip s which makes rotor reactance per phase equal to rotor resistance per phase. This slip is sometimes written as sb and the maximum torque as Tb.

Slip corresponding to maximum torque is s = R2/X2

Putting R2 = sX2 in the above equation for the torque, we get

From the above, it is found

1. that the maximum torque is independent of rotor resistance as such.

2. however, the speed or slip at which maximum torque occurs is determined by the rotor resistance. As seen from above, torque becomes maximum when rotor reactance equals its resistance. Hence, by varying rotor resistance (possible only with slip-ring motors) maxi- mum torque can be made to occur at any desired slip (or motor speed).

3. maximum torque varies inversely as standstill reactance. Hence, it should be kept as small as possible.

4. maximum torque varies directly as the square of the applied voltage.

5. for obtaining maximum torque at starting (s =1), rotor resistance must be equal to rotor reactance.

Example 34.11. A 3-phase, slip-ring, induction motor with star-connected rotor has an induced e.m.f. of 120 volts between slip-rings at standstill with normal voltage applied to the stator. The rotor winding has a resistance per phase of 0.3 ohm and standstill leakage reactance per phase of 1.5 ohm.

Calculate (i) rotor current/phase when running short-circuited with 4 percent slip and (ii) the slip and rotor current per phase when the rotor is developing maximum torque.

(Elect. Engg.-II, Pune Univ. 1989)

Incoming search terms:

• the condition for maximum running torque is
• condition for maximum running torque
• condition for maximum torque under running condition
• condition for maximum torque in induction motor
• the condition for maximum running torque
• The condition for maximum running torque is?
• the condition for maximum running torque is:
• when maximum torque is obtained the rotor power is-
• Maximum running torque
• condition for maximum running torque is
• when maximum torque is obtained the rotor power is?
• torque equation of induction motor
• when maximum torque is obtained the rotor power is ?
• starting torque of induction motor
• when maximum torque is obtained the rotor power
• condition for maximum torque
• torque equation of 3 phase induction motor
• the condition for maximum runing torque is
• torque under running condition
• what is the condition for maximum torque in induction motor
• what is the condition for maximum running torque
• when the maximum torque is obtained the rotor power is
• the comdition for maximum running torque
• the condition for maximum running torque is ?
• derive maximum running torque expression and its condition of induction motor by electrical for u
• maximum running torque is
• torque equation of three phase induction motor
• the condition of maximum running torque is
• condition for maximum running torque is ?
• the condition for maximum running torqe is
• why resistance of the rotor of an induction motor varies widely between starting and running conditions
• condition for maximum torque of induction motor
• the condition formaximumruning torque is
• maximum torque under running condition
• www the condition for maximum running torque is
• at what slip the torque developed in an induction motor will be maximum
• conditions for maximum torque for 3phase induction motor
• Torque equation of 3phase induction motor
• the condition for maximum running torque is :
• running torque of induction motor
• condition of maximum running torque
• when maximum torque is obtained the rotor power is what
• condition for maximum torque of 3 phase induction motor
• maximum starting torque under running condition
• the conditions for maximum running torque is
• state the condition at which the starting torque developed in a slip ring induction motor is maximum?
• What are the condition for maximum torque during starting and running
• torque condition under running condition
• condition and expression for maximum starting torque
• max torque at starting conditions