Induction Motor Torque
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Induction Motor Torque (induction motor torque formula)
Torque of a three phase induction motor is proportional to rotor current, flux per stator pole and the power factor of the rotor.
T ∝ ɸ I2 cosɸ2 OR T = k ɸ I2 cosɸ2 .
where, ɸ = flux per stator pole,
I2 = rotor current at standstill,
ɸ2 = angle between rotor emf and rotor current,
k = a constant.
Now, let E2 = rotor emf at standstill
we know, rotor emf is directly proportional to flux per stator pole, i.e. E2 ∝ ɸ.
therefore, T ∝ E2 I2 cosɸ2 OR T =k1 E2 I2 cosɸ2.
Starting Torque of Induction Motor
Starting torque refers to the torque created when a motor first turns on. In some circumstances, starting torque may be higher or lower than running torque.
We know, T =k1 E2 I2 cosɸ2.
let, R2 = rotor resistance per phase
X2 = standstill rotor reactance
Therefore, starting torque can be given as,
The constant k1 = 3 / 2πNs
Maximum Starting Torque of Induction Motor
Flux ɸ and E2 remain constant if supply voltage V is kept constant. Hence,
Therefore, it can be demonstrated that the maximum starting torque occurs when the rotor resistance equals the rotor reactance at rest. i.e. R22 + X22 =2R22 .
Torque Under Running Condition
T ∝ ɸ Ir cosɸ2 .
where, Er = rotor emf per phase under running condition = sE2. (s=slip)
Ir = rotor current per phase under running condition
reactance per phase under running condition will be = sX2
as, ɸ ∝ E2.
Maximum Torque Under Running Condition
The value of slip (s), where torque is maximum under running conditions, causes rotor reactance per phase to be equal to rotor resistance per phase.