Condition for Maximum Starting Torque
Condition for Maximum Starting Torque:
It can be proved that starting torque will be maximum when rotor resistance/phase is equal to standstill rotor reactance/phase.
Differentiating the above equation w.r.t. R2 and equating the result to zero, we get,
Hence starting torque will be maximum when:
Rotor resistance/phase = Standstill rotor reactance/phase
Under the condition of maximum starting torque, Φ2 = 45° and rotor power factor is 0.707 lagging [See Fig. (1 (ii))]. Fig. (1(i)) shows the variation of starting torque with rotor resistance. As the rotor resistance is increased from a relatively low value, the starting torque increases until it becomes maximum when R2 = X2. If the rotor resistance is increased beyond this optimum value, the starting torque will decrease.
Effect of Change of Supply Voltage:
Therefore, the starting torque is very sensitive to changes in the value of supply voltage. For example, a drop of 10% in supply voltage will decrease the starting torque by about 20%. This could mean the motor failing to start if it cannot produce a torque greater than the load torque plus friction torque.