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The starting torque of a D.C. motor is independent of which of the following?
Flux
Speed
Armature current
Flux and armature current
Speed
Quick Summary: The starting torque of a DC motor is directly proportional to the product of flux per pole and the armature current. Since at the instant of starting, the speed of the motor is zero, the starting torque is independent of speed.
The starting torque of a DC motor is directly proportional to the product of flux per pole and the armature current. Since at the instant of starting, the speed of the motor is zero, the starting torque is independent of speed.
TaтАЛ=KaтАЛ╬жIaтАЛ тАФ Basic torque equation for DC motors
EbтАЛ=VтИТIaтАЛRaтАЛ тАФ Back EMF equation which relates to speed
The electromagnetic torque developed in a DC motor is governed by the relation TaтАЛ=KaтАЛ╬жIaтАЛ. At the moment of starting (N=0), the back EMF (EbтАЛ=60AP╬жZNтАЛ) is zero. The torque depends solely on the interaction between the field flux and the armature current flowing through the conductors, regardless of the rotational speed.
Starting torque is maximal at the moment of start when N=0.
Back EMF (EbтАЛ) is directly proportional to speed, making torque calculation at start independent of EbтАЛ.
For shunt motors, high starting armature current IaтАЛ provides high starting torque.
High starting torque is desirable for traction applications.
Easier control of starting current using external resistors.
High starting current can damage commutator segments if not controlled.
Thermal stress on windings during direct-on-line (DOL) starting.
DC series motors in electric locomotives.
DC shunt motors in industrial drives requiring constant speed.
Option A is incorrect because Torque is proportional to Flux.
Option C is incorrect because Torque is proportional to Armature current.
Option D is incorrect as it identifies factors that the torque is actually dependent on.
B is correct тАФ Starting torque is determined at zero speed, hence it is independent of the rotational speed of the motor.
Remember that while torque is independent of speed at start, the armature current itself is limited by the external starter resistance until the speed picks up and generates back EMF.