The starting torque of a D.C. motor is independent of which of the following?

Question

Examoogle · Accepted Answer

Speed

Explanation: 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.

Working Principle: The electromagnetic torque developed in a DC motor is governed by the relation $T_a = K_a \Phi I_a$. At the moment of starting ($N=0$), the back EMF ($E_b = \frac{P \Phi Z N}{60 A}$) 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.

Key Formulas: $T_a = K_a \Phi I_a$ — Basic torque equation for DC motors; $E_b = V - I_a R_a$ — Back EMF equation which relates to speed

Key Points: Starting torque is maximal at the moment of start when $N=0$.; Back EMF ($E_b$) is directly proportional to speed, making torque calculation at start independent of $E_b$.; For shunt motors, high starting armature current $I_a$ provides high starting torque.

Advantages: High starting torque is desirable for traction applications.; Easier control of starting current using external resistors.

Disadvantages: High starting current can damage commutator segments if not controlled.; Thermal stress on windings during direct-on-line (DOL) starting.

Applications: DC series motors in electric locomotives.; DC shunt motors in industrial drives requiring constant speed.

Additional Info: 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.

Answer: B is correct — Starting torque is determined at zero speed, hence it is independent of the rotational speed of the motor.

Exam Tip: 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.

Related Questions

Discussion (0)