The demand for a large increase in torque of a d.c. shunt motor is met by

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Large increase in current

Explanation: In a d.c. shunt motor, the torque is directly proportional to the product of flux per pole and the armature current. Since a shunt motor is essentially a constant flux machine (provided the supply voltage remains constant), any significant increase in torque must be achieved by a corresponding increase in the armature current.

Working Principle: The torque equation is $T = K_a \Phi I_a$. In a shunt motor, the field winding is connected in parallel with the armature, maintaining a constant flux $\Phi$ as long as the terminal voltage is steady. Consequently, when the mechanical load increases, the motor slows down slightly, reducing back EMF $E_b$, which allows more current $I_a$ to flow to generate the required increase in electromagnetic torque.

Key Formulas: $T_a = \frac{1}{2\pi} \cdot P \cdot \frac{Z}{A} \cdot \Phi \cdot I_a$ — Electromagnetic torque equation; $E_b = V - I_a R_a = \frac{P \Phi Z N}{60 A}$ — Back EMF and speed relationship

Key Points: DC shunt motors are known as constant-speed motors.; The torque-armature current characteristic is linear in a shunt motor.; A large increase in torque is strictly dependent on an increase in armature current because the flux is kept constant by the parallel field connection.

Advantages: Constant speed operation under varying loads.; Simple control circuit.

Disadvantages: Low starting torque compared to series motors.; Torque is limited by armature current rating.

Applications: Lathe machines; Centrifugal pumps; Fans and blowers

Additional Info: Option A is incorrect because speed only drops slightly, not largely, to maintain torque equilibrium.. Option B is incorrect because speed decreases, not increases, when load torque increases.. Option D is incorrect because a large torque increase requires a proportionally large increase in armature current, not a small one.

Answer: C is correct — Since the magnetic flux in a d.c. shunt motor is constant, the motor increases its torque by drawing more armature current when the mechanical load demand increases.

Exam Tip: Always remember that in a DC Shunt Motor, the flux is roughly constant, making current the primary control variable for torque. In contrast, for a DC Series Motor, flux increases with current, leading to a torque proportional to $I_a^2$.

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