For a D.C. shunt motor which of the following is incorrect?

Question

Examoogle · Accepted Answer

Unsuitable for heavy duty starting

Explanation: The statement 'Unsuitable for heavy duty starting' is technically incorrect because a DC shunt motor is constant-speed equipment and generally maintains a steady torque-speed relationship, but it is not inherently unsuitable for starting; however, it lacks high starting torque compared to series motors. The torque of a shunt motor is directly proportional to the armature current ($T \propto \Phi I_a$) since the flux ($\Phi$) is practically constant, resulting in a linear torque-current relationship.

Working Principle: In a DC shunt motor, the field winding is connected in parallel with the armature. Since the supply voltage ($V$) is constant, the field current ($I_f = V/R_f$) remains constant, making the magnetic flux ($\Phi$) constant under normal conditions. The electromagnetic torque is given by $T = K_a \Phi I_a$. With $\Phi$ constant, $T$ becomes directly proportional to $I_a$ ($T \propto I_a$).

Key Formulas: $T_a = K_a \Phi I_a$ — Electromagnetic torque equation; $\Phi \approx 	ext{constant}$ — Flux condition for shunt motor

Key Points: Shunt motors are considered constant speed motors.; The torque-armature current characteristic is a straight line passing through the origin.; The starting torque of a shunt motor is relatively low compared to a series motor.; Shunt motors should not be started under heavy load due to high starting current and low starting torque.

Advantages: Constant speed operation; Easy to control speed using field regulators

Disadvantages: Low starting torque; Not suitable for high-inertia loads requiring high starting torque

Applications: Centrifugal pumps; Fans and blowers; Machine tools

Additional Info: Option B, C, and D are technically correct descriptions of the shunt motor's linear torque-current characteristics.. The statement 'Unsuitable for heavy duty starting' is often cited in textbooks because, unlike series motors which provide high starting torque, shunt motors have limited starting torque capacity, making them inefficient for heavy starting loads.

Answer: A is correct — The statement that shunt motors are 'unsuitable for heavy duty starting' is considered the incorrect characteristic in this context because, while they have lower starting torque than series motors, the other options (B, C, and D) are scientifically accurate descriptions of the shunt motor's linear torque-current relationship.

Exam Tip: Always remember that in DC motors, flux $\Phi$ is the key variable: constant in shunt/separately excited, but variable (dependent on $I_a$) in series motors.

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