The speed-torque characteristics of which motor whose speed falls when there is increase in load torque is sui

Question

The speed-torque characteristics of which motor whose speed falls when there is increase in load torque is suitable to the application?

Examoogle · Accepted Answer

The correct answer is Option B: DC series motor. A DC series motor is characterized by a speed that is inversely proportional to the load current. As the load torque increases, the armature current increases, resulting in a significant drop in speed to maintain the power balance, making it ideal for high-starting torque applications.

Step-by-Step Solution & Concepts:
A DC series motor is characterized by a speed that is inversely proportional to the load current. As the load torque increases, the armature current increases, resulting in a significant drop in speed to maintain the power balance, making it ideal for high-starting torque applications.

Working Principle: In a DC series motor, the armature and field windings are in series. Therefore, the flux $\Phi$ is directly proportional to the armature current $I_a$. Since speed $N \propto \frac{E_b}{\Phi}$ and $E_b \approx V - I_a R_a$, an increase in load torque increases $I_a$, which increases $\Phi$. Because $N$ is inversely proportional to $\Phi$, the motor speed drops sharply.

Key Formulas: $N \propto \frac{V - I_a(R_a + R_{se})}{\Phi}$ — relationship between speed, voltage, and flux; $\Phi \propto I_a$ — magnetic flux dependency in series configuration

Key Points: DC series motors exhibit a variable speed characteristic that is heavily dependent on load.; High starting torque makes them suitable for heavy mechanical loads like traction.; They should never be started at no-load as the speed would reach dangerous levels.

Advantages: High starting torque; Robust construction for industrial heavy-duty use

Disadvantages: Speed cannot be maintained constant with fluctuating loads; Risk of mechanical runaway at no-load condition

Applications: Electric locomotives (Traction); Cranes and hoists; Elevators

Additional Info: DC shunt motors (Option D) are essentially constant speed motors and are unsuitable for applications requiring speed reduction under high load.. DC differential compound motors are rarely used because their speed can increase with load, leading to instability.

Answer: B is correct — the DC series motor's speed drops significantly as the load torque increases due to the direct dependence of flux on armature current.

Exam Tip: Always remember the 'Universal' rule: Series for high starting torque, Shunt for constant speed, and Cumulative Compound for varying loads that require a balance between both.

Examoogle · Answer

Option A: DC differential compound motor

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Option C: DC compound motor

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Option D: DC shunt motor

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