No-load speed of which of the following motor is highest?

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Examoogle · Accepted Answer

Option C: Series Motor

Explanation & Concepts:
The DC series motor exhibits the highest no-load speed because its field flux is directly proportional to the armature current, which approaches zero at no-load conditions. According to the speed equation $N \propto \frac{E_b}{\Phi}$, as the flux $\Phi$ becomes extremely small, the speed $N$ tends towards infinity, making it dangerous to start a series motor without load.

Working Principle: In a DC series motor, the armature and field windings are connected in series. At no-load, the current drawn is minimal, resulting in very low magnetic flux. Since $N \propto \frac{V - I_a(R_a + R_{se})}{\Phi}$ and $\Phi$ is nearly zero at no-load, the motor speed increases drastically to reach a steady-state equilibrium, which is physically destructive to the machine.

Key Formulas: $N = K_b \frac{V - I_a R_a}{\Phi}$ — Fundamental speed equation for a DC motor; $\Phi \propto I_a$ — Flux relation in a DC series motor

Key Points: A DC series motor should never be started without a mechanical load.; The torque produced is proportional to the square of the armature current ($T \propto I_a^2$).; Shunt motors have nearly constant speed characteristics due to the constant field flux.

Advantages: High starting torque; Robust construction

Disadvantages: Cannot run at no-load; Variable speed characteristics

Applications: Electric locomotives; Cranes and hoists

Additional Info: Differentially compound motors can also exhibit high speeds, but series motors have a theoretical speed limit defined only by mechanical friction and windage losses at no-load.. Option D (Shunt Motor) is considered a 'constant speed' motor.

Answer: C is correct — The DC series motor is prohibited from running at no-load because its speed tends to reach a dangerous, destructive value as the field flux approaches zero.

Exam Tip: Always remember that in DC motors, speed is inversely proportional to flux ($N \propto 1/\Phi$). Since $\Phi$ is low at light loads for series machines, $N$ must spike.

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Option A: Differentially compound motor

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Option B: Cumulative compound motor

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Option D: Shunt Motor

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