What is the armature-circuit-resistance speed control method for DC motor?

Question

Examoogle · Accepted Answer

The correct answer is Option C: Variation of resistance in the armature circuit. The armature-circuit-resistance control method regulates the speed of a DC motor by inserting an external variable resistance in series with the armature circuit. This modification directly affects the voltage drop across the armature, thereby altering the back EMF and the motor speed.

Step-by-Step Solution & Concepts:
The armature-circuit-resistance control method regulates the speed of a DC motor by inserting an external variable resistance in series with the armature circuit. This modification directly affects the voltage drop across the armature, thereby altering the back EMF and the motor speed.

Working Principle: The speed of a DC motor is given by $N = \frac{V - I_a R_a}{k_a \Phi}$. By adding an external resistance $R_{ext}$ in the armature circuit, the effective resistance becomes $R_{total} = R_a + R_{ext}$. Since the terminal voltage $V$ and flux $\Phi$ are held constant, the increase in voltage drop $I_a (R_a + R_{ext})$ reduces the back EMF ($E_b$), resulting in a decrease in speed $N$.

Key Formulas: $N = K \frac{V - I_a(R_a + R_{ext})}{\Phi}$ — Speed equation with external armature resistance; $E_b = V - I_a(R_a + R_{ext})$ — Back EMF equation with external resistance

Key Points: This method is used to obtain speeds below the rated speed.; It is a constant torque, variable power method of speed control.; A large amount of power is wasted as $I_a^2 R_{ext}$ loss in the external resistor.; The speed regulation becomes poor as the resistance is increased.

Advantages: Simple control circuitry; Easy to implement for speed reduction below base speed

Disadvantages: Large power loss in external resistance; Efficiency decreases significantly at lower speeds; Poor speed regulation under varying load conditions

Applications: Used in DC series motors for crane controls; Used in applications where speed reduction is required for short durations

Additional Info: Option A (Variation of armature terminal voltage) is known as Ward-Leonard control.. Option B (Variation of resistance in the field circuit) is known as Flux control method for speeds above rated value.. Option D (Variation of field flux) is also known as Field weakening method.

Answer: C is correct — Variation of resistance in the armature circuit reduces the back EMF, which lowers the speed below the rated value.

Exam Tip: Always remember that armature resistance control is for speeds BELOW rated speed, whereas flux control is for speeds ABOVE rated speed.

Examoogle · Answer

Option A: Variation of armature terminal voltage

Examoogle · Answer

Option B: Variation of resistance in the field circuit

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Option D: Variation of field flux

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