Question:
The only disadvantage of field control method for controlling the speed of a DC shunt motor is that it _______.
The only disadvantage of field control method for controlling the speed of a DC shunt motor is that it _______.
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Always remember: weakening field → instability and arcing → poor commutation.
Updated On: Jun 24, 2025
- gives speeds lower than the normal speed
- is wasteful
- needs a large rheostat
- adversely affects commutation
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The Correct Option is D
Solution and Explanation
Step 1: Field control method basics.
Speed $\propto \frac{V - IaR_a}{\phi}$
In field control, $\phi$ is decreased to increase speed.
Step 2: Side effects of reduced field.
- Weakening field reduces stability
- Causes poor commutation
- Arcing at brushes due to improper neutral plane
Step 3: Why others are incorrect?
- Method allows higher speed, not lower
- It’s not wasteful
- Does not use large rheostat (that’s in armature control)
Conclusion: $\boxed{\text{Adversely affects commutation}}$
Speed $\propto \frac{V - IaR_a}{\phi}$
In field control, $\phi$ is decreased to increase speed.
Step 2: Side effects of reduced field.
- Weakening field reduces stability
- Causes poor commutation
- Arcing at brushes due to improper neutral plane
Step 3: Why others are incorrect?
- Method allows higher speed, not lower
- It’s not wasteful
- Does not use large rheostat (that’s in armature control)
Conclusion: $\boxed{\text{Adversely affects commutation}}$
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