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Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Back to Practice Questions
ElectricalElectric Drives
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______________  has a relatively wider range of speed control.

A

Synchronous motor

B

shunt motor

C

Ship ring induction motor

D

Squirrel cage induction motor

Correct Answer

Concept & PrincipleElectricalElectric Drives
Option B

shunt motor

Quick Summary: A D.C. shunt motor allows for a wide and smooth range of speed control by adjusting the field flux or armature voltage. This makes it superior to induction or synchronous motors, which generally operate at fixed speeds governed by supply frequency and pole pairs.

💡 Explanation

A D.C. shunt motor allows for a wide and smooth range of speed control by adjusting the field flux or armature voltage. This makes it superior to induction or synchronous motors, which generally operate at fixed speeds governed by supply frequency and pole pairs.

🔢 Key Formulas

N=KV−IaRaΦN = K \frac{V - I_a R_a}{\Phi}N=KΦV−Ia​Ra​​ — Fundamental speed equation for a DC motor

Pmech=EbIaP_{mech} = E_b I_aPmech​=Eb​Ia​ — Mechanical power developed

⚙️ Working Principle

The speed of a DC motor is governed by the relation N∝EbΦN \propto \frac{E_b}{\Phi}N∝ΦEb​​. By controlling the shunt field current using a rheostat, the flux Φ\PhiΦ can be varied, resulting in speed variations above the rated value (field control). Alternatively, varying the applied armature voltage VVV allows for speeds below the rated value (armature control), providing a versatile operational envelope.

📌 Key Points
  • ▸

    Speed control in DC shunt motors is achieved without the need for complex power electronics (though they are now commonly used).

  • ▸

    Field control provides constant power drive characteristics.

  • ▸

    Armature control provides constant torque drive characteristics.

  • ▸

    Synchronous motors run at a constant speed defined by Ns=120fPN_s = \frac{120f}{P}Ns​=P120f​.

✅ Advantages
  • ▸

    Wide range of speed control

  • ▸

    Smooth and precise speed regulation

  • ▸

    Simple control circuit

❌ Disadvantages / Limitations
  • ▸

    Requires DC supply or rectifier

  • ▸

    Maintenance due to brushes and commutator

  • ▸

    Lower efficiency compared to brushless AC motors

🛠️ Applications / Uses
  • ▸

    Machine tools

  • ▸

    Lathes

  • ▸

    Conveyor belts

  • ▸

    Fans and blowers

🔄 Comparison Table
FeatureDC Shunt MotorSynchronous Motor

Speed Control

Wide and smooth (Flux/Voltage control)

Fixed speed (Synchronous)

📄 Additional Information
  • ▸

    The Squirrel cage induction motor is primarily a constant speed motor; speed variation requires Variable Frequency Drives (VFDs).

  • ▸

    Slip ring induction motors allow speed control via rotor resistance, but it is less efficient due to heat loss in resistors.

📊 Diagram / Illustration
DC Shunt Motor Speed Formula
N=K⋅V−IaRaΦN = K \cdot (V - I_a R_a / \Phi)N=K⋅ΦV−Ia​Ra​​
NNN: Speed, VVV: Terminal Voltage
Φ\PhiΦ: Flux, RaR_aRa​: Armature Resistance
✅

B is correct — The D.C. shunt motor provides a relatively wider range of speed control through armature voltage and field flux variation.

Core Concepts Used
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Electromagnetic induction Back EMF Speed-Torque characteristics
💡 EXAM TIP

Always remember that in DC motors, field weakening (Φ↓\Phi \downarrowΦ↓) increases speed, whereas in AC induction motors, changing frequency (fff) is the primary method for speed control.

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