Which of the following motors are preferred when smooth and precise speed control over a wide range

Question

Which of the following motors are preferred when smooth and precise speed control over a wide range is desired?

Examoogle · Accepted Answer

D.C. motor

Explanation: DC motors are preferred for applications requiring smooth and precise speed control over a wide range because their speed is directly and linearly related to the applied voltage and field flux. Unlike AC motors, which depend on frequency or rotor resistance for speed control, DC motors allow for independent control of armature voltage and field current, enabling fine adjustments across a continuous range.

Working Principle: The speed of a DC motor is governed by the equation $N = k \frac{V - I_a R_a}{\Phi}$. By varying the armature voltage ($V$) or the field flux ($\Phi$), the back EMF and consequently the rotor speed can be controlled smoothly. Armature voltage control (Ward-Leonard system or Chopper control) allows speed control from zero up to the rated speed, while field flux control enables operation above the base speed (field weakening region).

Key Formulas: $N = \frac{V - I_a R_a}{k \Phi}$ — Relationship between speed (N), Armature Voltage (V), and Flux ($\Phi$); $T_e = k \Phi I_a$ — Electromagnetic torque proportional to flux and armature current

Key Points: Armature voltage control provides constant torque drive for speeds below base speed.; Field flux control (field weakening) provides constant power drive for speeds above base speed.; DC motors offer high starting torque and high overload capacity.; Precise speed control is achievable using power electronics like DC-DC choppers.

Advantages: Wide range of speed control; High starting torque; Excellent transient response; Smooth and linear speed adjustment

Disadvantages: Requires maintenance due to commutator and brushes; High initial cost compared to induction motors; Risk of sparking at brushes in hazardous environments

Applications: Electric traction and locomotives; Rolling mills and paper mills; Robotics and servo systems; Lifts and cranes

Additional Info: Squirrel cage induction motors have limited speed control options, usually requiring expensive Variable Frequency Drives (VFDs) for wide-range control.. Wound rotor induction motors allow speed control via external rotor resistance, but this results in significant power loss and poor efficiency at lower speeds.. Synchronous motors are constant speed machines (N = 120f/P) and are typically not used for variable speed applications.

Answer: A is correct — DC motors allow for independent and continuous control of speed by varying the armature voltage or magnetic field flux, which is not easily achieved in AC motors.

Exam Tip: For competitive exams, remember: DC Motors = Armature/Field control; Induction Motors = Frequency/Slip control; Synchronous Motors = Constant speed (utility as synchronous condensers).

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