Unit 5.0 Special Electrical Machines

 

Unit 5.0 Special Electrical Machines

5.1 Construction, Working, Speed–Torque Characteristics, and Applications

(i) AC Servo Motor



  • Construction: Two‑phase motor with high resistance rotor; designed for precise control.

  • Working: Operates on error‑signal amplification principle; responds quickly to control signals.

  • Speed–Torque Characteristics: Linear torque vs. control voltage; high torque at low speeds.

  • Applications: Robotics, CNC machines, aerospace control systems.

(ii) Stepper Motor



  • Construction: Stator with multiple windings; toothed rotor.

  • Working: Rotates in discrete steps when windings are energized sequentially.

  • Speed–Torque Characteristics: Torque decreases with speed; excellent for precise positioning without feedback.

  • Applications: Printers, CNC machines, 3D printers, disk drives.

(iii) Linear Induction Motor (LIM)



  • Construction: Stator and rotor laid flat to produce linear motion.

  • Working: Generates thrust instead of torque by creating a traveling magnetic field.

  • Speed–Torque Characteristics: Thrust decreases with slip; efficiency lower than rotary motors.

  • Applications: Rail traction, conveyors, amusement rides, magnetic levitation systems.

(iv) Reluctance Motor



  • Construction: Stator similar to induction motor; rotor has salient poles without windings.

  • Working: Rotor aligns with stator field due to minimum reluctance principle.

  • Speed–Torque Characteristics: Poor starting torque; runs at synchronous speed once started.

  • Applications: Electric clocks, signaling devices, synchronous applications requiring constant speed.

(v) Hysteresis Motor



  • Construction: Stator similar to induction motor; rotor made of hard steel with high hysteresis loss.

  • Working: Torque produced by hysteresis and eddy current effects.

  • Speed–Torque Characteristics: Smooth torque, constant speed, noiseless operation.

  • Applications: Sound recording instruments, timing devices, precision equipment.

(vi) Universal Motor



  • Construction: Similar to DC series motor; can operate on AC or DC supply.

  • Working: Series field winding and armature winding connected in series; commutator used.

  • Speed–Torque Characteristics: High starting torque; speed decreases with load.

  • Applications: Portable tools, household appliances (mixers, vacuum cleaners, hair dryers).

5.2 Criteria for Selection of Special Motor

  • Nature of Load: Continuous, intermittent, or variable.

  • Required Speed and Torque: High starting torque, constant speed, or variable speed.

  • Control Requirements: Precision positioning, feedback control, or open‑loop operation.

  • Supply Availability: AC, DC, or both.

  • Environment: Noise level, maintenance needs, operating conditions.

  • Cost and Efficiency: Balance between performance and economics.

  • Application Specific Needs:

    • Robotics → Servo/Stepper

    • Traction → LIM

    • Household appliances → Universal motor

    • Precision instruments → Hysteresis motor

✅ Key Takeaway

Special electrical machines are chosen based on application‑specific requirements such as precision, torque, speed, and supply type. Each motor has unique characteristics making it suitable for particular industrial, commercial, or domestic uses.

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