Notes on Repulsion-Type Motors

Repulsion-Type Motors

Overview

  • Repulsion-type motors are early forms of single-phase induction motors, used substantially from the 1930s to the 1950s.
    • Today, they are being replaced by more efficient split-phase and capacitor-start motors.
    • Still produced by select manufacturers and remain operational in various applications.

Efficiency and Starting Current

  • Compared to split-phase and capacitor-start motors, repulsion-type motors exhibit:
    • More efficient starting under load.
    • Lower inrush or starting current, which is beneficial under low-voltage conditions.

Key Types of Repulsion Motors

  1. Repulsion Motor:

    • Features a stator with windings connected to a power source and a rotor with winding connected to a commutator.
    • Brushes on the commutator are short-circuited and positioned to align with the magnetic axis of the rotor winding, allowing variable-speed operation.

  2. Repulsion-Start Induction Motor:

    • Similar winding structure as a repulsion motor, beginning operation as a repulsion motor before transitioning to induction motor characteristics at a predetermined speed.
    • Starts under high torque, making it suitable for applications like commercial refrigerators and pumps.

  3. Repulsion-Induction Motor:

    • Combines features of both the repulsion motor and an induction motor, featuring a squirrel-cage winding in addition to the primary winding.
    • Capable of constant-speed or varying-speed characteristics.

Construction Elements

  • Common components across all types include:
    1. Stator: Similar structure to split-phase motors, usually a two-section winding.
    2. Rotor: Slotted core housing a winding connected to a commutator; analogous to a DC motor's armature.
    • Skewed slots help maintain consistent starting torque and reduce magnetic hum.
    1. Commutator: Can be axial or radial, with specific designs affecting the motor's operation.
    2. End Plates/Brackets: Support for rotor shaft bearings.
    3. Brushes: Carbon brushes conduct current through the armature winding.

Types and Operations

Repulsion-Start Induction Motor
  • Operates in two designs:
    1. Brush-Lifting Type: Brushes disengage from the commutator at approximately 75% of full speed, utilizing a centrifugal lifting mechanism.
    2. Brush-Riding Type: Brushes remain in contact with the commutator throughout motor operation.
Operation Mechanism

  • For Brush-Lifting Type:

    • At start, stator winding induces current in the rotor winding generating a repulsion torque.
    • When the centrifugal switch activates at 75% speed, brushes are lifted, allowing the motor to behave like a squirrel-cage rotor.

  • Centrifugal Mechanism: Key components include governor weights, spring mechanisms, and brush holders that facilitate the transfer from repulsion to induction operation.

Stator Winding and Dual Voltage Operation

  • Stators are typically arranged for dual-voltage capability with appropriate wiring configurations that accommodate both high and low voltage demands.

Troubleshooting and Maintenance

Common Issues
  1. Failure to Start: Can result from open circuits in either stator or rotor, worn bearings, or poor connection settings.
  2. Excessive Heat: May indicate overload conditions, poor winding insulation, or worn components causing increased current draw.
  3. Brush Problems: Sticking or worn brushes can hinder operation, necessitating inspection and replacement as needed.
  4. Shifting Configuration: Incorrectly set brush holders can lead to improper torque and rotation issues.

Repairs

  • Regular maintenance procedures include checking winding integrity, verifying com-mutation connections, and ensuring proper brush tension and contact.
  • When rewiring, adhere to voltage and inductive changes while maintaining original construction standards and layouts.