Static Testing of DC Machines

Key Testing Methods for DC Motors

1. Field Frame Tests:

   • Winding resistance, inductance, impedance.
   • Insulation-to-earth tests (e.g., Megger tests).
   • Surge tests for turn-to-turn insulation weakness.

2. Armature Tests:

   • Includes winding resistance, commutator checks, and equalizer coil tests.
   • Specialised tests: span testing and bar-to-bar surge testing.

Modern Testing Equipment and Techniques

• Bar-to-Bar Testing:

  • Detects faults like short circuits, weak insulation, and open circuits.
  • Low-resistance measurements between adjacent bars using Kelvin methods.
  • Modern devices produce reports and bar charts, eliminating manual error.

• Surge Testing:

  • Detects insulation weaknesses between turns using potential differences.
  • Relies on Parian’s Law, which requires a voltage differential between conductors to identify faults.
  • Demonstrated with two examples:
    • No potential difference: insulation withstood 15kV without breakdown.
    • With potential difference: insulation breakdown occurred below 8kV.

Advanced Techniques for Armatures

1. Span Testing:
   • Compares groups of bars.
   • Effective for smaller armatures but less sensitive for large, low-impedance armatures.
2. Bar-to-Bar Testing:
   • Required for large armatures to create necessary potential differences.
   • High-current surge testers ensure precision and reliability.
   • Recorded data allows easy comparison and traceability for customers.

Modern Test Equipment Features

• Automated recording and reporting for speed and accuracy.
• Adjustable tolerances to flag deviations.
• High-current surge testers for large armatures.
• Traceable results enhance customer trust and ensure compliance.