The DC Winding Resistance Tester under Wuhan UHV can help many power workers conduct various power tests more conveniently.
I. Preparation
Power Off & Discharge:
Disconnect all power sources (armature, field windings, control circuits).
For large motors, discharge the armature: Short the positive and negative terminals to ground using an insulated wire for 10-15 seconds.
Tools:
Digital multimeter (low resistance range): used for rough measurement (limited accuracy).
Micro ohmmeter (or DC resistance tester): Recommended tool with an accuracy of 0.1 μ Ω~1 Ω, eliminating the influence of wire resistance.
Infrared thermometer: Record the temperature of the winding (resistance needs to be converted into temperature).
Insulated gloves and goggles.
II. Measurement Steps
1. Armature Winding (Rotor)
Lift the brush holders or remove the brushes to isolate the rotor circuit.
Measure using the micro-ohmmeter's 4-wire (Kelvin) method:
Single turn: Place probes on two adjacent commutator bars.
Parallel path: Place probes on commutator bars 180 degrees apart.
Overall: Measure between the positive and negative brush terminals (ensure good contact).
Key Action: Slowly rotate the rotor and take multiple measurements at different positions; calculate the average.
2. Field Winding (Stator)
Disconnect external wiring and measure directly across the winding terminals.
Note: Shunt/series field resistance is higher (tens of Ω to kΩ), separate excitation is lower (a few Ω to tens of Ω).
III. Key Considerations
Always use the 4-wire (Kelvin) method:
Connect separate current and voltage leads to eliminate lead resistance error (voltage contacts <1cm from the terminal).
Temperature Correction (Copper α=0.00393/°C):
Formula: R₂₀ = Rₜ / [1 + 0.00393 × (t - 20)] (t is measured winding temp in °C).
Eliminate Interference:
Clean terminals for good contact; pause measurement if humidity >80%; use the meter's current reversal function to cancel thermal EMF.
IV. Fault Diagnosis
Infinite Resistance (OL): Open winding or broken lead.
Significantly Low Resistance: Turn-to-turn short or insulation degradation.
Fluctuating Readings: Cold solder joints on the commutator or carbon dust buildup.
>±3% Difference between parallel branches: Asymmetrical winding or localized short.
V. Safety
Verify no residual voltage on high-voltage motors (use voltage tester); ground instruments to prevent ESD; follow IEC 60034-27 or GB/T 1029 standards.
Tip: Investigate abnormal resistance readings alongside insulation resistance tests (500V/1000V megger) and no-load current tests. For precision diagnostics, use a winding analyzer (e.g., Megger).