Standards and Necessity for DC Resistance Testing of Three-Phase Asynchronous Motors

The DC resistance test of a three-phase asynchronous motor is a fundamental step in evaluating its electrical performance and safety. The following section provides a systematic explanation from three aspects: standards and specifications, the necessity of the test, and implementation methods.

I. Standard Requirements for DC Resistance Testing

1. Limits on three-phase resistance imbalance

General standard ( GB 501502006 ): requires that the deviation of the resistance of any one phase from the average value of the three phases shall not exceed ± 5% (i.e., the unbalance ≤ 5% ).

High-voltage / high-capacity motors: Motors with rated voltage ≥ 3 kV or power ≥ 100 kW , phase-to-phase deviation ≤ 2% (line resistance deviation ≤ 1% when neutral point is not brought out ).

Special scenarios: For explosion-proof motors in coal mines (such as the YBK3 series), the deviation is required to be ≤ 2% , while for low-voltage, small-capacity motors, the requirement can be appropriately relaxed, but historical data consistency must be maintained.

2. Measurement Condition Specifications

Cold test: Conduct the test after the motor has stopped and the temperature has dropped to ambient temperature ( 20 ℃± 5 ℃ recommended) to avoid the temperature rise affecting the resistance value.

Instrument accuracy: A double-arm bridge or a high-precision DC resistance tester (resolution above 0.1% ) is required. For large-capacity motors, the four-wire method should be used to eliminate contact resistance errors.

3. Temperature correction requirements

If the ambient temperature deviates from 20 ℃, the resistance value needs to be corrected.

II. The Necessity and Core Role of Testing

1. Fault Diagnosis and Prevention

Inter-turn short circuit detection: Phases with low resistance values ​​may have inter-turn short circuits, leading to localized overheating.

Open circuit or poor contact: A high resistance value indicates an open circuit in the winding, poor soldering, or loose terminals.

Welding quality assessment: Deviation >5% often reflects poor winding welding (such as missing welds or cold welds).

2. Ensure operational performance

Current balance: Imbalanced resistance can cause three-phase current imbalance, increase motor vibration and noise, and accelerate bearing wear.

Thermal stability verification: The rate of change of resistance with temperature should be ≤ 1% . Abnormal changes indicate insulation aging or heat dissipation failure.

3. Safety and Compliance Basis

  Compliance with mandatory testing standards such as GB/T 12497-2006 is a necessary item for the acceptance of motors after leaving the factory and after major repairs.

  To prevent the risk of electric shock or fire caused by poor grounding or phase loss.

III. Testing Methods and Key Operation Points

1. Selection of Measurement Method

Direct phase resistance measurement method: Applicable to motors with all 6 winding terminals led out, measuring the resistance of each phase separately.

2. Operating Procedures

 1. Safety preparation: Disconnect the power and test for voltage, remove external connections, and discharge to ground.

 2. Instrument wiring: Clamp the test leads tightly to the terminals to avoid the influence of contact resistance (for the four-wire method, the current / voltage terminals need to be separated).

 3. Data recording: Each phase is measured 3 times and the average value is taken. The ambient temperature is recorded simultaneously.

IV. Typical Problems and Diagnostic Reference

V. Summary

DC resistance testing is a core part of motor testing:

At the standards level: Strictly adhere to the ± 5% deviation limit (≤ 2% for high-voltage motors ) to ensure design compliance;

Technical aspects: Through cold-state precision measurement, electrical defects can be identified early, preventing the escalation of faults;