Using a Multimeter to Test Motor Coils

YouTube：https://youtube.com/shorts/YP54caZw5Kc

1. **Preparatory Work**
- First, ensure that the multimeter is in normal working condition and the range is suitable for measuring relevant parameters of the motor coil. Generally, the multimeter needs to be set to the resistance measurement range. You can first select a moderate range. For example, for small motors, you can first select the 200Ω range. If the measured value exceeds the range, gradually adjust to higher range settings, such as the 2kΩ, 20kΩ range, etc.
- For safe and accurate measurement, it is necessary to disconnect the power supply of the motor and remove the connecting wires between the motor and other electrical equipment to ensure that the motor coil is in an independent state.

2. **Measuring Coil Resistance**
- **Three-Phase Motors**:
- A three-phase induction motor has three windings. Use the resistance range of the multimeter to measure the resistance values of the three sets of windings U-V, V-W, and W-U in the motor terminal box respectively. Under normal circumstances, these three resistance values should be approximately equal. Due to differences in motor power, winding materials, and design, the resistance values will vary. Generally speaking, the winding resistance of small three-phase motors (for example, with a power of 1 - 5kW) may be between a few ohms and several tens of ohms. For example, for a 3kW three-phase induction motor, its winding resistance may be around 3 - 8Ω. If the deviation of the three measured resistance values is large (for example, exceeding 10%), it may indicate problems such as inter-turn short circuits, open circuits, or poor contact in the motor windings.
- **Single-Phase Motors**:
- A single-phase motor generally has a main winding and an auxiliary winding. First, find the terminals of the main and auxiliary windings of the motor. Usually, the resistance value of the main winding is relatively small, and the resistance value of the auxiliary winding is relatively large. Taking a common capacitor-start single-phase motor as an example, the resistance of the main winding may be between a few ohms and a dozen ohms, and the resistance of the auxiliary winding may be between a dozen ohms and several tens of ohms. For example, for a capacitor-start single-phase motor, the resistance of the main winding is about 5Ω, and the resistance of the auxiliary winding is about 20Ω. If the measured value is significantly different from the motor's rated parameters, or the resistance ratio of the main and auxiliary windings is abnormal, it may also indicate that there is a fault in the motor coil.

3. **Judging the Insulation Performance of the Coil (It is more accurate to use the insulation resistance range of the multimeter or a megohmmeter)**
- Although the multimeter can also preliminarily judge the insulation performance, a megohmmeter is more suitable for high-voltage and high-precision requirements. However, the insulation resistance range of the multimeter can also provide some reference.
- Set the multimeter to the insulation resistance range (if available), and generally, a higher range can be selected, such as the 1000MΩ range. Measure the insulation resistance between the motor winding and the motor housing. Under normal circumstances, the insulation resistance should be a relatively large value. Generally, the insulation resistance is required to be greater than 0.5MΩ (in low-voltage motors). If the measured value is very small (for example, less than several hundred kilohms), it may indicate that the insulation between the motor winding and the housing has been damaged, and there is a risk of leakage.
- For three-phase motors, it is necessary to measure the insulation resistance between the U-phase, V-phase, and W-phase windings and the housing respectively; for single-phase motors, measure the insulation resistance between the main and auxiliary windings and the housing.

4. **Judging Whether the Motor Coil is Short-Circuited (Between Phases or Between Turns)**
- In addition to judging whether there is a short circuit between the windings of a three-phase motor by measuring the resistance value, the induction method can also be used for auxiliary judgment. For a three-phase motor, short-circuit the two terminals of any one-phase winding in the motor terminal box, and then use the milliampere range of the multimeter (the range can be appropriately selected according to the motor power, such as the 0 - 500mA range) to connect the terminals of the other two-phase windings. Slowly turn the motor shaft. If the multimeter pointer swings significantly, it may indicate that there is an inter-turn short circuit in the motor winding. Because under normal circumstances, the magnetic fields between the windings of a three-phase motor are balanced. When there is an inter-turn short circuit, the magnetic field will change, thereby generating an induced current that causes the multimeter pointer to swing.
- For a single-phase motor, you can preliminarily judge whether there is an inter-turn short circuit by comparing the resistance ratio of the main and auxiliary windings. If the resistance ratio is significantly different from the normal situation, there may be an inter-turn short circuit. At the same time, you can also observe whether there are abnormal heating, smoking, and other phenomena during the operation of the motor, which may also be manifestations of an inter-turn short circuit.