One can use a multimeter to check the winding resistance of a three-phase motor where the normal value should fall between 0.5-10 ohms, and insulation resistance should be checked with a megohmmeter; a value below 1MΩ indicates an insulation problem. Current balance should be monitored: a difference of more than 15% is unacceptable.
Measuring Winding Resistance with a Multimeter
One of the most common ways to check if a motor is short-circuited is by measuring the winding resistance using a multimeter. First, the motor should be completely powered off and isolated from the power supply to avoid any misoperation that could result in electric shock or damage to the equipment. When using a multimeter to measure the resistance between the three-phase windings of the motor, measure the U, V, and W windings. If the motor is running normally, the resistance values of these three-phase windings should be fairly similar. If the resistance of one phase is significantly lower than the other two, it may indicate a short circuit between the internal turns of the motor. If the resistance of one phase is near infinity, this indicates a broken circuit in that phase.
In general, the resistance of three-phase motor windings should be between 0.5 and 10 ohms, depending on the motor’s power and model. If the measured resistance is close to 0 ohms, the possibility of a short circuit in the winding is very high. In this case, the motor should be stopped immediately to avoid more serious damage. This simple and quick method can effectively detect initial motor faults.
Checking Insulation Resistance
Insulation is one of the key indicators of motor condition, especially in high-moisture or high-temperature environments where insulation materials may degrade, leading to winding short circuits or leakage. Insulation resistance between the motor windings and ground can be measured using a megohmmeter. During measurement, the two ends of the megohmmeter are connected to the winding and ground, respectively, to measure the insulation resistance of the three-phase windings to the ground. If the resistance is less than 1MΩ, the motor may have serious insulation issues and must be shut down for further examination.
Normally, the insulation resistance of new motors is over 100MΩ. If the insulation resistance of an older motor is less than 10MΩ, it requires special attention. If a motor has been running in a humid environment for a long time, and the insulation resistance drops below 1MΩ, it becomes highly dangerous. Such a low value indicates that the motor is at risk of leakage or short circuits during operation and requires repair or replacement of insulation materials. Insulation resistance checks help prevent electrical failures and unnecessary safety hazards.
Listening to Sounds and Observing Vibrations
Changes in sound and vibration can help diagnose the motor’s health. A motor running normally should emit a steady, low-pitched hum. If abnormal sounds such as metal friction, knocking, or crackling occur during operation, they may indicate internal problems. For example, the motor’s bearings might be damaged, the windings might have short-circuited, or foreign objects may have entered the motor, causing mechanical friction.
Excessive vibration is another indicator of motor failure. Vibration can be checked by feel or with professional vibration detection instruments. Excessive vibration usually indicates rotor imbalance, damaged bearings, or unstable installation. Generally, a motor’s vibration velocity should be less than 1.8mm/s. If it exceeds this standard, further inspection is needed to determine if there are mechanical or electrical issues within the motor.
Checking Motor Temperature
Abnormal motor temperature rise is another common indicator of motor problems. Excessive motor temperature typically suggests winding short circuits, excessive load, or poor ventilation. If not addressed in time, it can cause further damage to the motor and even result in production stoppage. Motor temperature can be quickly checked using an infrared thermometer or through thermocouples installed inside the motor.
In general, the stator winding temperature of the motor should not exceed 85°C, and the housing temperature should be kept below 65°C. If the motor’s surface temperature exceeds these limits, it could indicate internal short circuits or inadequate ventilation that is preventing proper heat dissipation. If the motor’s temperature rises abnormally and is accompanied by a burning smell or smoke, the motor is already in a dangerous state and should be shut down immediately for inspection.
Monitoring Current Imbalance
When a three-phase motor is running normally, the current of the three phases should be balanced. An imbalance in the three-phase current typically indicates that there is a short circuit or turn-to-turn short circuit inside the motor windings. A clamp ammeter or a current monitoring device installed in the distribution cabinet can monitor the motor’s current in real-time.
When measuring the running current of a three-phase motor, the current values of U, V, and W should be approximately equal. If the current of one phase is significantly higher than the other two, it may indicate a short circuit or turn-to-turn short circuit in that winding. Additionally, the motor’s running current should not exceed its rated value. If a motor runs overloaded for a long time, it not only causes current imbalance but also accelerates damage to the windings.
Based on experience, the current difference among the three phases should not exceed 10%. If the current imbalance exceeds 15%, the motor is already in a dangerous state and should be stopped immediately for inspection. It is recommended to install a current imbalance alarm device in the electrical control system, which will issue a real-time warning when the actual deviation exceeds the preset value.
