Commonly used electrical parameter monitoring methods include current monitoring and voltage monitoring.
Excessive voltage deviation
As an electrical device, a reliable power supply is a key indicator for the normal operation of a motor. Excessive undervoltage can cause the motor to stall, and excessive current can lead to prolonged undervoltage, causing the motor and power lines to operate under overload conditions for extended periods, resulting in damage to both. Conversely, overvoltage can damage the insulation of the motor and power lines, potentially causing serious accidents in a short period. Compared to overvoltage and undervoltage, phase loss is equally fatal, also causing serious accidents in a short time. Therefore, real-time voltage monitoring and early warning are crucial indicators for ensuring the normal operation of equipment.
The reliability of voltage amplitude is generally determined by the power supply. If the matching power supply margin is insufficient, the motor will experience a significant voltage drop when encountering abnormal load fluctuations, which will cause the motor to stall and the equipment to stop. On the other hand, if the matching margin is too large, it will occupy space and have poor economic benefits.
When a motor experiences undervoltage, there are generally two causes. One is a power supply problem, where fluctuations in the power grid can lead to fluctuations in the supply voltage, resulting in undervoltage. The other is a load problem, where too many devices are running simultaneously or the motor is running under short-term overload, causing an excessive load and resulting in undervoltage. Most motors have a working voltage range, and the working voltage should be kept within this range as much as possible.
When an overvoltage occurs in a motor, it is caused by power supply and load problems due to fluctuations in the mains voltage. In a power supply system with multiple electrical devices operating simultaneously, if multiple devices stop working at the same time, it may cause a sharp increase in the supply voltage. Therefore, planned shutdowns are also very necessary.
When a motor experiences a phase loss, it is necessary to stop the machine immediately, disconnect the power supply circuit, and troubleshoot. Generally, the power transmission circuit should be checked first to confirm its reliability, and then the power supply should be checked to confirm whether there are any abnormalities. If both of the above are working normally, the problem usually lies in the connection parts between the two.
When there is a significant three-phase imbalance in the motor's power supply voltage, troubleshooting is usually done by analyzing the current.
Monitoring voltage is equivalent to monitoring the electrical environment of the motor. A stable operating environment is the primary condition for ensuring reliable motor operation. Generally, the required power supply voltage is within ±10% of the rated voltage. Traditional motors are also designed to account for power supply voltage fluctuations (also known as energy fluctuations). When the voltage is within ±10% of the rated voltage, the motor can operate stably. Exceeding this range may damage the motor insulation, while falling below this range may result in excessively high motor current, ultimately leading to excessive heat and damage to the motor.
Excessive current deviation
Voltage monitoring primarily reflects the power supply status, while motor power consumption is reflected by current monitoring. Motor overload will cause excessive current, while loose load will cause insufficient current. Damaged winding insulation or electric shock will cause current imbalance. Conversely, excessive current will cause the motor temperature to rise significantly, leading to motor damage. Insufficient current indicates a loose connection on the motor load side or at the load connection, which may cause more serious problems if not addressed promptly. Current deviations mostly indicate motor insulation problems or even personal accidents.
Therefore, timely detection and resolution of abnormal current deviations is not only a guarantee for the reliable use of motors, but also a necessary condition for their safe use.
The value of the current can accurately reflect the usage of electrical equipment, and there are generally several types of abnormal current deviations.
Excessive current: When the motor current is found to exceed the limit value, the power supply of the motor should be checked first to see if there is undervoltage or overvoltage. When the power supply is normal, the occurrence of overcurrent means that the motor load has increased abnormally. Bearing wear and lack of lubrication can cause the motor load to increase. Problems on the motor load side can also cause the motor load to increase and lead to overcurrent. Secondly, when there is a problem with the motor insulation, it will lead to phase-to-phase short circuit or short circuit to ground. These problems will also lead to overcurrent. Insulation problems are generally accompanied by current imbalance.
Insufficient current: When the connection between the motor and the load becomes loose, the current will decrease significantly. In this case, the integrity of the connection between the motor and the load should be checked to confirm whether any connecting parts are loose, such as a loose coupling or scratched threads. At the same time, the working condition of the load should also be checked.
Excessive current deviation: Current deviation is divided into two categories. When the current value deviates, there are generally three reasons: First, the power supply is problematic. An imbalance in the power supply voltage will directly lead to an imbalance in the motor current. Repairing the power supply is a feasible way to solve this problem. Second, the windings are aging. After the motor has been running for a long time, the windings may age, causing changes in the winding values and resulting in current imbalance. This problem can be investigated by stopping the machine and checking the inter-turn and winding resistance values. Third, the insulation of the windings is problematic. When this problem occurs, the current in one or more phases of the motor will increase significantly. The machine should be stopped immediately to check the phase-to-phase insulation and insulation to ground of the windings.
Current imbalance: Unlike the current imbalance in a motor, when there is a problem with the motor's insulation to ground, the changes in the three-phase current values may not be obvious. The three-phase current values may not increase significantly, nor are they unbalanced. However, analysis of the motor current using instruments such as a power analyzer will reveal the generation of zero-sequence and negative-sequence currents, indicating a significant discharge from the motor to ground. Early detection of this type of problem can resolve these potential risks before complete insulation breakdown.
Almost any abnormality in any part of the motor can be reflected in the current value at any time. Therefore, this article gives a limit on the deviation of the motor current. When the current deviates from the rated value by more than +15% to -30%, or the three-phase imbalance exceeds 10%, the motor operation may be abnormal and the motor operation needs to be carefully checked.
The motor is emitting smoke while running, with excessively high local temperatures and a burning smell. The three-phase current of the motor is unbalanced, and several turns or one coil have become bare wire.
If several turns or one coil burns out, and the slot fill factor is low, it can be repaired by rewinding. For the remaining parts, refer to the handling method in section 4. To prevent accidents and ensure smooth production, electrical equipment must be inspected and maintained regularly, and operational faults must be accurately identified and dealt with promptly to reduce equipment failure losses.
