Any electrical product, including motor products, has specified the rated voltage for its normal operation, and any voltage deviation will cause adverse consequences to the normal operation of the electrical appliance.
For relatively high-end equipment, necessary protective devices are used. When the power supply voltage is abnormal, the power supply is cut off for protection. For very precise instruments, constant voltage power supply is used for adjustment. However, for motor products, especially industrial motor products, the possibility of using constant voltage devices is very small, and there are more situations of power outage protection.
For single-phase motors, there are only two types of voltage: high and low, while for three-phase motors, there is also a voltage balance problem. The direct impact of these three voltage deviations is an increase in current or an imbalance in current.
According to the technical specifications of the motor, the deviation between the rated voltage of the motor cannot exceed 10%, and the torque of the motor is proportional to the square of the motor terminal voltage. When the voltage is too high, the motor core will be in a magnetic saturation state, and an increase in stator current will cause serious heating of the winding, even leading to quality problems such as winding burnout; For situations where the voltage is low, one possibility is that there may be problems with the starting of the motor, especially for motors operating under load. In order to meet the load operation of the motor, the current must also increase. The consequence of the current increase is also the heating or even burning of the winding, especially for long-term low-voltage operation, which is even more serious.
The voltage imbalance of three-phase motors is a typical power supply problem. When the voltage is unbalanced, it will inevitably lead to the imbalance of motor current. The negative sequence component of unbalanced voltage generates a magnetic field in the motor air gap that is opposite to the rotation of the rotor. The small negative sequence component in the voltage may cause the current flowing through the winding to be much larger than the current at voltage balance. The frequency of the current flowing through the rotor conductor is almost twice the rated frequency, so the current squeezing effect in the rotor conductor causes the increase in losses of the rotor winding to be much greater than that of the stator winding. The temperature rise of the stator winding is higher than that when operating under balanced voltage.
When the voltage is unbalanced, the locked rotor torque, minimum torque, and maximum torque of the motor will all decrease. If the voltage imbalance is severe, the motor will not function properly.
When the motor operates at full load under unbalanced voltage, the speed will slightly decrease due to the increase in slip rate as the additional loss of the rotor increases. As the degree of voltage (current) imbalance increases, the noise and vibration of the motor may increase. Vibration may damage the electric motor or the entire drive system.
To effectively identify the cause of uneven motor voltage, methods such as power supply voltage detection or current changes may be used. Most devices are equipped with voltage monitoring instruments, which can be analyzed through data comparison. However, for situations without monitoring devices, regular testing or current measurement should be used; For motors that can rotate in both directions, without affecting the dragged equipment, the two-phase power supply lines can be freely swapped to observe the current changes, and indirectly analyze the voltage balance. After eliminating the problem of uneven voltage, quality issues such as turn to turn and phase to phase may be involved.
