Three-phase voltage imbalance and the effect of overload on the motor
When the three-phase voltage imbalance occurs when the motor is running (that is, the three-phase line voltages are not equal), when the voltage deviation is large, the rotor and stator winding currents are additionally increased, and a serious imbalance is also generated.
For example, when the line voltage unbalances is only 4%, the motor line current imbalance can reach 25%, which causes additional heating of the motor. The three-phase voltage imbalance also reduces the electromagnetic torque and increases the electromagnetic noise.
In the grid of lighting and power mixing or single-phase and three-phase power mixing, when the single-phase load is large (such as the electric furnace, electric welder, drying oven, etc.) and power failure, etc., three-phase can be caused. The voltage is not balanced. But as long as any two phases.
When the line voltage difference between the two is not more than 5%, the motor can operate for a long time under the rated load. However, when the power supply voltage is seriously unbalanced, long-term operation is not allowed. If such a fault is encountered, a professional electrician must be required to perform line maintenance. You can operate the motor again after the circuit fault is solved.
So what are the consequences of long-term overload operation of the motor on the motor? Motor overload operation refers to the operation of the output power greater than the rated power, that is, overload operation.
When the motor is overloaded, it must overcome the resistance torque of the overload and output a larger torque, which forces the current of the rotor and stator circuits to increase. The short-time overload motor can also automatically adjust the bearing. Still, in the long-term overload operation, the current exceeds the rated value for a long time. The temperature rise exceeds the allowable value, the motor insulation is damaged, and finally, the motor winding is short-circuited and burned. Therefore, the motor is not allowed to operate for a long period of time. You can install an overload protection device (such as a thermal relay) during circuit maintenance to avoid the overloading of the motor.
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