Dalan Motor analyzes the harm caused by three-phase unbalance to the motor
What are the hazards of three-phase unbalance on the motor? Following the specific analysis of Dalan Motor's small series, mainly reflected in the following six aspects of the three-phase imbalance caused to the motor? Following the specific analysis of Dalan Motor's small series, it is mainly reflected in the following six aspects:
1, increase the power loss of the line
1 In a three-phase four-wire power supply network, when a current passes through a line conductor, power loss is generated due to the presence of an impedance, and the loss is proportional to the square of the passing current.
2 When the low-voltage power grid is powered by a three-phase four-wire system, the three-phase load imbalance is inevitable due to the presence of a single-phase load.
3 When the three-phase load is unbalanced, there is current flowing through the neutral line. In this way, not only the phase line is depleted, but also the neutral line is also depleted, which increases the loss of the grid line.
2. Increase the power loss of the distribution transformer
The distribution transformer is the main power supply equipment of the low-voltage power grid. When it is operated under the unbalanced three-phase load condition, it will cause an increase in the distribution loss. Because the power loss of the distribution varies with the imbalance of the load.
3, the distribution of output reduction
1 When the design is changed, the winding structure is designed according to the load balance operation condition, and the winding performance is basically the same, and the rated capacity of each phase is equal. The maximum allowable output of the distribution is limited by the rated capacity of each phase.
2 When the distribution transformer is operated under the condition of three-phase load unbalance, the one phase with light load has a surplus capacity, so that the output of the distribution transformer is reduced. The degree of reduction in output is related to the imbalance of the three-phase load. The greater the three-phase load imbalance, the more the distribution output is reduced.
3 The distribution transformer operates when the three-phase load is unbalanced, and its output capacity cannot reach the rated value, and its spare capacity is also reduced accordingly, and the overload capability is also reduced. If the distribution is operated under overload conditions, it will easily cause the distribution to heat up, and even in severe cases, it will cause the distribution to burn.
4, the distribution produces zero sequence current
1 Distribution transformer operation under three-phase load unbalanced conditions will produce zero-sequence current, which will change with the degree of unbalance of three-phase load. The greater the imbalance, the larger the zero-sequence current. If there is a zero-sequence current in the running distribution, a zero-sequence flux will be generated in the core.
2 The winding insulation of the transformer is accelerated due to overheating, resulting in a decrease in equipment life. At the same time, the existence of zero-sequence current will also increase the loss of the distribution.
5. Affect the safe operation of electrical equipment
1 The distribution transformer is designed according to the three-phase load balancing operation condition, and the resistance, leakage reactance and excitation resistance of each phase winding are basically the same. When the distribution transformer operates in the three-phase load balance, the three-phase currents are basically equal, and the voltage drop per phase in the distribution transformer is also substantially the same, and the three-phase voltage of the distribution transformer output is also balanced.
2 The distribution transformer operates when the three-phase load is unbalanced, and the output currents of the phases are not equal. The internal three-phase voltage drop of the distribution transformer is not equal, which will inevitably lead to three-phase unbalance of the distribution transformer output voltage.
3 In the case of voltage imbalance, the power supply is easy to cause the user of one phase of the high voltage to burn out, and the user of the low voltage one phase may not be used. Therefore, when the three-phase load is unbalanced, the safe operation of the electrical equipment will be seriously jeopardized.
6, the motor efficiency is reduced
1 Distribution transformer operation under three-phase load unbalanced conditions will cause the output voltage three-phase unbalance. Since the unbalanced voltage has three voltage components of positive sequence, negative sequence and zero sequence, when the unbalanced voltage is input to the motor, the negative sequence voltage generates a rotating magnetic field opposite to the rotating magnetic field generated by the positive sequence voltage, and acts as a brake. effect. However, since the positive sequence magnetic field is much stronger than the negative sequence magnetic field, the motor still rotates in the direction of the positive sequence magnetic field.
2 Due to the braking effect of the negative sequence magnetic field, the output power of the motor will be reduced, resulting in a decrease in motor efficiency. At the same time, the temperature rise and reactive loss of the motor will also increase with the imbalance of the three-phase voltage. Therefore, it is very uneconomical and unsafe for the motor to operate under three-phase voltage imbalance conditions.
----- Editor-in-Chief: Dalan Oil Pump Motor 02-Procurement Consultant
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