In the process of using the motor, there are often encountered three-phase current imbalance, combined with our usual experience, today I will sort out the causes of three-phase current unbalance.
Three phase voltage unbalance
If the three phase voltage is unbalanced, there will be a reverse-sequence current and reverse-sequence magnetic field in the motor, which will produce larger reverse-sequence torque, resulting in the unbalanced three-phase current distribution of the motor and increasing the current of a phase winding.
When the three-phase voltage unbalances reach 5%, it can make the motor phase current exceed 20% of the normal value.
Three-phase voltage unbalance is mainly manifested in:
1. abnormalities occur in one phase of the transformer's three-phase winding, which transmits asymmetric supply voltage.
2. long transmission lines, uneven wire cross-section size, and different impedance voltage drop, resulting in unbalanced voltage in each phase.
3. Power, lighting mixed common, which single-phase load, such as: household appliances, electric furnace, welding machine, etc. too concentrated in a phase or a two-phase, resulting in uneven distribution of each phase load, so that the supply voltage, current imbalance.
Overload
The motor is in overload running condition, especially when starting, the motor stator and rotor current increases and heats up. If the time is slightly longer, the winding current unbalance phenomenon will occur easily.
The overload is mainly manifested in:
1. belt, gear and other transmission mechanism is too tight or too loose.
2. the coupling mechanism is skewed, the transmission mechanism has a foreign body stuck.
3. dry lubricating oil, bearing jamming, mechanical rust (including the motor itself mechanical failure).
4. The voltage is too high or too low, which increases the loss.
5. Improper load matching, the rated power of the motor is less than the actual load.
Stator, rotor Fault of the winding
Stator winding inter-turn short circuit, local grounding, disconnection, etc., will cause a phase or its two roots in the walker winding current is too large, so that the three-phase current is seriously unbalanced. Walking son, rotor winding fault manifested in.
1. the fixed in the chamber of dust, debris, hard trauma, resulting in inter-turn short circuit.
2. a phase of the stator winding broken circuit.
3. the stator winding is damp, there is leakage current phenomenon.
4. Bearing, rotor damage and deformation, rotor and rotor winding rubbing each other.
5. Squirrel cage rotor winding breakage weld crack, produce unstable current.
Improper operation and maintenance
Operators can not regularly do a good job of inspection and maintenance of electrical equipment, is the main factor that artificially caused the motor leakage, out-of-phase operation, resulting in unbalanced current.
Improper operation and maintenance are mainly manifested as follows;
1. the operator and installer will reverse the phase and zero line connection.
2. The incoming line touches the junction box and there is current leakage.
3. the connection switch, contact loosening, oxidation and other reasons caused by the lack of phase phenomenon.
4. frequent starting, starting time is too long or too short, resulting in fuse breakage.
5. Long-term use, lack of maintenance, making the motor aging, local insulation degradation.
Three-phase motor current imbalance may occur in the motor insulation breakdown.
Whether the breakdown depends on the size of the current in the motor winding, the motor starts when the starting inrush current is very large, the possibility of breakdown is greater, but not absolutely, this is related to the size of the current, insulation level, etc..
Three-phase current imbalance will certainly produce instability of motor torque.
Generate motor three-phase current imbalance, I believe that the main reason is due to the motor three-phase winding imbalance.
So in the electric motor production, quality control and factory testing is also very important.
One of the main faults is the unbalanced three-phase current (in extreme cases, the motor is out of phase).
Unbalanced three-phase current may cause difficulty in starting.
The motor will make noise when running, and in serious cases, the motor will vibrate and roar violently.
Current increases, if not stopped in time, may also cause the motor winding burned.
Opinions and thoughts are always generated in interaction! What do you think about the market situation of electric motor? welcome to leave comments.
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