First.Causes of phase failure of three-phase asynchronous motor
The three-phase asynchronous motor often burns out when it runs out of phase. The main reason is that the reverse sequence magnetic field produces large braking torque after one-phase power failure, which reduces the output torque of the motor. When the external load remains unchanged, the slip rate increases, and the current in the stator winding is much higher than that in normal operation, resulting in increased copper and iron consumption, resulting in the increase of motor temperature and burning out the stator winding. During my electrician maintenance, I found that there are many reasons for the lack of phase operation of three-phase asynchronous motor caused by human negligence.
Second.Condition analysis of motor in single-phase operation
1. When the power supply and load are star connected, the line voltage is equal to √ 3 times the phase voltage, and the line current is equal to the phase current. When one phase of the power supply or load is disconnected, the current is related to the load carried by the motor and increases in proportion.
2. When the motor is △ connected, the line voltage is equal to the phase voltage and the line current is √ 3 times the phase current If the external (power supply) is disconnected, the motor is actually connected with the third phase in parallel after two-phase windings are connected in series, and then connected with the two-phase power supply lines without disconnection. The line current is equal to the sum of the parallel circuits. When the load remains unchanged, the series connected two-phase current remains unchanged, the parallel third-phase current increases by 0.5 times, and the line current at full load is 1.7-2.0 times of the rated current.
3. When the motor is △ connected, if the internal phase of the motor is broken, the motor becomes V-shaped wiring, the phase current and line current increase in proportion to the motor load, the two-phase current increases by 1.5 times, the one-phase line current increases to 1.5 times, and the other two-phase line current increases by about 0.9 times.
Third.Phase failure analysis of power supply
It accounts for a certain proportion in the proportion of phase failure operation of motor, which is manifested in circuit breaker and fuse
1. Circuit breaker:
Many times, the operator of the equipment likes to use the circuit breaker as the shutdown switch. Once it is opened, the circuit breaker belongs to the electrical equipment that is not normally moving. If it is opened with load and the type of protection motor is wrong, it is very easy to produce phase loss. A new production line was replaced in a workshop. The production line is equipped with two 2.2 kW three-phase motors, 24 40 Watt fluorescent lamps and 24 60 watt electric fans, which are connected in three phases. After two months, all the two motors were burned out. After inspection, the fluorescent lamp power supply is connected to one phase, the fan power supply is connected to one phase, and the motor power supply is connected to three phases. The circuit breaker is used to directly turn on and off all electrical equipment during work. After disassembling the circuit breaker, it was found that many carbon deposited contacts in one-phase arc extinguishing chamber were burned out. It was analyzed that the circuit breaker was caused by the lack of phase due to the arcing and burning of contacts due to the opening of the circuit breaker with load.
2. Fuses:
Fault fusing, due to the frequent forward and reverse rotation of the motor, causes the single-phase grounding of the main circuit of the motor. For non fault fuse, the fuse melt capacity is too small and the installation is unreasonable.
3. Contactor:
Unreasonable selection and installation.
4. Thermal relay:
The selection is not matched, the installation is not up to standard, and the commissioning is not in place.
5. Quality reasons of motor itself:
It is mainly manifested in poor butt joint of copper and aluminum and poor welding of external leads of winding.
Fourth.Preventive measures for motor phase loss
1. Circuit breaker:
The circuit breaker is used to connect and disconnect the circuit infrequently. When protecting the motor, there are two points that need special attention: one is its overload capacity, and the other is its large starting current. Therefore, when selecting the type of motor to be protected, the instantaneous setting current value of the circuit breaker is 8-15 times the rated current of the motor, When protecting the wound rotor asynchronous motor, the instantaneous setting current is 3-6 times the rated current of the motor. It is strictly prohibited to use the circuit breaker as the operating mechanism for stopping the operation of electric equipment In the case mentioned above, the power supply of fans and fluorescent lamps was started by adding contactors and the three phases were evenly distributed. It was strictly stipulated to stop the electrical equipment first and then pull the circuit breaker, and there was no open circuit and lack of phase again.
2. Fuses:
For fault fusing, install equipment and electrical circuits in strict accordance with standards. Select the motor that can adapt to the surrounding environment, carry out technical transformation for the motor that needs frequent forward and reverse starting, and install a delay device to stagger the peak impulse current. Non fault fusing: in terms of non fault fusing, it is necessary to select reasonably and avoid some misunderstandings. It is considered that avoiding the general starting current is the best standard value to protect the motor melt. Therefore, the standard fuse close to the starting current value of the motor shall be selected as far as possible. In fact, the function of installing fuse protector is to protect the phase to phase short circuit accident or single-phase grounding accident of the motor, It is not used for overload protection of the motor. The overload protection function is completed by the thermal relay.
３. Contactor:
For different working systems, select contactors with the capacity matching the working system. For example, the contactor with silver alloy or silver inlaid contact shall be selected as far as possible in the long-term working system, and the capacity of copper contact shall be reduced to less than 50% of the rated capacity of intermittent long-term working system. The influence of arc energy must be considered in frequent operation. Choose contactors that can adapt to different environments For example, humid, corrosive gas, dusty, vibration and other environments will rust the iron parts of the contactor, increase the arcing distance between the dynamic and static contacts, reduce the suction force, cause jamming, inequality of the pole surface of the iron core, etc.
4. Thermal relay:
Select the matching thermal relay to distinguish the long-term and short-term protection functions. Generally, it is selected according to 0.95-1.05 times of the rated current of the motor.
5. Motor:
For newly purchased motor or motor with newly repaired winding, resistance measurement shall be carried out to measure the quality of motor winding, and conductive paste shall be coated between the wiring terminal of electric power machine and the contact surface of power line crimping lug. Because the resistance values of the three-phase windings of the motor are equal, especially when the aluminum wire winding is externally connected to the copper terminal, it is necessary to apply conductive paste to prevent the oxidation of aluminum by air and increase the conductivity, so that there will be no phase loss due to oxidation disconnection.
