Essential knowledge for factory circuit and equipment maintenance

Jun 09, 2023

1、 General fault handling

1. Voltage circuit breaker failure

The contact is overheated, and there is a smell in the distribution control cabinet. After inspection, it was found that the moving contact was not fully inserted into the static contact, and the contact pressure was insufficient, resulting in a decrease in the switch capacity and causing the contact to overheat. At this point, the operating mechanism needs to be adjusted so that the moving contact is fully inserted into the stationary contact.

When powered on, the flash and explosion were caused by long-term overload and loose contact of the contacts. When repairing this fault, it is necessary to pay attention to safety and strictly prevent the harm of electric arcs to people and equipment. After repairing the load and contacts, the no-load power should be turned on normally before checking the operation with load until it is normal. Attention must be paid to the daily maintenance work of the equipment used for this fault to avoid unnecessary harm.

2. Fault of contactor

The contact is out of phase. Due to poor contact of a certain phase contact or loose screws on the wiring terminal, the motor runs out of phase. At this time, although the motor can rotate, it emits a buzzing sound. Immediately stop the vehicle for maintenance. Contact welding, connected to the "stop" button, the motor does not stop rotating and may emit a buzzing sound. This type of fault is caused by the melting phenomenon of two-phase or three-phase contacts due to high overload current. It should be immediately powered off, and the contactor should be replaced after checking the load. The energized armature does not engage. If there is no vibration or noise when powered on after inspection, it indicates that the moving part of the armature is stuck along the edge, and it is just a fault of a coil open circuit. The coil can be removed and rewound according to the original data before being soaked in paint and dried.

3. Thermal relay failure

The Mechanical equivalent of heat element is burnt out. If the motor cannot be started or there is a buzzing sound when starting, the fuse in the thermal element of the thermal relay may be burnt out. The cause of such faults is that the action frequency of the thermal relay is too high, or overload occurs on the negative side. After troubleshooting, replace the appropriate thermal relay, pay attention, and readjust the setting value. The thermal relay malfunctions. There are generally several reasons for this type of fault: the setting value is too small, resulting in action without overload; The long starting time of the motor causes the thermal relay to act during the starting process; The operating frequency is too high, causing frequent impact on the thermal components. Readjust the setting value or replace it with a suitable thermal relay to solve the problem. The thermal relay does not operate. This type of fault usually occurs when the current setting value is too high, causing the overload to remain inactive for a long time. The setting current should be adjusted according to the working current of the load. The thermal relay has been in use for a long time and its operational reliability should be regularly verified. When the thermal relay acts and trips, it shall be reset after the Bimetal cools down. Do not use excessive force to press the reset button, otherwise it may damage the operating mechanism.

2、 Troubleshooting and Essentials of Commonly Used Voltage Electrical Appliances

Voltage appliances with contact action mainly consist of three parts: contact system, electromagnetic system, and arc extinguishing device. It is also a key point in maintenance.

1. Troubleshooting of contacts

The faults of contacts generally include overheating, welding, etc. The main reasons for contact overheating are insufficient contact pressure, surface oxidation or uncleanness, and insufficient capacity; The main reason for contact welding is that the contact generates a large arc when closed, and the contact is severely jumpy. Check for oxidation and dirt on the contact surface. The contact points have dirt and have been cleaned with gasoline. The oxide layer of silver contacts not only has good conductivity, but also can be reduced to metallic silver during use, so it cannot be repaired. If there is an oxide layer on copper contacts, they can be flattened with an oil smooth file or gently scraped off with a small knife.

Observe the surface of the contact points for burns and burrs. Copper contact points can be trimmed with an oil smooth file or a small knife for burrs. The surface of the repaired contact does not need to be too smooth, and it is not allowed to use sandcloth for repair to avoid residual sand particles embedded on the contact during contact closure, which may cause poor contact. But silver contact singeing does not need to be repaired. If there is fusion welding on the contacts, they should be replaced. If it is caused by insufficient contact capacity, an electrical appliance with a higher capacity should be selected for replacement. Check if the contacts are loose and tighten them to prevent them from jumping. Check for mechanical damage to the contacts that causes spring deformation, resulting in insufficient contact pressure. If there is, the pressure should be adjusted to ensure good contact between the contacts. The empirical measurement method for contact pressure is as follows: for the measurement of initial pressure, a paper strip about 0.1mm wider than the contact is placed between the bracket and the moving contact. The paper strip is compressed under the action of the spring, and then the paper strip is pulled out with one hand. When the paper strip can be pulled out and has a strong feeling, the initial pressure can be considered appropriate. For the measurement of final pressure, the paper strip is clamped between the moving and stationary contacts. After the contact is energized and closed by the electrical appliance, the paper strip is pulled using the same method. When the paper strip can be pulled out, the final pressure can be considered more appropriate. For large capacity electrical appliances, such as 100A or above, when using the same method to pull the paper strip, if there is tearing phenomenon when the paper strip is pulled out, it can be considered that the initial and final pressure are more suitable. The measurement method for contact pressure above has shown good results in multiple repair tests. All can be carried out normally. If the measured pressure value cannot be restored by adjusting the spring, the spring or contact must be replaced.

2. Maintenance of fire extinguishing devices

Remove the arc extinguishing cover, check the integrity of the arc extinguishing disc, and remove smoke marks and metal particles from the surface. The shell should be intact and undamaged. If there are cracks in the arc extinguishing cover, it should be replaced in a timely manner. It should be noted that electrical appliances with arc extinguishing covers are never allowed to use Phoenix short-circuit protection without arc extinguishing covers. There are many types of commonly used low-voltage electrical appliances. The above are some representative and most commonly used methods and essentials of electrical failure. By analogy, the maintenance of other electrical appliances has certain commonness.

3、 Reasons and Preventive Measures for Single-phase Operation of Electric Motors

1. Fuse blown

(1) Fault fusing: mainly caused by single-phase grounding or interphase short circuit in the main circuit of the motor, causing the fuse to blow.

Preventive measures: select motors and correctly installed low-voltage appliances and lines that are suitable for the surrounding environment, and regularly check them, strengthen daily maintenance, and eliminate various hidden dangers in time.

(2) Non fault fusing: mainly due to improper selection of melt capacity, the capacity is too small, and when starting the motor, the fuse may blow due to the impact of starting current.

Non fault fusing of fuses can be avoided. Do not assume that the capacity of the melt should be chosen as small as possible to avoid the starting current of the motor, in order to protect the motor. We need to make it clear that fuses can only protect the single-phase grounding and interphase short circuit accidents of the motor, and they must not be used as overload protection for the motor.

2. Correct selection of melt capacity

The general formula for selecting the rated current of a melt is:

Rated current=K × Rated current of electric motor

(1) The K value of fuse with large heat resistance capacity (with filler) can be 1.5~2.5.

(2) The K value of fuse with small heat resistance capacity can be 4~6.

For different loads carried by the electric motor, the K value also varies accordingly. For example, if the electric motor directly drives the fan, the K value can be chosen to be larger. For example, if the electric motor has a small load, the K value can be chosen to be smaller. The specific situation depends on the load carried by the electric motor.

In addition, there must be good contact between the melt of the fuse and the fuse holder, otherwise it will cause heat at the contact point, causing the melt to be subjected to external heat and causing non faulty melting.

During the installation of the motor, appropriate wiring methods and correct maintenance methods should be used.

(1) For copper and aluminum connections, copper aluminum transition joints should be used as much as possible. If there is no copper aluminum joint, tin can be hung on the copper joint for connection.

(2) For plug-in fuses with larger capacity, a thin copper strip (0.2mm) can be added to the wiring for better results

(3) Check and adjust the contact pressure between the melt and the melt seat.

(4) When wiring, avoid damaging the fuse, tighten it moderately, and add spring washers at the wiring points.

The relationship between wire, square, current, and power

1、 Calculation of wire cross-sectional area and current carrying capacity:

The safe current carrying capacity of a general copper wire is determined based on the maximum allowable core temperature, cooling conditions, and laying conditions.

The safe current carrying capacity of copper conductors is generally 5-8A/mm2, while that of aluminum conductors is 3-5A/mm2.

Calculate the upper and lower range of the cross-sectional area S of copper wire: 0.125 I (mm2)<s=i="" (5-8)<="" 0.2="" i (mm2)=""<="" span="">

S - cross-sectional area of copper wire (mm2) I - load current (A)

Recommended value for safe current carrying capacity of 2.5mm2 BVV copper wire 2.5 × 8A/mm2=20A 220V × 20A=4400W 4mm2

Recommended values for safe current carrying capacity of BVV copper conductors 4 × 8A/mm2=32A 220V × 32A=7040W 6mm2

Recommended values for safe current carrying capacity of BVV copper conductors 6 × 8A/mm2=48A 220V × 48A=10560W

2、 The relationship between wire square and current - Safety calculation method for copper wire

The maximum current that a national standard 1 square meter wire can withstand is 19 amperes, but in reality, it still needs to be considered according to the usage environment, temperature, installation method, etc. The most rough estimation method is 5 amperes for single-phase 1000W and 2 amperes for three-phase. The general safety calculation method for copper wire is:

If it is aluminum wire, the wire diameter should be 1.5-2 times that of copper wire.

The safe current carrying capacity of a 2.5 square millimeter copper power cord is -28A.

Power 220 × The safe current carrying capacity of a 6160W 4 square millimeter copper power cord is -35A.

Power 220 × The safe current carrying capacity of a 7700W 6 square millimeter copper power cord is -48A.

Power 220 × The safe current carrying capacity of a 48=10560W 10 square millimeter copper power cord is -65A.

Power 220 × The safe current carrying capacity of 65=14300W 16 square millimeter copper power cord -91A.

The safe current carrying capacity of a 25 square millimeter copper power cord is -120A.

3、 Safety current carrying capacity formula for wires:

Ten down five, one hundred up two, two five three five four three realms,

Seventy nine five and a half times, copper wire upgrade calculation.

The threading temperature is 80% off, and the bare wire is increased by half.

For wires of 1.5, 2.5, 4, 6, and 10mm2, the cross-sectional area can be multiplied by 5 times.

For wires of 16 and 25mm2, the cross-sectional area can be multiplied by 4 times.

For wires of 35 and 50mm2, the cross-sectional area can be multiplied by three times.

For wires of 70 and 95mm2, the cross-sectional area can be multiplied by 2.5 times.

For wires of 120, 150, and 185mm2, the cross-sectional area can be multiplied by two times.

Explanation: Multiply the cross-section of aluminum wire below 10 mm2 by 5. If it is copper wire, it will be upgraded by one level. For example, if it is 2.5 mm2 copper wire, it will be calculated as 4 mm2. Multiply everything above 100 by 2, everything below 25 by 4, everything above 35 by 3, and everything between 70 and 95 by 2.5.

4、 Power calculation

The general load for power calculation (also known as electrical appliances such as lighting, refrigerators, etc.) can be divided into two types: resistive load and inductive load. The calculation formula for resistive loads: P=UI

For the calculation formula of fluorescent lamp load: P=UIcos ф, The power factor cos of the fluorescent lamp load ф= 0.5. Different inductive loads have different power factors, so when calculating household appliances uniformly, the power factor cos can be adjusted ф Take 0.8.

If the total power of all household appliances is 6000 watts, the maximum current is I=P/Ucos ф= 6000/(220 × 0.8)=34 (A).

In general, it is not possible to use household appliances simultaneously, so adding a common coefficient results in a common coefficient of 0.5.

So the above calculation should be rewritten as I=P * common coefficient/Ucos ф= 6000 * 0.5/220 * 0.8=17 (A), the total current value of this household is 17A.

The air switch of the main gate should be greater than 17A. If the above is inaccurate, such as a total power of 10000 watts (maximum power of 6000 watts) and a total brake of 5000 watts calculated based on the common coefficient, it should be taken as 0.5 of the maximum power plus the remaining power.

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