As a general mechanical equipment, air compressor is widely used in the field of industrial and agricultural production. The problem is that the failure damage rate of the motor as the power equipment of the air compressor is generally higher than that of the motor of other equipment. Why? This paper will put forward the basic concepts and arguments about the damage causes of the air compressor motor and how to protect it, hoping to play the effect of throwing bricks and attracting jade, attract the attention of the air compressor manufacturing industry and the majority of users, and jointly participate in the discussion.

Analysis of damage causes of air compressor motor

There are many and comprehensive reasons for the damage of air compressor motor. We went deep into the work and production site of the air compressor and conducted field investigation. We believe that the reason for the damage of the air compressor motor is not only common with other motor power equipment, but also related to the fault factors caused by the specific operation of the air compressor itself. The main points are summarized as follows:

Requirements for working conditions of air compressor

The mechanical operation and electrical control of the air compressor are relatively simple. Generally, it works independently in a single unit and is in an unprofessional operation situation. The occurrence and treatment of the fault is not timely. Relatively speaking, the repetition and expansion rate of the fault increases.

Reliability and sensitivity of automatic pneumatic switch

The start and stop of the air compressor is usually controlled automatically by the AC contactor directly driven by the air pressure automatic switch. Most of the pneumatic automatic switches adopt mechanical structure, and use the spring to compress and release energy to operate the contacts. It may be the congenital deficiency of the automatic air pressure switch or the influence of different reasons such as improper adjustment and mechanical shock. In the process of controlling air pressure, two abnormal actions may occur from time to time:

1. The automatic air pressure switch refuses to act when the air pressure exceeds the air pressure, so that the motor cannot operate normally, and the air pressure continues to rise, resulting in overload and overload operation of the motor. In serious cases, fault blocking may occur.

2. The air pressure automatic switch has the phenomenon of transient shock jump, which makes the AC contactor produce synchronous shock jump at the same time. Cause the main contact of AC contactor to burn out or stick under the impact of frequent starting current, resulting in shutdown or phase failure operation.

Automatic exhaust valve failure

Generally, the air compressor does not have an automatic exhaust device. When the air pressure reaches the set pressure, it will automatically exhaust and depressurize after shutdown. It is preset that the air pressure in the cylinder of the air compressor cannot be released after the next no-load start. It is equal to the on-load start of the motor, which may cause starting difficulty or locked rotor failure.

Sudden power cut-off during operation

When the power supply of the air compressor controlled by automatic gear suddenly stops, it is restarted because the air pressure in the cylinder of the air compressor is too late to release. When the motor is started under heavy load, it is likely to form locked rotor fault.

Start too frequently

Due to irregular peripheral gas consumption, when the gas consumption is large, the motor is forced to start frequently. When starting, the accumulation of heat surplus generated by large current is not easy to be dissipated, so that the temperature rise of the motor is overheated, which is likely to cause the motor to burn out.

Heavy load shutdown

Unlike other equipment, the shutdown of the air compressor is when the maximum pressure of the set upper limit is reached. AC contactor operates in sections under heavy load and large current. At this time, the arcing spark is the most serious, which poses a great threat to the damage of the AC contactor. Due to the damage of the AC contactor, it will lead to the property failure of the power supply. Operating conditions such as imbalance.

Mechanical property fault of air compressor

The air compressor is of non-uniform load nature, with large mechanical vibration noise and blocked transmission parts, resulting in mechanical faults such as wear and damage, which is very easy to cause motor overload.

Current situation of air compressor motor protection

The above typical fault factors show that the correct and reasonable protection of air compressor motor is very urgent and necessary. However, so far, the protection device of air compressor motor is still mainly thermal relay. Of course, as a traditional protection product, thermal relay has the advantages of simple structure, convenient use, low price and certain inverse time limit characteristics. However, its working principle and performance characteristics determine its shortcomings and defects, such as small function, high power consumption, low precision, rough setting, easy to be affected by the environment, poor repeatability, large error and so on. For the faults such as locked rotor and long-term overload during motor starting, reliable protection can not be realized, so it can not meet the actual protection requirements of air compressor. Therefore, the failure of protection is also an undeniable fact. With the development of society and the progress of science and technology, the backward products will be eliminated, and the thermal relay has completed its historical mission.

Protection starter

The new generation of electronic motor protector is the product of the era of advanced electronic technology. It has incomparable and impossible functional characteristics of thermal relay. The protection starter, hereinafter referred to as starter, is a derivative series of a new generation of electronic motor protection products developed for the special protection needs of air compressor. The protection of air compressor motor has more scientific, reasonable and correct protection effect. The utility model solves the problems that the AC contactor is momentarily and continuously vibrated and the heat cannot be dissipated due to the poor contact of the switching element pushed by the air pressure automatic switch, the incorrect displacement of the free end of the elastic element and the frequent action of the air pressure automatic switch during the use of the air compressor, resulting in the burning out of the main contact of the AC contactor or the adhesion of the contact, resulting in the lack of phase or overcurrent operation of the motor, So as to make the motor burn out; It fills the gap of domestic magnetic starter with special motor protector module.

Functional features

a. The starter is designed with operation, phase failure and overload fault status indication to provide convenience for user maintenance.

b. The starter carries out signal detection and memory for the whole process of starting, running and shutdown of the air compressor, automatically adapts to the thermal power requirements of the air compressor motor, and can take different protection action time for different thermal states of the motor.

c. Since the starter is equipped with cold state and hot state overload protection characteristic k-coefficient curve detection circuit and imbalance detection circuit, it can protect the possible phase failure, overload, locked rotor, blocking, three-phase imbalance and other faults of the air compressor motor during starting and operation.

d. The starter is designed with self inspection function, which can distinguish whether the power input and itself work normally, and take corresponding self-locking measures.

e. The starter effectively solves the problem that the automatic air pressure switch refuses to act when the air pressure is too high, resulting in overload of the motor or blocking behavior that may occur when it is serious, as well as the transient shock of the automatic air pressure switch. At the same time, it makes the AC contactor produce continuous synchronous shock, resulting in the main contact of the AC contactor under the impact of large current, The heat cannot be dissipated and burned or adhered, resulting in the problem of phase failure operation.

f. The starter effectively solves the problem that the residual air pressure in the cylinder cannot be released after the check valve fails, and the air compressor cannot be shut down or unloaded, resulting in motor overload and burnout.

g. The starter effectively solves the problem that the air compressor is forced to start frequently due to the irregular peripheral air consumption. In addition, due to the generation of large current during starting, the heat surplus accumulation is not easy to be distributed, which makes the motor temperature rise too high and difficult to protect.

Application example of protection starter

1. Why does the protective starter have a good protective effect on the air compressor motor? The following examples are given for interpretation:

Example 1: the motor of the air compressor is burnt out for many times

Fault phenomenon: a motor of an air compressor in a shoe company had two burning accidents in a short time. The power switch tripped, and the burning time was on the occasion of power conversion. When checking the motor, it was found that there were signs of overheating, bonding and carbonization of three-phase windings, which was preliminarily determined to be caused by serious overload of the motor

To.

Cause analysis: the air compressor works in automatic gear. When the power is switched, the air pressure of the air compressor reaches the maximum upper limit. Due to the failure to discharge the air pressure in the cylinder during shutdown and the direct self starting during power transmission, the air pressure is high and the motor is difficult to start, which is regarded as the nature of locked rotor fault. The working circuit of the motor has no reasonable protection and is only provided with a power switch instead. Therefore, when serious overload and locked rotor faults occur, although the circuit breaker trips, it is too late, at the cost of burning the motor.

Improvement and treatment: after the reason is clear, the new type of protection starter is replaced. In case of various faults afterwards, the protection module plays a due protective role and avoids the motor from burning again.

Example 2: the AC contactor of the magnetic starter of the air compressor is frequently damaged

Fault phenomenon: an air compressor in a factory, with a power of 18kw, failed to start and operate normally after one year of operation. After preliminary inspection, it was found that the main contact of the contactor had been obviously damaged. Similar faults occurred a few days after replacement, and it was replaced twice continuously in more than a month.

Fault analysis: the AC contactor is damaged continuously in a short time, which is a very abnormal phenomenon. What is the reason? Through careful inspection and observation, the maintenance personnel found that at the moment of shutdown of the air compressor, the AC contactor occasionally had rapid shock and the arc spark of the main contact was very obvious. Then what causes the contactor to vibrate? Check the wiring contacts in the measurement control circuit. There are no obvious fault doubts about the relevant electrical appliances. After analysis, it may be caused by the abnormal air pressure automatic switch contactor.

Treatment method: therefore, the occurrence rate of such fault phenomenon is accidental, with strong concealment and difficult to find. Finally, it was decided to replace the special protection starter. After the replacement, the fault phenomenon disappeared in the startup test. After half a year of use, no similar fault phenomenon occurred.

Example 3: Starter action protection caused by mechanical failure of air compressor

Fault phenomenon: in a plant, an air compressor trips and stops during operation, and the fault indication is "overload". When it is restarted after power-off delay reset, it is found that the motor is difficult to start, with obvious electromagnetic and vibration noise. In a very short time, the starter protection module acts and the power supply trips.

Inspection: after stopping the machine for inspection, the circuit and electrical appliances are basically normal and there is no doubt about the fault. It is more flexible to rotate the motor shaft manually. When rotating the flywheel of the air compressor, it is found that the transmission is stuck and seriously blocked. It is suspected that the mechanical fault of the air compressor is found. Disassemble the cylinder head and find that the piston connecting rod in the cylinder is damaged.

Cause analysis: after serious mechanical damage of the air compressor, the transmission is blocked and the motor cannot be started. In terms of protection, it is called locked rotor fault. When the motor is locked rotor, the current is greater than the normal starting current, because the starter protection module has strong inverse time limit characteristics and starts the protection action in a very short time. Locked rotor is regarded as serious overload fault, and this type of starter protection module shows locked rotor like "overload" indication.

Treatment: after the mechanical fault of the air compressor is eliminated, it will turn to normal start-up and operation.

Example 4: protection of frequent starting of air compressor

Fault phenomenon: an air compressor in a certain plant has action tripping for many times during operation, and the fault indication "overload" can restart normal operation after each reset.

Inspection: the repair personnel shall check the circuit and electrical appliances, and there is no doubt of fault damage. The transmission parts shall be rotated manually, which is relatively easy and free from obstruction. The starting current and running current measured with a clamp multimeter are within the normal range, but it is found that the temperature rise on the surface of the motor is high and feels hot.

Fault cause analysis: since there are no faults in electrical appliances, circuits, machinery and current, the doubt is basically normal. What is the reason for the "overload" action? Through field observation, it is considered that it is caused by frequent starting of motor. Due to the large peripheral gas consumption, the motor is forced to start frequently. The accumulation of heat surplus generated by short-time and frequent starting current cannot be dissipated to overheat the motor, while the starter protection module is designed with thermal memory function, which can identify the actual thermal state of the motor and make protection action like "overload". The possibility of motor burning due to overheating is avoided.

Treatment: adjust and control the external gas consumption, and restore the normal operation after startup. No frequent action tripping is found.

2. Why can the abnormal operation of the automatic air pressure switch of the air compressor be resolved by protecting the starter?

Solution: in the protection and control of the original thermal relay, the contacts of the pneumatic automatic switch and the "95 and 96" contacts of the thermal relay are connected in series in the AC contactor circuit. When the pneumatic automatic switch has abnormal shock jump, there is a certain time difference, which can eliminate this abnormal phenomenon. Theoretically, it can be solved by using the delay principle.

Firstly, the working power supply of the starter protection module is changed to be controlled by the air pressure automatic switch of the air compressor with a specific wiring mode.

Normal working procedure flow: when the air pressure of the air compressor is lower than the lower limit of the set pressure, the automatic air pressure switch is closed, the starter protection module has the working power supply, the internal control protection drives the AC contactor to pull in, and the air compressor motor starts to run.

When the air pressure rise exceeds the upper limit of the setting value, the contact of the automatic air pressure switch will be closed and turned to open, the AC contactor in the circuit will lose power and release at the same time, and the motor will stop running. In this way, the start and stop of the air compressor motor will be automatically controlled repeatedly.

The specific wiring mode of the working power supply of the protection starter is to obtain the discharge time condition for the internal delay circuit of the starter protection module. Therefore, when the automatic air pressure switch has abnormal shock action, the internal control delay action of the starter protection module. So as to make up for the improvement of the shock frequency of the automatic air pressure switch and stabilize the operation of the circuit.