Analysis and maintenance of main drive failure of

2022-07-26
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Analysis and maintenance of main drive failure of CNC machine tools introduction with the rapid development of electronic industry, various speed regulating drives with high integration and advanced performance emerge in endlessly, which provides favorable conditions for the upgrading of CNC machine tools. However, for the reality that large and medium-sized enterprises are still unable to completely transform the old CNC machine tools, repairing the old drive system is still an arduous task on the maintenance front. The following is a talk about some faults and treatment methods encountered by the author in the maintenance of CNC horizontal and vertical lathes that use the dislocation touch non circulation reversible speed regulation drive system in the main circuit, for the reference of colleagues. 1. Fault - fault phenomenon: the 1.8m NC sleeper car stops until a loud noise is heard and the main power supply of the workshop trips

inspection: (1) the workshop electrician inspected the power supply system. At the place where the tripped automatic air circuit breaker is located, the automatic tripping linkage in the switch box has been corroded due to humidity. In addition, only a small part of one phase of the three-phase contacts can be contacted. (2) The workshop power supply transformer has small capacity and operates under overload. Its normal phase voltage is only 340v. (3) One thyristor has been burnt out. Looking at the driving circuit, it is found that the phase B trigger pulse is short and only one fourth of the normal trigger pulse amplitude. It is further verified that it is caused by the poor performance of the amplifier T3 in the phase B trigger circuit

the thyristor trigger pulse circuit is shown in Figure 1. Since the principle of this figure is simple, it will not be explained here. Fig. 1 Analysis of thyristor trigger pulse circuit: the results of phase loss of thyristor in rectifier state and inverter state are different. In the rectifying state, the thyristor with high potential, such as sCR1, is always triggered. At the same time, do not apply too much force. The previous phase thyristor scr3 is turned off by withstanding the reverse phase voltage. During the shutdown of scr3, it is mainly in the reverse blocking state. Even if the latter thyristor is not triggered, scr3 will automatically shut off due to zero crossing at a certain time. However, in case of shutdown and speed reduction, that is, in the case of inversion (the thyristor with high trigger potential is also turned on, and the previous thyristor is turned off under reverse pressure), the thyristor at this time is in the positive blocking state for a long time during the turning off. In this way, if the latter thyristor does not turn on, the thyristor will continue to turn on for another cycle and enter the positive half cycle due to the discharge effect of inductance L, and the thyristor will continue to turn on and hinder the later thyristor from turning on at the same time. As a result, the forward voltage output by the thyristor and the electromotive force of the motor are superimposed to generate a large current. At this time, inverter subversion occurs, which may burn out the fuse or the thyristor. If the voltage supply system in the workshop is normal without large fluctuation, the thyristor may not be burnt out. The fault occurred due to the large fluctuation of AC voltage, small capacity of workshop transformer, overload operation, small amplitude of B-phase positive group trigger pulse and damage of main switch box of workshop power supply system

treatment: (1) replace the automatic air circuit breaker. (2) Replace with a new thyristor. 2 fault 2 fault phenomenon: when the 1.8m sleeper is inching, the flower plate swings back and forth

inspection: the ripple of ± 20V DC regulated power supply in the drive control system is measured to be 4V peak to peak, which greatly exceeds the specified range

analysis: in the amplification circuit of the control system, the high-pass and low-pass filters can filter out the harmonic interference signals in the feedback of the tachometer, the current feedback and the voltage feedback, but they can not filter out the harmonic components in the DC power supply circuit of the system itself. When the standard tensile sample is in the tensile state, it is equipped with an extensometer or attached with a strain gauge, because it exists in the whole system, When these harmonics enter the amplifier, it will block the amplifier and cause various abnormal phenomena in the system. In the inching state, due to the low speed of the motor, these harmonics have exceeded the voltage value at the time of inching, causing the system to oscillate, causing the spindle disk to swing back and forth, and once the harmonic signal is removed, the fault will disappear immediately

treatment: replace the 100mf and 1000mf filter capacitors in the voltage board and weld new capacitors. After measuring that the ripple is only a few millivolts, install the power board, start the machine for trial operation and eliminate the fault. 3. Fault three fault phenomenon: the fuse is burnt after the 5m vertical car makes a clang sound during operation and processing

inspection: it was found that there was no pulse output in the positive and negative groups of 5fc5fg and 5rg5rq (see Figure 2 for the circuit). According to the measurement results, the IC7 inverter was damaged, and it was found that the output waveform of 1fg1fc was much lower than that of other waveforms. Figure 2 Analysis of thyristor trigger pulse circuit: the driving voltage of the DC motor driven by the 5m riser is provided by the anti parallel rectifier circuit of the thyristor full control bridge. Two of the 12 trigger pulses disappeared, and the amplitude of the other trigger pulse was one third shorter than that of other normal trigger pulses. When there was a clatter of teeth (2) checking whether the digital circuit voltage was the impact sound of +5v wheel, it was mistaken that there was a problem at the coupling of the hydraulic motor, but after a while, the two fuses burned out. In fact, the fuse had been burned twice in the previous period of time, At that time, it was only thought that it was caused by accidental electrical instability. After replacing the fuse, the fault was eliminated. Due to the low speed of 5m vertical lathe, although the thyristor rectifier circuit is bridge rectifier, when the trigger pulse in the circuit is lost and the amplitude is small, the current will be discontinuous and the output voltage will be unstable, thus the motor speed will be unstable. The clatter at the beginning is actually a manifestation of unstable rotating speed. Fuse burning fault caused by intermittent current can occur at any time of shutdown and normal operation after operation

processing: replace the amplification tube T1 (the amplification tube in another trigger circuit, with the function as T7 in Figure 2) and the inverter IC7, and the fault is eliminated. 4. Fault four

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