1. The power supply of CNC machine tools divides the power supply used by CNC machine tools into three levels. From primary power supply to tertiary power supply, the derivation relationship is in turn, and the frequency and difficulty of failure are also increased. The specific rating is as follows:
(1) Primary power supply. The primary power supply is a three-phase 380 V power supply supplied by the plant grid, which is the total energy supply for the operation of the CNC machine. The power supply is required to be stable. Generally, the voltage fluctuation range should be controlled at 5% to 10%, and there must be no high frequency interference.
(2) Secondary power supply. It is derived from the primary power supply by a three-phase power supply via a transformer. Its main uses are:
1) Derived single-phase AC 220 V, AC 110 V, power supply to CNC unit and display unit, as power supply for heat exchanger, machine control loop and switching power supply.
2) Some CNC machine tools derive three-phase low voltage power supply for DC 24 V rectifier bridge. Some CNC machine tools generate three-phase AC 220 V from a three-phase transformer and supply the servo amplifier power supply unit as its working power supply.
(3) Three power supplies. The three-time power supply is a variety of DC power supplies used in CNC machine tools, which are converted from secondary power supplies. There are several main types:
1) The DC voltage supplied by the servo amplifier power supply unit, the three-phase alternating current that is inverted by the servo amplifier component to a variable frequency and voltage amplitude to control the rotational speed of the AC servo motor.
2) The AC 24 V provided by the rectifier bridge is used as the “ready†and “controller enable†signal source for the hydraulic system solenoid valve, motor brake solenoid power supply and servo amplifier unit.
3) Low-voltage DC voltage provided by switching power supply or DC/DC power supply module. These voltages are: +5 V, ±12 V, ±15 V, which are used as power supply for measuring grating, numerical control unit and servo unit electrical board.
2. Devices used in the power supply loop of CNC machine tools The devices used in the CNC machine tools from primary power supply to tertiary power supply are:
(1) Workshop power distribution equipment, generally including: three-phase AC voltage regulator and circuit breaker (also known as air switch, or knife switch) connected to the workshop power grid.
(2) Machine tool components, including: filters, reactors, three-phase AC transformers, circuit breakers, rectifiers, fuses, servo power components, DC/DC modules and switching power supplies.
3, power failure example analysis (1) grid fluctuations too large plc does not work. The performance is that the PLC has no output. Check the input signal (power signal, interference signal, command signal and feedback signal) first. For example, a CNC lathe with the SINUMERIK 3G-4B system does not work with its built-in PLC. Using the observation method, first check the grid voltage waveform with an oscilloscope, and find that the grid fluctuation is too large, and the undervoltage noise jump duration is >1 s (external cause). Since the machine tool is in the commissioning phase, component failures in the power supply system should be excluded. Internal internal power grid interference measures (filtering, isolation and voltage regulation) show that the conventional power system can not cut off or filter out the grid undervoltage for too long. Noise, which is caused by insufficient anti-grid measures (internal cause), causes the PLC to fail to obtain normal power input and cannot work. Add an AC voltage regulator to the system power input, and the PLC works normally.
(2) Power failure. For a double-station CNC lathe, each station is controlled by a separate NC system, and the NC system uses the SINUMERIK 810/T system from Siemens. The NC system of the right station often stops the electric shutdown when the parts are automatically machined. After restarting the system, the NC system can still work automatically. Check the 24 V power supply load without a short circuit problem. The pattern was analyzed and two NC systems shared a 24 V rectified power supply. There are two possible reasons for this failure:
1) The power supply quality is not high, the power supply fluctuates, and the faulty NC system is more sensitive to the power supply requirements.
2) The problem of the NC system itself, the system is unstable.
According to this judgment, the 24V power supply voltage is first monitored, and the voltage amplitude is found to be low, only about 21V. It has been observed that this voltage fluctuates downward at the moment of failure, and the voltage immediately rises to about 22V after the NC system is powered off. Faults generally occur when the spindle is started. The cause may be a problem with the 24V rectifier transformer, insufficient capacity, or short-circuit between turns, causing the rectified voltage to be low, and the grid voltage to fluctuate, affecting the normal operation of the NC system. In order to determine the cause of this fault, the AC 380V supply voltage was increased to 400 V with an AC regulated power supply, and this failure did not occur again. To replace the 24V rectifier transformer for this problem, the problem is completely solved.
(3) A PNE480L CNC lathe produced by VDF.BOEHRINGER (Germany), when the main switch is started to start the CNC system, the other indicators are all on the display panel except the READY light is off. The CNC system of this machine is Siemens SYSTEM5T system. Since the fault occurs at the moment of power on, it should be checked whether the power-on clear signal RESET is abnormal. Because the DP6 light on the main board is on, and the DP6 monitors the DC power supply, it is necessary to check the relevant circuit for driving the DP6 and the related DC power supply. The steps are as follows:
Because the DP6 light is on the alarm display, first check the relevant circuit of DP6. After checking, it is confirmed that the logic state of the flip-flop LA10 driving the DP6 is incorrect and has been damaged. After replacing with new parts, although the DP6 indicator is not lit, the fault still exists and the CNC box still cannot be started. Check the *RESET signal and the connection of each connector in the CNC box is good, but the *RESET signal is not normal, and it is found that the logic relationship between the LA01 and the NOT circuit in the A38 position is not correct. Then each DC current is checked.
Check ±15V, ±5V, ±12V, +24V, and find that the voltage is -5V to 4.0V, and the error is over ±5%. Further inspection revealed that the printed circuit board copper foil was broken at the solder fillet of the filter large capacitor C19 after the rectifier bridge of the circuit. After soldering it, the voltage is normal, the logic relationship of the LA01 circuit and the *RESET signal are correct, the fault is eliminated, and the CNC box can start normally.
(4) Return to the reference point exception. This is because the condition that the reference point must be returned does not satisfy the direction that must be returned to the reference point and cannot be too close to the reference point (more than 128 pulses) and the return reference point cannot be too low. The processing steps for this type of failure are [2, 3]:
1) >128 pulses from the reference point position, returning to the reference point. 1 When the motor is turned less than 1 revolution (that is, no 1 rotation signal is received), the start position at the time of return is first changed, and in the state where the position deviation amount is >128 pulses, 1 rotation or more is performed in the direction of returning to the reference point. Fast feed, detecting whether a 1 turn signal has been input. 2 The motor has been rotated more than 1 revolution, which is a separate pulse encoder. At this time, check whether the 1-turn signal of the pulse encoder is input to the axis card when the position is returned. If it is, the axis card is defective. If it is not input, first check whether the power supply voltage for the encoder is low (allowing voltage fluctuations at Within 0.2V), otherwise the pulse encoder is defective.
2) <128 pulses from the reference point position. 1 Check the feedrate command value, rapid feedrate override signal, and return to the reference point deceleration signal and the external deceleration signal. 2 Change the start position at the time of return so that the position deviation amount exceeds 128 pulses. 3 Return to the reference point speed is too low. The speed must be a speed at which the position deviation exceeds 128 pulses. If the speed is too low, the motor 1 signal is scattered and it is impossible to perform correct position detection.
(5) A machining center is equipped with the F-0M system. When the automatic operation is started, the tool magazine and the work table suddenly rotate at the same time. After resetting, adjusting the tool magazine and working table, it works normally. However, when the machine is restarted after the power is turned off, the servo alarm No. 410 appears on the CRT. Check the L/M axis servo PRDY, VRDY two indicators are on; the feed axis servo power AC100V, AC18V is normal; the x, y, z servo unit PRDY indicator is not on, the three MCC are not connected; measurement The voltage on it is found to be abnormal 24V, ±15V; the resistance of the power fuse on the axis servo unit is too large. After the replacement, the DC voltage returns to normal, the machine is re-run, and the alarm No. 401 disappears.
(6) Fault phenomenon: A company produces VF2 vertical milling machining center. After one year and seven months of operation of the machine tool, an x-axis over current or drive fault occurred during machining, and the machine stopped running. The alarm can be cleared with the "RESET" button and the machine can resume operation. This failure phenomenon occurs occasionally. After the machine is running for two years, the frequency of failure increases frequently, and the phenomenon of failover occurs: when the reset button is used to clear the alarm No. 161, the alarm information is reported to No. 162 (Y-axis over current or drive fault). ), if cleared again, then forward the z-axis again, and so on. The machine is no longer able to maintain operation.
Fault analysis and inspection: According to the fault alarm information, the phenomenon is transferred between several servo axes. It is easy to see that the fault occurs in the common link related to each servo axis, that is, it is the "position control board" or servo unit of the numerical control unit. The power component has failed. The position control board is one of the numerical control unit components. According to empirical analysis, the probability of failure of the numerical control unit electrical board is very low, so analyzing and checking the servo power supply component is a more feasible troubleshooting entry point. The inspection found that the servo power supply of this machine is divided into two parts, of which the output of low voltage DC ± 12 V is the switching power supply. The measurement results are: +11.73 V, -11.98 V. Analysis of this result shows that the positive voltage output is 0.27 V lower and the voltage is reduced by 2.3%. Due to the lack of a quantitative concept, it is assumed that this switching power supply is faulty if other sources of failure are not found.
Troubleshooting: In order to verify the output voltage deviation is the root cause of machine faults, replace the original power supply with a WYJ type dual transistor DC voltage regulator, adjust the two output voltages symmetrically, and adjust the amplitude to 12V. disappear. In the next 20 working days of the test run, the fault will not recur. It is fully confirmed that the fault is caused by damage to this servo power supply unit.
Theoretical analysis: Operational amplifiers and comparators, some are powered by a single power supply, some are powered by dual power supplies, and dual-powered operational amplifiers require positive and negative power supply symmetry, the difference is generally not greater than 0.2 V (except for operational amplifiers with adjustment function) Otherwise it won't work. The faulty power supply, the two output voltages differ by 0.25 V, which exceeds the error tolerance range, which is the root cause of the fault.
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