Analysis of PLC and frequency converter analog interference - processing

by:Coolmay     2020-06-16
Instance ( A) Description:

phenomenon of Siemens PLC in AO points 4 - all the way 20 ma current control signal, the output to the Siemens inverter, unable to control frequency converter. Fault lookup

1, the suspected problem, analog output interface card 4 - is measured with a multimeter 20 ma output signal, the signal is normal!

2, began to suspect that frequency converter control signal input end is the problem, in a same frequency converter, the problem still.

3, made 4 - a handheld transmitter 20 ma output signal source, standard output current signal to frequency converter, the converter under launched, so we exclude the analog output interface card and the fault frequency converter.

4, this theory is the interference of frequency converter caused by signaling to the analog channel.

5, in order to verify that the PLC analog 4 - Equipped with a 20 ma output channel TA3012 signal isolation module, the input terminal of TA3012 5, 6, the analog output module, inverter output terminals 1, 2, terminal, 3, 4 terminals connect external 24 VDC power supply, inverter normal boot.

6, accordingly concluded that the root of the problem lies in the inverter interference caused by analog channel. Attention

in the automatic control system of PLC and frequency converter to be used at the same time, should focus on pay attention to the contents:

PLC power supply and power system power supply ( Inverter power supply) Configuration, respectively, and the PLC power supply should choose isolation transformer;

power line as far as possible to separate from the signal lines, signal lines to do shielding;

both analog signal input analog signal output, analog channel are using signal isolation module;

software in the PLC program do filter design;

signal with the power to separate design.

instance ( 2)

some time ago to see an analog interference problem analysis and solving, we will encounter a lot of similar problems in practical application. Share: 'workshop has 10 units of 250 kw motor, load for high pressure pump. Inverter with PLC controller by DP with schneider ATV71 connection, use of PLC of Siemens 300, pressure transmitter for Siemens, transmitter for 4 - to the PLC Among 20 ma analog, input use the shielded wire.

debugging good run a week after everything is ok. Manufacturer, boot suddenly appear 8 pump, set 40 kg pressure, the actual value is 70 kg. Set 80 kg pressure actual value is 110 kg. Just began to suspect that the sensor fault, replace to the other pump all normal. Frequency converter after open, no. 3,4,5,6,7,9,10 pump appear similar problem. Speculation for pressure sensor disturbance by frequency converter. Manufacturers suggested adding metal shielding. But considering the construction difficulty, Control room away from the machine more than 30 meters, all go underground line LanGou) 。 And I think the inverter harmonic interference should be fluctuating pressure value, rarely seen have caused increases linearly. Just began to suspect that the factory program has a problem, because the screen side always show the pressure value is 40 kg, but the inverter output frequency of 70 kg. Factory don't agree with this point of view, said he absolutely with Siemens standard PID block.

a mystery. Inadvertently found that negative manufacturer sensor with shielding layer at the same time, access to the PLC analog input M. Remove the shielded wire after its access devices to fault elimination. Speculation: 2 wire sensors, the anode with PLC provides 24 v voltage, cathode is 4 - sensor is used to output 20 ma current place. Shielded wire with negative received after blocking online induction electromotive force with an electrical current into the PLC controller input caused a stack current, thus forming pressure value linear increase. Result no 2 days again appear the same situation, and is more serious is that one of the pressure sensor is dismantled, 40 kg pressure, final inspection found that the PLC input side anode stripping off long, short circuit between each other, causing other channel signal strung out. This just to think of it, just asked me if I start to debug manufacturer equipment with not a cabinet on the ground. Pressure sensor after the grounding, shielding line 2 interference is particularly severe. Cannot be displayed. I didn't want to more freely to a sentence, single side grounding. What they say. Now that I think about it, it should be at the beginning of each sensor output current side all through the shielding wire together caused a short circuit, after dismantled the sensor side of the earth. Because of the shielding wire missed along the normal of the signal.

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