Analog Output From Telemecanique Vfd

[gauravtri79]

I have an Altivar 71 VFD telemecanique make. I have given its analog output voltage to 2 nos. speed indicators in parallel. Now I have to show the same speed in SCADA screen. For doing this, I have given AO of VFD to AI of Siemens S7-300 PLC. But on checking I found that AI of PLC keeps fluctuating at max. value and actual value in Simatic manager. IThere is no fluctuation in indicators reading but on SCADA it keeps fluctuating every second. Please help me if there is some interference. There is one parameter 'Analog output interference filter' in VFD. If it would be helpful and what should be its value (Range from 0.00s to 10.00s)

[marke]

It sounds as though you are using the analogue output in voltage mode.

If the cable from the VFD to the SCADA is unscreened, you will pick up a lot of noise.

Even with a screened cable, you can still pick up noise. You would be able to see the noise on a broadband oscilloscope.

You could try connecting a capacitor across the SCADA analogue input. This may reduce the problem.I would suggest a 100nF ceramic in parallel with a 10uF capacitor.

 

Best regards,

Mark.

[gauravtri79]

Thanks Mark,

I am using screened cable. Moreover, When I give analog voltage signal through voltage source externally through same cable, there is found no any fluctuation. The problem is only with VFD analog output. The interference may be coming from VFD. I have cross checked with AO of another nearby VFD but result is same.

It sounds as though you are using the analogue output in voltage mode.

If the cable from the VFD to the SCADA is unscreened, you will pick up a lot of noise.

Even with a screened cable, you can still pick up noise. You would be able to see the noise on a broadband oscilloscope.

You could try connecting a capacitor across the SCADA analogue input. This may reduce the problem.I would suggest a 100nF ceramic in parallel with a 10uF capacitor.

 

Best regards,

Mark.

[marke]

I would almost guarantee that the noise is coming from the VFD, the question is how to quieten it down.

4 - 20 mA is much more noise independent.

It is possible that you have a multiple earth situation and are therefor seeing earth loops. i.e. the HMI 0V is connected to the VFD and connected to another device which is also earthed. You may need a signal isolator between the VFD and the HMI.

 

You could try about a 100 ohm resistor is series with each line from the VFD to the HMI, that may help.

 

Best regards,

Mark.

[jOmega]

Here are some additional points to consider.

 

1. Some VFD's have Two AOs so that one can deliver a voltage (0-10v) and the other can deliver a current (0 or 4-20 mA); each of which is proportional to output frequency (relative speed).

 

2. The ATV71 provides ONLY a single 10-bit DAC to generate the analog signals. User programming choice determines whether the DAC output is configured in a pull-down voltage scheme or as a

current-sinking scheme to source 0-10v or 4-20 mA respectively to the AO terminals.

 

As MarkE pointed out, the analog voltage output is designed to terminate into a nominally high impedance as compared to the analog current output which is designed to terminate into

a nominally low impedance. Because of the higher impedance, the voltage output will support ambient noise picked up from the environment and wiring; the analog current output with

its lower terminating impedance is less susceptible (less likely to support)..... noise.

 

3. The DAC in the ATV71 has the following specifications:

 

• analog output 0 to +10 Vdc, min. load impedance 470 Ω

or

• analog output X - Y mA, X and Y can be programmed from 0 to 20 mA

• max. load impedance 500 Ω

• 10-bit resolution, reaction time: 2 ms ± 0.5 ms

• Accuracy ± 1% for Δθ = 60°C (140°F), linearity ± 0.2% of max. value

 

From this it is important to note that the total impedance, as seen by (connected to) the AO terminals must NOT be < 470 ohms (higher is better).

 

You have three loads connected to the AO terminals. The impedance of each load is not stated in your post. However, you do need to assure that

 

when all three loads are connected to the AO terminals, that the effective total impedance is NOT LESS THAN 470 ohms.

 

If it is less.... then you will need to obtain and install an interposing signal conditioner as MarkE suggets.

 

Consider: if each of the two Speed Indicators has an impedance of 1000 ohms, then connecting them in parallel to the ATV71 AO terminals, results in

 

an effective total impedance (as seen by the AO terminals) of 500 ohms.

 

Assume that the SCDA has an impedance of 1250 ohms. Then adding it to the ATV71 AO terminals in parallel with the two Speed Indicators, results in a

 

total impedance of 357 ohms; which is 113 ohms LESS than the absolute minimum value specified. That's a problem !

 

 

 

4. Here is a diagnostic experiment you can try:

 

Disconnect the two meters, leaving only the analog input of the SCADA unit connected to the ATV71 AO terminals.

 

Do you still have the problem? If not then I would reason that there is an impedance problem as discussed above.

 

Connect the meters one at a time ..... see if the problem comes back when the first meter is reconnected... and then when the

 

second meter is reconnected. If the problem returns at this stage ... you most likely have an impedance problem.

 

 

HOWEVER, if you see the problem with ONLY the SCADA unit connected to the ATV71 AO terminals, then the

 

problem could be a common-mode noise loop existing between the VFD and the SCADA system.

 

 

5. In the past, I've had success adding .47mfd low-inductance capacitors to filter out the noise.. I've added them from each of the terminals to earth ground

 

With just the SCADA unit connected to the ATV71 AO terminals, you can try unconnecting the screening from earth... at both ends, then try reconnecting

 

at the ATV71 end and observe. If that doesn't make a difference, unconnect it and connect it at the other end (SCADA) ... see if that makes a difference.

 

 

6. Adding additional components to the circuit can affect linearity and range.

 

 

7. An Oscilloscope will be most useful in pursuing the problem and filtering/earthing/isolation will most likely be required to eliminate it.

 

 

 

Kind regards.