Simple one; HMI rpm display wrong

[plutoman 0]

Hello everyone, its been a long time. I have a question regarding incorrect rpm reading on my display, I have a Panel View 600 on an extruder with a large DC motor driven by a DCS800 ABB drive. The problem is that it shows 19 rpm when it should be 0 rpm, so basiclly it is of by 19. Can I just go and correct this on the display program or is this a drive setting issue. There is no encoder or tach on the unit currently. Any feedback would be appreciated. Thanks, Robert

[MrAutomation 20]

Is the panelview connected directly to the drive, or is it connected to a PLC, which is connected to the drive (which is what I would expect). Open the application and see what the tag is and where it points to. That's the first step. I would suspect it's probably just pointed to the wrong tag, or it's a PLC error.

[plutoman 0]

Yes, drive is connected to PLC, I am not at work currently and will have to wait until monday to verify

[JRoss 84]

Could be a lot of things, but here's one I commonly see. Assuming there is a PLC in the mix, the speed feedback would be communicated from the drive to the PLC using an analog channel. This is a linear scaling of the speed to a variable voltage or current range. For example, say the analog signal is 0-10V and the motor speed range is 0-3000 rpm. At a motor speed of 1500 rpm, the analog signal should be 5V. Now that's in an ideal world, and if the analog signal is not calibrated, this could be off by a percentage. Let's look at your case. Given the above scaling, 19 rpm would equate to 0.63% error, which is fairly low. So what could be happening is that the analog signal is a little off, and the PLC is seeing a little voltage (or current) at zero speed. If the signal is voltage, you could easily verify this with a multimeter. You could do one of several fixes. If the drive lets you calibrate the analog output, then use that to adjust the zero and span of the signal so the PLC sees the correct value. If the drive doesn't have the adjustment, then you could put a scaling block in the PLC to do the same think. Or if the speed seems to be fairly accurate and you just don't like the floating value at zero speed, you could just push a 0 to the speed when the analog input signal drops below a certain threshold.