Analog Outputs on the GS4 Variable Frequency Drive (VFD) from AutomationDirect

GS4 Variable Frequency Drives have 2 analog outputs. And either analog output can be used to represent any of these things: the output frequency, motor speed, power factor. They can show you what the analog inputs are doing, be commanded from Serial Modbus, Modbus TCP or Ethernet/IP and you can even enter values directly from the keypad which is handy when you are trying to calibrate your system and just want to force a known output for test. Analog output 1 is voltage only. Switch 1 on the terminal board selects if it’s positive voltage or plus and minus voltage. Analog output 2 can be switched between voltage and current using switch 2 on the terminal board. Could you use Analog output 1 as a current output? Sure – you just have to provide an external 500 ohm load in series to get a 0 to 20 mA current output. Using parameter 4.53, you can even change it to 4 to 20 mA loop. Again, that assumes you connected an external 500 ohm load. Is this parameter really changing the output from 0 or 4 to 20 mA? No, not really. It’s just scaling the voltage from 0 to 10 volts to 2 to 10 volts. So when you add that external that 500 ohm load you get the correct currents. If you just want to use analog output 1 as a straight voltage output like the switch says, make sure that parameter 4.53 is at its default scaling setting of zero so you get the full range you expect. Analog output 2 is identical to analog output 1 with one exception: The switch really does switch output 2 between voltage and current. Other than that the same exact rules apply. If you are using the voltage setting, make sure the scaling is set to 0 to 20 so you get the full voltage range. If you have the switch set to current mode, then parameter 4.57 switches between 0 and 4 to 20mA. Analog output 2 has one other benefit: A bias control. That is, you can tweak the output level to accommodate anomalies in your system. Let’s do an example. Suppose we need analog output 2 in current mode, we’ll use the default 0-20mA range and we want to output a fixed level. If we tell output 2 to go to 50%, sure enough it gives us 10 mA. Perfect. But suppose our system really needs for the 50% level to be at 11mA. Using parameter 4.59 we can add in an offset – or bias – to make the value whatever we want. Let’s bump it a little, a little bit more, a little bit more – bingo. Now when any source that’s driving analog output 2 is at 50%, Analog output 2 will send out 11mA. Cool. You can also change the gain of the output from 0 to 500%. So if we double the gain, the output would look like this. If we reduce the gain to 50%, the output would look like this. One side note: If you are writing this value from Modbus TCP or Ethernet/IP using one of the optional com cards on the integrated PLC, note that there is an implied decimal – so to get 50%, you would write a 500. To get 500%, you would write a 5000. You can even change the behavior of the output. These parameters tell the drive to only output positive numbers, zero for negative numbers and or automatically shift the mid-point to the mid-point of the range to represent plus and minus. This is really intended to modify how to present the analog outputs when the motor is going in reverse. Do you just want to see positive values for forward and reverse, no value for reverse or do you want use half the range to indicate when it is going in reverse and half the range when it is going forward? Before we go any farther, take a look at this chart in Appendix C. It shows you everything you need to know about Analog output 1 in one quick reference. All the associated parameters, their options and default values. In one convenient place. There is an identical chart for analog output 2. The only difference is it has that extra bias parameter. We just did an example of using the fixed value as the source. We just put a 13 in parameter 4.50 or 4.54 for output 2 and then the value we want output in parameter 4.60 or 4.61. If you make the source one of the optional com cards, then you can set the output value simply by writing to this memory address via Modbus TCP or EtherNet/IP. If you are sending commands in via RS485 Modbus, you write to the same address. In both cases there are two implied decimal places. So to output 50% of the max range you would write 5000. It’s important to understand you only get one or the other: If you choose the RS485 port by selecting an 11 for this parameter, then writes coming from the any of the com cards will be ignored. And vice versa. If you choose a 12, then the RS485 port writes will be ignored. You can also output the analog inputs, the power factor and anything listed here. Most of the time you will probably want to monitor the output speed or amps or frequency. Regardless of what you are monitoring, all of them end up in one place that then goes through the same gain, bias, and negative value handling regardless of the source. That includes writes from the PLC. When the PLC writes to one of the dedicated analog output addresses, it also goes through all of this stuff. And remember: If the PLC references an analog output, whatever you select here will be ignored. Which raises an interesting question. Instead of talking to its dedicated output register, could the PLC tell the Drive parameters to go to fixed value and then send its output by writing to this guy? Sure! The cool thing about that is now the PLC doesn’t take the output control away from drive control so the drive analog output source parameters can still be used. And the PLC can switch these to anything at any time. Maybe for one operation you need the analog output to represent output frequency, but for another you need it to represent output current. The PLC can switch between these all day long BUT only if the PLC never references its own dedicated output register. Once that happens, nothing you or the PLC select here will work. Which is why this parameter is so important – at a glance you can see if the drive thinks the PLC owns the analog output. If this block is solid analog output 1 is owned by the PLC and all of these will be ignored. Whether you selected them or the PLC did – it doesn’t matter. If this block is solid, analog output 2 is owned by the PLC and any function you select here will be ignored. So keep an eye on this analog output mask and consider using the trick of having the PLC control the drive parameters instead of its own dedicated register – that way you get both PLC and Drive control. And there is one other side benefit to controlling the analog output parameters from the PLC: If the PLC writes to its dedicated register – that’s a write only operation. If the PLC tries to read that value back it will get zero. On the other hand, if the PLC uses the drives the drive parameters to output a fixed value, it CAN read the output value back. This snippet of a chart in section 2 of the user manual shows how the analog outputs share an analog common – which is the same one as the analog inputs by the way – and that any cable shields should be connected to chassis ground. There is also a reminder to make sure the switches on the terminal board are set to give you the output range you need. Positive voltage or plus and minus voltage for analog output 1 and voltage or current for analog output 2. The analog outputs are located here on the terminal strip and remember if you remove these two screws you can pull out the terminal board to make your wiring easier. Slide it back in, re-insert the screws and you are ready to go. Click here to learn more about GS4 drives, here to learn about AutomationDirect’s free support options and here to subscribe to Automationdirects YouTube channel so you will be notified when we post new video content.