GS4 VFD Sleep Mode 2 - PID Frequency Cmd Controlled from AutomationDirect

Sleep Mode 2 uses the PID Generated frequency command to decide when to automatically turn the motor on and off. This is especially important in applications like PID Driven tank filling. I have PID setup to fill this tank to this level. Let’s turn water level LEDs on to make that easier to see and open the valve a little to create some demand. When I hit run, PID sees the water level is down so it cranks up the blue Frequency Command and that exits the drive as the yellow frequency output – which has the acceleration ramp added in. This red line is the error – the difference between where the water level is and the setpoint where we want it to be. The error gets smaller and smaller as the water level approaches the setpoint. And as the water level starts to get close to the setpoint, PID automatically slows the motor down. Looks like our demo needs to maintain the output frequency at around 16 Hz to maintain the water level at the setpoint for the particular demand I created by opening the drain valve a little bit. But if we’re just filling a tank, do we really need for the drive to be running at 16Hz to maintain the water level? That’s a lot of extra electricity and motor and pump wear and tear that really isn’t accomplishing much. And, since centrifugal pumps aren’t real efficient at mid to low speeds any way and they’ll actually stop pumping altogether if the frequency gets too low, why not just turn the motor off when the PID Commanded Frequency starts to get small? And then let the water level drain to some lower level before turning the motor back on? That’s where sleep mode comes in handy. Sleep Mode 2 monitors the PID Command frequency – the blue line on our chart here - and when it sees it slow down enough, it disconnects the Blue PID command frequency from the Yellow Output frequency which turns the motor off. And as the water level falls, Sleep Mode 2 waits for the PID command frequency to get large again before re-connecting it to the output frequency to the motor. That way the drive isn’t running the motor forever burning electricity and wearing out pump and motor parts. Let’s do a live example so we can see how this works. These are the PID parameters we are using. There is a separate video showing you how this was done so we won’t do it again here. And these are the Sleep Mode Parameters. We only need a couple of these to get started. First, we need to tell Sleep Mode to watch the PID Frequency Command to make its decisions. That puts us in Sleep Mode 2. Centrifugal pumps get pretty inefficient below 30Hz, so let’s have sleep mode turn the motor off at 30 Hz. As the water level falls, PID is going to automatically raise the command frequency to try and bring the water level back up. It can’t because sleep mode turned the motor off. PID doesn’t know that so it will keep increasing the output frequency to try and get the water level to rise. Let’s have sleep mode turn the motor back on when PID pushes the command frequency past 50Hz. That’s all there is to enabling sleep mode 2. Let’s try it. Suppose we want our setpoint to be when the tank is 95% full. 95% of our max drive speed of 60Hz is 57 Hz so we enter that here. Remember – because we are using PID this isn’t a drive frequency any more, it’s the setpoint. I’ve already setup the GSOFT2 scope function with the PID generated frequency command in blue and the actual output frequency in yellow. These are just analog potentiometers I connected to analog inputs 2 and 3 so we can use them as cursors. Let’s adjust those potentiometers to remind us that sleep is at 30 Hz and Wake up level is at 50 Hz. The tank is currently low on water, and the valve is closed, so when I hit RUN, PID sees the low water level so it would normally crank up the blue drive frequency command. Of course, it was already at 60hz when I started this trace, so the yellow output frequency immediately ramped to match it As the water level approaches the setpoint, the Frequency commands – which are on top of each other right now - start to slow down and when the blue PID command frequency command hits the 30Hz sleep level we specified, the yellow output frequency goes to zero which turns the motor off. Perfect – we are now NOT wasting energy on filling that last little bit. If I open the valve to create some demand, the blue PID frequency command goes up to try and compensate and when it crosses the green 50Hz wake up level, sleep mode allows that to pass through to the motor. Exactly what we expect. So now the GS4 drive is automatically filling the tank as needed by watching the PID Generated frequency command and only letting that through to the pump when it is efficient to do so. You can also delay sleep and wake up. Let’s the sleep delay to two seconds and the wake-up delay to two seconds. I’m also going to set this min drive speed to 30 Hz. You’ll see why in a minute. Run the same test, and this time Sleep Mode saw the PID frequency command hit the sleep level, then waited 2 seconds and then turned the motor off. So by adding an appropriate sleep delay you can get a little extra water in the tank that you didn’t get before because sleep mode was shutting the motor down before we reached our setpoint. This plateau is the output frequency you want the motor to run at during the delay. Maybe you don’t want to run the motor at full speed during this time while you are topping off the tank. Or maybe you don’t want it to run at all. Well, you can set that speed using parameter 6.26 – the min drive output frequency. That will only be enabled during this sleep delay time. One final note: PID is still running while Sleep Mode has the motor turned off. It can’t do anything because the motor is off, but it is still trying. Which means the integral term is still integrating the entire time the motor is off. Depending on your system, that can get quite large which means it will take a long time for PID to settle out when the motor gets turned back on. If it is bad enough, the process will completely miss the setpoint and possibly create a huge mess. The good news is, the GS4 drive has this Integral Limit parameter where you can specify a limit for the max duration of the integral term. That way you can control how quickly the process recovers after the motor has been off for a while. That should be enough to get you going with Sleep Mode 2 where the GS4 Drive monitors the PID generated Command Frequency to make it’s sleep and wake decisions. Click here to learn more about the GS4 drive. Click here to learn about AutomationDirect’s free award-winning support options and click here to subscribe to AutomationDirect’s YouTube channel so you will be notified when we publish new videos.