Analog Inputs Part 2: Monitoring Temperature with the GS4 VFD at AutomationDirect

Using the Analog inputs for Temperature monitoring and control is a really cool feature of the GS4 drive. Normally you would have a motor with a sensor built in that you wire to the GS4 drive. But you could use it for what ever you want. For example, could you use this feature to monitor the temperature of a room so you know if there is a fire and need to shut the motor down? Sure! Of course, you would normally use the drives Fire Mode for that, but this would be another way to do it. Where ever the temperature sensor comes from, the drive can use that to monitor the temperature and issue warnings, faults and automatically back off the motor speed or even shut the motor down. The GS4 can use either PTC’s or RTD’s for temperature sensing. At a certain temperature, a PTC’s resistance changes dramatically. When you buy one, you just pick one that changes at the temperature you want. The GS4 drive just looks for that rapid change to know when things have gone south. RTDs on the other hand have a very linear response to temperature change so you can just buy one device and then pick whatever temperature you want the GS4 to react to. RTDs give you a lot of control over the fault reporting and drive behavior. When the temperature rises to Level 1 which you specify, a delay timer starts. If the temperature is still above Level 1 at the end of the delay, the GS4 can drop the motors speed to a value you set. If the temperature falls back below level 1 for greater than the delay time, then the motor will resume normal operation. If the temperature reaches Level 2 which you also set, the drive does whatever parameter 6.39 says it should do. It can just issue a warning, Issue a Fault and Ramp to stop, Issue a Fault and coast to stop, or do nothing. Once the temperature reaches level 2, the only way to get the motor to resume normal operation is to clear the fault. It doesn’t automatically recover like the level 1 condition did. Let’s do an example using an RTD so we can see how this works. For both types of temperature sensors, connect the device to analog output #2 because it can be setup as a true current source that can drive a fixed current through the sensor to create a voltage we can measure with an analog input. And since the resistance changes with temperature the voltage we read will be proportional to that temperature. You can use any analog input, we’ll use input number 3 for this example because I have the other inputs setup for demos in other videos. By the way, could drive this sensor with your own current source? Sure. You might want to do that if you are already using analog output 2 for something else. But in general, using the GS4’s current output feature is so easy and convenient, there is usually no need to spend the extra money on an external supply. I’m using the RTD in this room temperature sensor from Automation Direct simply because that’s what I had laying around, but any PT100 RTD will do. Great – we’re wired up, so let’s configure the drive. Analog input 3 only reads voltages so there is no switch to flip. If we were using analog input 1 or 2 we would make sure the switch on the terminal board is set to the 0 to 10 volt position. In parameter 4.04 we tell the drive to use Analog Input 3 as the temperature monitor by selecting an 11. Analog Output number 2 needs to output a fixed current, so we need to make sure his switch is set for current output, and we want to set the fixed output at 9mA, which is 45% of the 0 to 20mA range. Of Course, make sure analog output 2 is set for 0-20mA, not 4-20. Why did we choose 9mA? Because it’s enough current to reliably drive the RTD without having to worry about the RTD heating up and throwing off our temperature measurements. Ok, our hardware is wired and configured, now let’s setup the fault conditions. We need to tell the GS4 the voltages these temperatures generate. If you have a motor with a built in RTD, then look at the manufacturer’s datasheet – that will tell you what temperatures to watch out for. The example in the GS4 Drive’s user manual has a chart that will give you some idea what to expect. These motor insulation classes will have max temperatures of this. You will want to start protecting the motor a good 15 to 20 degrees below that. If you pull up a PT100 RTD chart on line you will see that at these temperatures of the PT100 RTD will be at these resistances. We’re pushing 9mA through the RTD, so it’s easy to calculate the voltages created. By the way, it’s called a PT100 RTD because the RTD sensor will be at 100 ohms at 0 degrees Celsius. For our demo we’ll just pick some numbers in that range which correspond to these temperatures. Again, normally you would make these whatever you need for your system. Just be sure the level 1 parameter is non-zero and below level 2. When we hit level 1 let’s delay 5 seconds and if we are still at level 1 after the 5 seconds, drop the drives frequency to 2 Hz. This number has to be greater than zero if you are using an RTD. Putting a 0 in here turns off the RTD function and enables the PTC function. If we hit level 2, then immediately do whatever is in parameter 6.39. In our example we’ll fault and ramp to a stop. That was a lot of parameters, so here is a summary of what we did. Ok. Let’s try it! I’ll enable the motor and here is a volt meter that’s across the RTD so we can keep an eye on the temperature generated voltage going into the drive. Here are the voltages for levels 1 and 2 we set a moment ago. I’ll take a soldering iron and heat up the RTD and sure enough when we reach the level 1 voltage, the GS4 waits five seconds, then reduces the motors speed to the 2 Hz drop frequency we specified and generates an overheat fault in the drive. If we cool the sensor off the motor resumes speed and the fault goes away AFTER the voltage has stayed below level 1 for that 5 second delay we set. That level 1 delay works on level one’s heating and cooling transitions. If we heat it up again, it passes level 1 and drops the frequency and generates a fault, just like before. Let’s keep heating to level 2. Sure enough the motor ramps to a stop with no delay. Exactly what we asked for. One thing to be aware of: once the motor hits level 2, it doesn’t automatically recover. Even if the temperature drops all the way back to normal. You have to clear the fault. I’ll hit stop on the keypad but you could also clear it remotely from PLC for example. Everything is pretty much the same when using a PTC device, except - since the PTC rapidly changes when it reaches its limit, you only have one level to set to look for that transition and the default value of 50 % is usually fine. The only other difference for PTC devices is you need to make sure the RTD Level 1 drop frequency is zero – that tells the drive we’re using a PTC sensor, not an RTD. That should be enough to get you started with temperature monitoring on the GS4 drive. Click here to learn more about the GS4 drive. Click here to learn about AutomationDirect’s free support options. And be sure to click here to subscribe to the channel so you can be notified when new videos become available.