The 4-20mA Current Loop
The 4-20mA current loop is used to transmit analog data representing various process variables like level, pressure, temperature or flow. Although conceptually simple, a 4-20mA current loop can be tricky to troubleshoot. Current flows in a single direction through each device in the loop (see Figure 1).
There are two types of instruments used in a current loop: loop-powered (2-wire) and non-loop-powered (4-wire) instruments. Loop-powered instruments are powered entirely from a DC voltage supply placed within the 4-20mA loop. Non-loop-powered devices require connection to a separate voltage source for the instrument to be used in the current loop.
Take care when setting up the loop to ensure that the impedance of the loop instruments and the available DC voltage supply allows for the full 20mA to flow through the loop. To ensure your loop will work properly, add up the voltage drops across each device in the loop, and make sure this number does not exceed the DC power supply voltage used to power the loop. While some controllers have internal DC power supplies for use with an analog level transducer input, these power supplies may not be sufficient when additional instruments are added into the current loop.
For a simple loop containing a controller and a transducer (see Figure 1), the internal supply shown is more than adequate to power the loop. For the example in Figure 2, however, the controller's internal 20VDC power supply is insufficient to power the loop. Most transducers require a minimum of 10-12VDC. A typical analog level input for a controller will have a voltage drop of 5VDC at 20mA. A dual-channel intrinsically safe barrier with a clamping voltage of 28VDC may have a voltage drop greater than 7 VDC at 20mA.
The loop in Figure 2 would require an external DC power supply rated at 24VDC for this application. If an external power supply with a DC voltage greater than 24VDC is selected, then use caution to ensure the clamp voltage of the barrier zener diodes is not exceeded when there is 4mA of loop current. A zener diode is a semiconductor device that is essentially off until the voltage across the diode reaches its zener, or clamping voltage. As the voltage level approaches the clamping rating, the zener diode begins to conduct, clamping the voltage at the voltage rating of the zener diode.
Two of the most common obstacles encountered when working with a 4-20mA loop are polarity and unintentional grounding. If an instrument in the loop is connected with an incorrect polarity, current may not flow in the loop. Grounding the loop in more than one place also creates a problem since the current does not flow through the entire loop.
When troubleshooting a 4-20mA current loop, carefully select the analog intrinsically safe barrier. Ensure the clamping voltage rating of the barrier is higher than the power supply voltage powering the loop. If the DC voltage powering the loop is greater than the clamping rating of the barrier, the loop will not achieve the full range of 4-20mA current flow due to the zener diodes of the barrier going into the conduction mode.
Isolated Versus Non-Isolated Analog Inputs
Isolation with regard to a controller's analog input has to do with whether the analog input (-) common is connected to the device's internal power supply common, which is typically connected to control panel ground.
The isolated analog input common (-) will be free from any connection to control panel ground and any connection to the common of any other analog loop. This fact is important, as a separately powered loop instrument may have its common tied to control panel ground in another loop.
For the majority of 4-20mA analog level input applications, isolation for the level input of the controller is not required. Figure 1 shows the level signal coming from a loop-powered transducer. Since the loop-powered transducer does not have a ground connection, no isolation is required at the controller analog input.
Here are four terms additional terms needed for a better understanding of the 4-20mA loop:
- Analog Common: The negative (-) terminal of the 4-20mA loop.
- Power Supply Common: The negative (-) terminal of the DC power supply associated with the source of power for the analog loop.
- Power Supply Ground: The ground connection for the DC supply. The power supply common may or may not be internally connected to the power supply ground.
- Control Panel Ground: The ground connection for the incoming power to the control panel.
If the 4-20mA level input current loop contains any externally powered instruments, ensure that the analog signal from this device is either isolated or that the analog common from this remotely powered instrument is not connected to a panel ground. If any of these externally powered instruments have their common lead for the 4-20mA circuit referenced to a control panel ground, then the analog level input for the controller should be specified as an isolated input.
Figure 3 shows an example of a 4-20mA loop with a remotely powered device that references the analog common to a panel ground. The analog input common of the controller is referenced to its internal power supply common, which in turn is connected to power supply ground and panel ground.
While the current flow in the 4-20mA loop would be read at the remote display, the current flow would bypass the controller analog input because the return path for the loop current would be the panel ground connections. To remedy the loop shown in Figure 3, the remote analog common must be disconnected from control panel ground, or the remotely powered device should be changed to a unit with an isolated analog circuit. Otherwise, the controller would require an isolated analog input circuit.
Another similar symptom requiring an isolated analog level input is an incorrectly displayed value. The analog input level signal, due to the difference in ground potentials, would divert some of the current loop into the controller analog input, and the remainder of the loop current would flow between the panel ground connections.
While it is rare that the analog level input requires isolation, any auxiliary analog inputs and outputs should always be isolated. Auxiliary analog current loops always have a possibility of being connected to field instruments and equipment that would have the common side of the auxiliary 4-20mA signal referenced to the control panel ground of the field instrument. A good set of field drawings for the person wiring the panel is helpful to reduce the chance of any of these problems occurring.
Properly Sizing a Submersible Pressure Transducer
The most commonly used submersible pressure transducers in the wastewater industry are 5-psi, 10-psi and 15-psi transducers. The 5-psi transducer has an operation range of 0- to 11.5-ft, over which the output will be 4 to 20mA. In other words, when there is no liquid above the unit (0-ft), then the output will be 4mA, and with 11.5-ft of water above the transducer, the output will be 20mA. The 10-psi transducer has a range of 0- to 23.1-ft for an output of 4-20mA, and the 15-psi transducer has a range of 0- to 34.6-ft for an output of 4-20mA. (1-psi = 2.31-ft.)
For peak performance in level control using a submersible pressure transducer, carefully determine the operating range of the wet well to match the correct transducer range to the operating range. Figure 4 shows a typical lift station wet well. The operating range is typically from the wet well bottom to the influent piping. Allowing the level to go higher than the influent pipe could cause backup into the system feeding the lift station wet well.
Figure 5 shows the resolution of the properly chosen submersible transducer for the lift station shown in Figure 4, where the 4-20mA signal corresponds to a level of 0- to 11.5-ft. Installing a submersible transducer with a greater range than the actual operating range (see Figures 6 and 7) results in a resolution loss. This resolution loss is due to the amount of useable 4-20mA signal from the transducer reduced to a point where reliable control may no longer be practical.
Linear Level Control Versus PID Level Control
There are two level control methods: linear and PID. Linear level control, also known as proportional control, is used for both VFD and constant speed applications, but PID (Proportional-Integral-Derivative) control is used only with VFD applications. Constant speed applications using a soft starting method offer benefits of reduced water hammer and reduced voltage sags on starting. A benefit to VFD application is a lower operating cost when the pump(s) are operated at speeds less than 100 percent.
For most lift station control applications, proportional control is sufficient. For a typical constant speed duplex lift station, the range of control would include the pump-off point, the lead pump-on point, the lag pump-on point and the high level point. The pump-off point does not initiate any control. Its purpose is to create a hysteresis from the lead pump-on point to the pump-off point, which allows for off time between pumping cycles and keeps a pump from being called just after it is turned off.
For linear level control, the relationship between the level in the wet well and pump control is proportional (i.e., if the level calls for a pump to run, a pump is called to run until the off point is reached). Advantages to linear level control in lift station applications are simplicity in setup, lower equipment cost and no "sludge ring" in the wet well since the level does not stay in one place for any length of time.
A typical setup for VFD applications using linear level control is shown in Figure 8. Setup of this speed versus level curve requires three parameters: VFD minimum speed, the level at minimum speed and the level at 100 percent speed. A VFD will operate in the variable speed mode until it reaches 100 percent speed. When an additional VFD is called to run, the additional VFD will be brought on at 100 percent speed, and both VFDs will ramp down together until their off points are reached. This control algorithm works well in lift stations where control over a range of level is desired.
When set up correctly, the control system will run one or more pumps at some speed between minimum speed and 100 percent, and continually make adjustments to the pump speed as needed to keep up with the flow into the lift station.
There are times when PID control, also known as setpoint control, is desired. PID control is designed to maintain a setpoint for a process variable such as level, flow or pressure. As the process variable begins to move away from the setpoint, the Proportional part of the PID control makes a change to the speed reference signal proportional to the error, or the difference between the setpoint and the controlled process variable.
The Integral part of the PID loop determines how fast the speed reference is being changed to move the process variable back to the desired setpoint. The Derivative part of the PID loop slows the rate of change of the speed reference signal by reducing the "overshoot" effect of the integral part of the PID control. PID control requires the control system to be "tuned" or adjusted for the best performance of the control system.
A typical example of setpoint control using flow would be controlling the flow from a master lift station feeding a treatment plant, or the flow of the treatment plant influent into the treatment plant. PID control will feed the treatment plant at a constant rate to maintain the process control within the treatment facility. An example of setpoint control using pressure would be the control of a booster system for maintaining building water pressure. PID control is only necessary when tight, accurate control is required for the application.
Backup Control and Why It Should Be Separate
Many controllers can perform various types of backup control. However, good engineering design practice dictates that backup control be a separate function from the main controls, and that backup control always be included when the main controller is electronic. After all, there is no guarantee that the controller's backup control features would continue to work in the event of a power surge or lightning strike.
The design of the backup control can be anywhere from a redundant controller to two floats or two single-point conductance probes operating between a "high" and an "off" point. Backup control should be kept as simple and reliable as possible since it is the key to the reliability of any pumping application.
Pumps & Systems, September 2008
