There’s a misconception that a VFD provides better resolution than a control valve. This isn’t always the case. Virtually every modern electronic control system today features some sort of digital or microprocessor control. There has to be an interface between the analog domain and the digital one. Both AC and DC drives use digital-to-analog (D/A) converters. These converters don’t provide infinite resolution — rather they quantize the signal. This produces a so-called quantization error. The number of digital bits is inversely proportional to the error. Process applications frequently use a “10 bit” D/A converter. This breaks up the output signal into 210 or 1,024 discrete levels from the zero point to the maximum.
For example, consider a DC motor speed control powering a gear pump. In a simulated flow control problem, with 10-bit output resolution for a 22-gpm maximum pumping system, this error amounts to 0.03 gpm for a separately excited motor and 0.2 gpm for a series wound motor.
In another case, a continuous process employed a positive displacement pump powered by a VFD-controlled AC motor. Several fluids were blended and the final product quality depended on an exact ratio of all fluids. The precision of this process can be compared to that of in-line pH control — a very small deviation in reagent ratio may result in very large pH changes. This process suffered quality problems. Mass-meter flow rate data at the same set point were archived in an historian and gave the histogram in Figure 3. The positive displacement pump curve has a slope of 0.1 gal/rpm flow. The motor synchronous speed is 1,800 rpm. The adjustable frequency AC drive specification has 514 frequency divisions for the published resolution. The flow controller was trying to find the right output, midway between these peaks, but couldn’t. This caused the process signal to hunt around the set point. Always suspect a quantization problem if a bi-nodal process variable histogram distribution occurs with a continuous steady-state output signal. The plant replaced the VFD with one that had better output resolution and solved the problem. This example is extreme; however it demonstrates that the problem can occur. Drives available today have much better resolution, 0.01%.
As the above illustrates, a VFD can affect the plant. Startup and shutdown operations differ from those with a full-speed pump and control valve. Remember with a VFD you’re controlling the power applied to the load while with a control valve you’re introducing a controlled restriction to the power already applied to the load.
Variable speed pumps offer a linear response within a given inlet and discharge pressure range. However, you must limit the minimum speed to ensure that the pump’s discharge pressure never drops below the static pressure downstream — otherwise, a disastrous flow reversal can occur. Also, you must add an on-off valve and coordinate it with the variable speed pump to provide positive flow shutoff.
Just as we have acceleration and deceleration lanes on and off highways, drives have acceleration and deceleration ramps. These ramps are used to limit the motor starting current. They act as integrators in the loop dynamics. The default setting usually is 5 sec. to 10 sec., a typical flow loop reset value. If the integrator in the PI or PID controller is set faster than the VFD ramp, the resulting closed-loop performance will exhibit a limit cycle. This cycle isn’t due to reset windup or saturation but, rather, occurs because the controller integrator is acting faster than the load can respond. Many controllers offer a feature that allows the reset action to be changed based on actual valve travel. Configure an auxiliary VFD output signal proportional to the actual load to the controller function block to eliminate the limit cycle.
Another factor is the process time constant. For drive-powered liquid flow control loops, the process time constant is larger during start up; in contrast, flow control loops with control valves have a smaller time constant. The loop will perform sluggishly during start up compared to a flow loop with a control valve and pump powered by an induction motor without a drive.