VFDs replace mechanical devices and PLCs for lead-lag pump control.
When a pump is started and run across the line, it will provide maximum flow at optimal efficiency. However, a pump motor operated continuously at full rated speed is inefficient during periods when maximum flow is not required, and the pump and motor will wear out faster than if the pump speed were regulated.
The affinity laws—which express the relationship between a pump's shaft speed, volumetric flow rate and power—state that running a pump at rated speed generates rated flow at rated power. However, a pump motor powered directly from the line will still be running near full power during periods of reduced demand. Operating in this way is inefficient, especially if the pump is oversized to accommodate future demand. One method of designing a better pumping system is to use lead-lag control.
Lead-lag control is nothing new in the pumping industry, and a common lead-lag design features a duplex booster pump system to maintain building water pressure. In these designs, lead-lag control is used so that a system that requires multiple pumps to maintain water pressure at peak demand will not have to use all the pumps when the demand is reduced or nonexistent.
Preferably, some of the pumps can be shut down when demand is low, and the system can be designed so that the water pressure can be maintained by a single pump during periods of low demand. Properly designed lead-lag systems will improve the efficiency, performance and longevity of the pumping system.
VFDs Beat Mechanical Control
A lead-lag system requires that the pump controls switch the pump motors on and off based on demand. The simplest way to switch the motors on and off is with an across-the-line starter in the form of a contactor. However, starting a motor directly across the line at full power presents issues. Powering a motor directly across the line will result in high starting currents. The current draw from a line started motor can be in excess of six to seven times the motor's full load amperage rating. This not only stresses the motor, but can result in excess demand charges from the electric utility.
Across-the-line starters can be used in combination with other mechanical devices to regulate the pump's output—including balancing valves, ball valves and butterfly valves. Some of these devices will require manual adjustments, while others are designed to automatically control pressure. These mechanical devices can reduce pressure to desired levels but do not solve the across-the-line starting issue nor significantly contribute to energy savings.
A better approach for maintaining the desired water pressure is to regulate the speed of the pump motor with a variable frequency drive (VFD). VFDs control the speed of AC induction motors by controlling the frequency and voltage supplied to the motor. While mechanical devices can be installed at a pump's output to adjust flow, a VFD regulates flow by adjusting the motor/pump speed. This approach is much better for a number of reasons:
- Uses a minimum amount of energy
- Reduces motor starting current
- Provides a degree of motor protection
- Cuts the wear and tear on the motor
- Simplifies the design of the flow control system
- Provides extensive diagnostics
- Reduces the required maintenance
When a VFD is used, the controlled motor's efficiency is optimized and runs at maximum efficiency regardless of the required flow and corresponding motor/pump speed. The VFD input current rises linearly with respect to output power because the VFD can slowly ramp the pump up to speed. As a result, the typical six to seven times motor rated current seen with an across-the-line started motor is nonexistent with a VFD.
As a result, the negative impact of frequent start/stop cycles is greatly reduced because the VFD limits the motor's inrush current, which and prevents the motor's thermal rises that are inherent with across-the-line starting.
Basic VFDs provide phase loss detection and motor thermal overload protection. Advanced pump-specific VFDs offer features such as loss of prime detection, detection of a pump in a no-flow (deadhead) condition, low/high-pressure level detection, broken pipe protection and pump over cycle protection. These factors make VFDs superior to mechanical devices for regulating pump flow, and other features of VFDs allow direct implementation in complex pump control applications.
Integrated PID Control Is Better Than External PLCs
(Moved later in the paragraph) Many VFDs have a built-in proportional-integral-derivative (PID) loop controller, which makes external PID controls unnecessary. These External controls are typically implemented with a programmable logic controller (PLC) or a stand-alone proportional-integral-derivative (PID) controller. In either case, an extra component must may need to be purchased, programmed, installed and interfaced to the VFD. However, many VFDs have a built-in proportional-integral-derivative (PID) loop controller, which makes external PID controls unnecessary. Performing PID and other control functions internal to the VFD saves money, reduces complexity and cuts installation effort. The advantages of this setup are:
- Costs less
- Simpler to configure
- Reduces integration among control components
- Requires less space
- Requires less maintenance
In pump control applications, the VFD's built-in PID feature is used to continuously monitor a feedback signal to maintain an output set point by adjusting motor/pump speed. In the case of a system used to maintain water pressure in a building, the VFD automatically increases the motor's speed and the pump's output as the feedback signal shows a drop in pressure.
Conversely, the VFD reduces motor/pump speed and eventually shuts the system down based on pump flow requirements as determined by the feedback from the pressure sensor. As a result, the VFD controls the motor/pump to generate the required flow to maintain the demanded pressure, all without the need for additional external automation components.
[[{"type":"media","view_mode":"media_large","fid":"670","attributes":{"alt":"","class":"media-image","height":"170","id":"1","typeof":"foaf:Image","width":"307"}}]]
Figure 1. VFD with across-the-line lag motors and pumps
Using VFDs to Implement a Lead-Lag System
A single VFD can be used in a basic lead-lag system to regulate pressure by directly controlling the lead pump motor's speed, while turning the lag booster pumps on and off through each pump's across-the-line starter (See
