The Latest Generation of VFDs


Written by:
Thomas R. Neuberger, Eaton Corporation

Reduce maintenance, improve efficiency and solve key pump application challenges with variable frequency drive technology.

Maximizing system efficiencies and protecting valuable assets help operators and pump manufacturers reach the overall goal—driving down energy consumption and improving system reliability. Increased awareness of the environmental and economic benefits of sustainable operations are driving change in industry regulations. It is critical to adapt quickly to evolving industry regulations and realities by adopting new technologies to control costs and reduce energy requirements.

Typically, energy consumption is 90 percent of the total cost of pump ownership. Best practices within pump applications today involve energy conservation, process efficiency and reliability, and a strong emphasis on long-term benefits.

 

Five Key Pump Application Challenges

Operators face many challenges that VFDs can help solve. They are detailed in this section.

 

Pump Cycling

Pumps turn on and off throughout the day. Starting a pump across the line creates mechanical stress on the system, which shortens the life of valuable assets. Variable frequency drives (VFDs) reduce flow, enabling the pump to run slower and continuously, which helps solve this problem.

Comparatively, other typical methods to reduce the impact of pump cycling involve valves or storage tanks, which either create maintenance issues or waste energy. Specifically, storage tanks allow the pump to fill extra liquid into a reservoir, buying time and avoiding some start/stop cycles. However, they can create ongoing maintenance issues by introducing another system, additional up-front costs and the need for extra space. Valves can be used to reduce flow, without making numerous starts and stops. However, that wastes a tremendous amount of energy because the pump continues to run at full speed even when it is unnecessary.

 

Water Hammer

The sudden shutoff of pumps and valves creates a backflow of water within the system that slams against the closed valve. Water hammer creates additional mechanical stress and wear on the pipes and valves, which reduces the life of valuable system assets. Reduced-voltage starting has evolved to help manage inrush currents and peak voltages. Solid-state soft starter technologies have also emerged to further eliminate shock to mechanical equipment. They reduce the load and torque applied to the motor powertrain during startup. Additionally, reduced voltage soft starters offer a wide range of current limit settings, providing greater control flexibility. For pumping processes, soft starters also help avoid water hammer in pipes by reducing line pressure so that valves can close gently and prevent a surge wave. Today, a wide range of motor starters offers high system configurability and flexibility for control gear design.

VFDs can eliminate water hammer by decreasing the pump speed rather than abruptly stopping the flow. VFDs offer all the protective features of reduced voltage and soft starters, while variable speed control allows the process to match energy consumption with process demands. By matching power consumption directly with process requirements and maintaining optimal operating parameters at all times, the energy savings realized by VFDs can generate investment payback periods of less than two years.Under Load, Torque Protection

In many pumping applications detecting under-load or under-torque conditions is important. When a pump is starved, deadheaded or stalled, the current and power demands drop. Typical overload devices, such as bimetallic relays, will not differentiate between full load and any value less than full load, therefore allowing a pump to run when one of these conditions is present. This can create mechanical wear on the pump and wasted energy, especially if demand requires that a second pump start. Beyond detecting current, VFDs measure power consumption or torque and allow the user to predefine thresholds. Demand curves produce a linear relationship between power or torque and load. This allows for a consistent sensitivity versus a sloped curve when compared with the relationship between current and load. With a sloped curve, the sensitivity on each end of the curve is lost. At full load, a slight change in that load results in a drastic change in current consumption. At a lower load, a slight change has almost no effect on the current, making the detection and prevention of an under-load condition difficult (see Figure 2).

 

Figure 1. The relationship between current consumption versus motor loading (top) and power consumption versus motor loading (bottom)

 

 

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See also:

Upstream Pumping Solutions

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