To ensure the best system setup, end users must first consider operating conditions, then conduct thorough cost-benefit calculations.
by Mark Berube (AutomationDirect.com)
July 24, 2015
  1. Determine the pump's minimum to maximum pressure/flow (system) curve.
  2. Include alternate flow paths and related system curves for all operating modes.
  3. Specify the motor and pump to meet both minimum and maximum requirements on the pump performance curve.
  4. Estimate time the pump runs at low, medium and high flow rates.
  5. Estimate cost of kilowatt-hours (kWh) at each flow rate, including a 3-percent VFD loss.
  6. Translate these costs into yearly savings compared with running the pump at full speed.
  7. Add in any rebates from the utility for VFD installations.
  8. Add in reduced maintenance and longer pump life resulting from running the pump at lower speeds.
  9. Determine the cost of the VFD after installation.
  10. Add the cost of required supplementary equipment for power factor correction, noise filtering, etc.
  11. Compare costs and benefits to determine feasibility.

To evaluate potential cost savings, determine the operating speed range. Pumping system characteristics are defined by the system curve, which describes flow rate at a specific pressure. To determine the system curve, static and friction head must be known.

In an existing system, the system curve is calculated using data from pressure and flow measurements, control valve position and pump motor electric current measurements. If valves are used to add or remove equipment from the system flow path, then system curves must be created for each configuration. Once that is complete, compare the manufacturer's pump curve with the operating points on the system curve to determine the correct pump speed for each configuration.

To estimate potential savings from reduced power consumption, determine the amount of time the pump runs at the different operating points on the system curve. Hours spent operating at lower flow rates and head pressures along the system curve offer the greatest opportunities for cost savings. Variations in on-peak versus off-peak cost of electricity should also be considered.

Calculations should include pump efficiency and motor efficiency, as well as VFD losses of about 3 percent. When estimating potential cost savings, compare the operating costs of a fixed-speed pump against those of a variable speed pump for one year.

Installation & Control Considerations

The installation of a VFD may require additional components. Electromagnetic interference (EMI) filters, line/load reactors and radio frequency (RF) filters may be needed as part of the installation. Because a VFD is typically larger than the motor starter it will replace, a larger electrical enclosure may be needed. When retrofitting a VFD, the cost of new power cables to the inverter and VFD-rated cable to the motor must also be taken into account. Typical installed costs of VFD systems range from $200 to $500 per horsepower (HP).

Suppliers can assist users in selecting a VFD that is properly sized and that includes any necessary filters and reactors. If the application involves an existing three-phase motor, the motor may be used if the winding insulation rating is sufficient. The motor should have an insulation class rating of F or higher.

Compare the savings resulting from reduced power consumption with the cost of the installed VFD to determine if the return on investment is sufficient to justify the expense. Operating the pump and motor at lower speeds may lead to increased service life and reduced maintenance intervals, and these savings should be included in the calculations.

Application Example

Assume a centrifugal pump operating with a 15-HP, three-phase AC motor has across-the-line starting at 460 volts AC, 60 Hertz. The pump typically turns at a constant speed of 1,750 rpm, consumes 10 HP and discharges 200 gallons per minute (GPM) with a head of 120 feet. A throttling valve is used to vary pump output from 200 to 100 GPM.

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