Evaluating Variable Speed Motors for Centrifugal Pump Applications

A VFD should be considered for every new industrial pump installation, and also for the number of industrial pump systems installed over the past decades that are still in operation.

The cost of electrical power to keep a 200 HP pump motor running continuously now exceeds $100,000 a year in most areas of the United States, and more in other countries where the cost of electricity is higher.

Fortunately, in the past decade variable frequency drives (VFDs) have become less expensive and more reliable. Thus, the need and opportunities to improve industrial pump system efficiencies through the use of VFD pump motors has never been higher.

A VFD can vary both voltage and frequency across a motor’s speed range to modulate shaft rotation speed. This makes it possible to reduce a motor’s electrical demand by adjusting motor speed to match pump output to system pressure/flow requirements.

Viscous resistance of fluid flow through piping systems can increase exponentially with fluid flow velocity. With a fixed speed synchronous motor, fluid flow is typically modulated with a throttling control valve. This is used in a pump system to increase fluid flow resistance and deliver desired lower flow rates by forcing the pump to back-up on its output curve.

The most obvious indication a control valve is throttling away significant pressure is excessive valve noise, sometimes better described as screaming or shrieking. High noise, vibration, erosion damage of valve internals and high pump or valve maintenance requirements are all indications that a control valve is throttling away significant pressure.

If a flow system spends significant time at reduced flow, a retrofit VFD for the pump motor should be evaluated. This retrofit would add a VFD to control motor speed and eliminate the throttling valve.

In high-pressure systems, a variable speed motor can reduce maintenance costs. This is especially true for high-speed pumps which use a gearbox to increase impeller speed to multiples of synchronous motor speeds. Some of these gear-driven centrifugal pumps spin the impeller at speeds in excess of 20,000 RPM. With a constant speed motor, their impellers and gearboxes are intolerant of rapid changes in flow rate.

Pump motors in piping systems with heat exchangers can be excellent candidates for variable speed drives. Heat exchange and temperature control is typically achieved by modulating fluid velocity and residence time through system heat exchanger. Small passages and large exchange surfaces contribute to significant increases in pressure drop through heat exchangers as fluid flow is increased.

As a result, most heat exchange systems require high pump pressure output at high flow rates, but need low pressure at low flow rates. If this type of fluid flow system operates at low flow rates for a significant part of its operating period, its monthly power bill will be much lower with a VFD motor, as opposed to operating with a single-speed motor and a throttling control valve.

The reason variable speed motors work efficiently for centrifugal pumps can be understood from the Pump Affinity Laws:

(Q2/Q1) α (N2/N1), Flow Is Proportional to Pump Speed

(H2/H1) α (N2/N1)2, Pressure Is Proportional to Pump Speed Squared

(HP2/HP1) α (N2/N1)3; However, Power Required Is Proportional to Pump Speed Cubed

From the Pump Affinity Law, the table shows how power draw decreases as pump speed is reduced.

As the table shows, pump HP requirements drop as pump speed is decreased, which is why a VFD can save significant amounts of power by matching pump speed to load requirements. This is more efficient than running a pump at full speed and throttling its output, and it also cuts maintenance costs.

To evaluate if a VFD is a cost-effective option for a specific new or retrofit pump application, the following should be considered:

1. Calculate the system pressure/flow curve from minimum to maximum flow. In an existing system, data to establish this curve can be taken from pressure and flow measurements, control valve position and pump motor electric current measurements. For an accurate detailed study of an existing system, arrangements should be made to record this data automatically over time.

2. If isolation valves or 3-way valves are utilized in the flow system to isolate or add additional equipment into the flow path for different operating modes, then as many pressure/flow curves will need to be generated as there are possible operational flow configurations.

3. Select a pump to match the maximum and minimum operating points on the curve. (Contact pump vendor(s) for pump curves.)

4. Estimate from collected data the amount of time, flow, pressure, pump horsepower, electric power and electric cost for major operating points along the system curve. The on-peak/off-peak variations in cost of electricity and any seasonal effects on operations will need to be considered. In addition to the pump efficiency and the motor efficiency in these calculations, VFD losses of approximately 3% will need to be included. Estimate and compare the operating costs for fixed-speed pump and variable speed pump options for a one-year period.

5. VFDs require electrical harmonic noise filters and potentially AC reactors in addition to the VFD inverter. Additional space for the VFD electrical enclosures will be necessary in addition to requirements for a fixed-speed motor starter. In a retrofit, upgraded power cables to the VFD inverter and to the motor will likely be required. Typical installed costs of VFD systems range from $200-$500/HP.

6. Contact VFD suppliers for details on a VFD sized adequately for the motor torque requirements including all required harmonic filters and reactors. In a retrofit application, it’s often possible to reuse an existing 3-phase motor if the winding insulation rating is high enough and the insulation is in excellent condition. VFDs are tougher on motor insulation than fixed-speed motors.

7. Compare the installed cost of the VFD to the annual power savings, and determine if the savings are sufficient to justify the expenditure. Be sure to factor in the reduced maintenance costs realized from running the motor and pump at speeds to match the load, as opposed to running at full speed and throttling the output with a control valve.

A VFD should be considered for every new industrial pump installation, and also for the number of industrial pump systems installed over the past decades that are still in operation. Higher electric power rates and reductions in VFD prices mean that all existing pump systems should be evaluated to see if a VFD retrofit makes sense.