Broader Definition of Pump-Drive System Efficiency Prompts Municipalities to Reconsider Drive Options
Since the early 1990s, electronic, variable frequency drives have become the commonly prescribed means of variable speed control, and often for good reason.
By Gary Garson
Since the early 1990s, electronic, variable frequency drives have become the commonly prescribed means of variable speed control, and often for good reason. Compared to hand-manipulated, mechanical valves and dampers, variable frequency drives can bring significant cost advantages to a broad range of motor control applications.
That said, "variable frequency" is not the only method of controlling "variable speed." There are other options when it comes to variable speed drive specification. In addition to mechanical drives and variable frequency drives, electromagnetic eddy-current drives can, for certain applications, be a more cost-efficient method of variable speed pump control. It all depends on the application, and definition of pump-drive system cost efficiency.
Mark Twain once said that the difference between the right word and the almost right word is the difference between lightning and a lightning bug. Just as "variable frequency" is not the same as "variable speed," the right definition of cost efficiency is important when calculating pump-drive systems costs. When cost efficiency is narrowly defined as "energy efficiency," a broader range of costs can go unmeasured.
Governed by today's most urgent economic reality, a fully informed variable speed drive specification is not limited to a pump system's energy usage costs, but also encompasses initial capital costs and ongoing ownership costs. Return on investment cannot be properly measured without the inclusion of these three cost variables.
Capital costs include application engineering, product procurement, system installation and commissioning. For VFD installations, costs can mount in each of these areas due to system complexity and required ancillary components, such as for controlled harmonics and drive system air conditioning. Even so, for many smaller horsepower pumping applications, VFD capital costs will be highly competitive, if not lower than electromagnetic, eddy-current drives.
It is among the larger horsepower pumping applications, beginning around 100 HP, where eddy-current drives offer capital cost savings compared to VFDs. Substantially higher capital cost savings can be achieved in medium voltage applications (2300 volts AC to 4160 volts AC), where the cost of an eddy-current drive can range up to 65% less than the cost of a variable frequency drive. This can mean an outset savings of hundreds of thousands of dollars, and possibly millions, on the purchase of larger drive systems.
In addition, from a plant floor installation standpoint, electromagnetic variable speed drives can be significantly smaller than comparable horsepower, medium voltage VFDs, so they can take up considerably less space.
The operating efficiency of an electromagnetic, eddy-current drive increases relative to the normal operating speed of a motor. This makes the drive well suited to most wastewater pumping applications, which typically require controlled speed within the range of from 75% to 100% of the motor's rated speed. In contrast, the operating efficiency of a variable frequency drive decreases across this range.
In addition, VFD related energy costs can be incurred due to the necessary integration of ancillary system components, such as for harmonic dampening and the air-conditioned cooling of larger AC drives.
In many wastewater plants, eddy-current drives that were first installed decades ago continue to perform reliably today, outlasting generations of VFDs by as much as 6:1.
Central to the design of an eddy-current type drive is an electromagnetic clutch that is highly tolerant of fluctuations in power quality. Unlike VFDs, electromagnetic drive systems produce virtually no harmonic noise into a plant's electrical system or onto the utility grid. The drives have few wear-prone parts, which are relatively low cost to purchase and replace. Service and repairs can often be done by on-staff maintenance.
The electromagnetic, eddy-current variable speed drive system consists of a constant speed AC induction motor and an electromagnetic clutch, governed by a small digital controller. The AC motor runs at its optimum rated speed and the electromagnetic coupling (clutch) is used to vary the output speed.
By regulating voltage to the clutch coil, a magnetic flux field is generated in the gap and distortion of the flux field creates torque. Output speed is governed by the digital control. The feedback signal from a tachometer is compared to a reference signal within the controller to maintain accurate speed within 0.5%. This closed-loop speed system typically uses less than 1% of the total AC input current to the motor.
Today's electromagnetic, eddy-current variable speed drives are available with simple yet versatile digital control options for simplified pump-drive system programming. The new controls can be retrofit to existing drives, and they are compatible with PLC and SCADA system integration.
As wastewater facility superintendents are rediscovering, they do have options when it comes to modern pump-drive system solutions. In addition to variable frequency drives (VFDs), electromagnetic eddy-current drives are proven for their durability, reliability and longevity. Moreover, depending on the application, these drives can be the most cost efficient choice, when specifications are based on a complete examination of costs.
About the Author: Gary Garson is the engineering manager at DSI Dynamatic, Inc. He can be contacted via email at firstname.lastname@example.org