Variable frequency drives control pumps and more

Oysters require a particular level of salinity and the right temperature range to be "happy", and those are the conditions typically found in the Chesapeake Bay.

Oysters require a particular level of salinity and the right temperature range to be "happy", and those are the conditions typically found in the Chesapeake Bay.

For more than two decades, however, oyster populations in the Chesapeake Bay and along many parts of the Atlantic and Gulf coasts have been battered.

Numbers tell it starkly - before the turn of the century, over 10 million bushels of oysters, yielding approximately 64 million pounds of meat, were harvested annually just in Maryland.

Nowadays, oyster harvests are tallied in terms of thousands, rather than millions of bushels.

What's happening? The decrease is the result of several factors, including over-harvest, loss of habitat due to environmental pollution, and two parasites that are harmless to humans but lethal to young oysters - MSX (Haplosporidium nelsoni), which thrives in the higher salinity brought on by dry years, and Dermo (Perkinsus marinus), which tolerates low salinity and therefore is the more damaging to the oyster population.

As a consequence, the famous native Chesapeake Bay oysters have dramatically come down in numbers.

In an attempt to bolster their population, the University of Maryland has been conducting a study to assess the consequences of introducing to the area the Suminoe oyster, an Asian species that has shown to be resistant to those two ravaging diseases.

In general, the use of non-native species has been scientifically accepted as a potential option to address a particular native species decline.

The actual implementation of this strategy, however, presents several risks.

In this particular case, the crucial steps are determining whether the Suminoe oyster can thrive in the low salinity, sediment-laden waters of the Chesapeake Bay; whether it can coexist with the native American species, and, last but not least, whether it can be controlled by the same natural predators.

Why is that a concern? If the Suminoe oysters have no natural enemies in this environment, they can quickly grow out of control and become a serious nuisance, competing for food and space, while not being vulnerable to the fish, crabs and flatworms that predate on the native species.

A University of Maryland 24-month study will examine the Suminoe oysters' eating and reefing habits as well as their vulnerability to the known native oyster predators, allowing researchers to assess project feasibility.

The tests require the Asian oysters to be cultured and observed under strictly controlled parameters, and will be conducted at the University's centre for Environmental Science Aquaculture and Restoration Ecology Lab.

The brand new facility has the unique advantage of offering both a closed aquatic system as well as a secure, flow-through system.

Other remarkable features include experimental temperature and carbon dioxide controls for climate change research, and a quarantine laboratory for the safe study of non-indigenous species.

Several operational requirements need to be in place for this type of research.

Reliable performance of the tank pumps, cooling towers and overall building HVAC systems has to be taken for granted; the temperature of the water has to be kept within certain pre-determined parameters at all times, and the filtration pumps need to function without fail.

In addition to efficiency and dependability, cost-effectiveness is also a concern.

Through their mechanical contractor, J.M.

Zimmer, the University of Maryland chose AC Technology's MCH Series Variable Frequency Drives to control their pumps, cooling towers, and building HVAC system.

These drives feature a built-in PID set-point controller function (see sidebar) as well as the versatile and popular automatic 3-contactor drive bypass, which assures full isolation of the VFD electronics and allows the motor to transfer from 'drive' operation and continue running across the line in 'bypass mode'.

Why is that important? Dependability is a key concern in this type of strictly controlled testing laboratories, and Joe Zimmer Jr.

of J.M.

Zimmer explains the advantage offered by the 3-contactor drive bypass.

"Let's say there's an external condition that will create a problem with variable frequency drive operation; for example, transient voltage harmonics from the utility causing the drive to trip off on a fault," says Zimmer, and he continues, "The MCH drive can be configured to attempt several restarts, but if the condition will not clear, the automatic-transfer-to-bypass function will transfer from drive operation to line power, on its own, without any human intervention.

"What would happen if the drive did not have this functionality? A trained operator would have to be on hand, at all times, first to ascertain there was a problem, and then, to manually put the drive into bypass mode.

"utilising these drives is a way to increase reliability, making sure we maintain uninterrupted performance of the laboratory's pumps, cooling towers and HVAC in general, so research results are not compromised," states Zimmer, who adds that they bought 24 MCH drives from Cummins-Wagner Co "With all power and control wiring easily accessible and ample wiring space, the MCH is considered to be one of the simplest to install, set up and use of all the HVAC variable frequency drives in the business," explains Richard Forrest at Cummins Wagner, who continues, "a single-point connection makes the MCH drives less error-prone, and the on-site warranty adds peace of mind." As AC Technology's local representative, Cummins-Wagner Co provided factory authorized start up service, on site training and technical assistance, as well as 24 hour emergency service, which according to Zimmer, has not been yet required after two years of operation, but certainly adds a valuable component.

Next time you sit down to enjoy an oyster dinner, take a minute to think about it.

There is a lot more research behind it than you probably ever knew - and, there is a lot to keeping research on track.

Process systems often require a system-controlled parameter, such as motor speed, to be able to react to variable situations in order to keep other system attributes constant, such as pressure, flow, temperature.

A simple example is a metering and dispensing system with a pump powered by a drive, and multiple discharge valves.

For flow to be repeatable at each valve, the pressure in the supply manifold must be held constant; the drive speed will need to increase as valves are opened, and be reduced as valves are closed.

A means to meet this requirement is to use a "set point controller," where the pressure in the manifold is measured with a pressure sensor and this value is compared with a "set point" indicating the value that you want the pressure to be.

A set-point controller compares the set-point value to the actual value and generates a speed command to the drive to correct the variance or error.

The AC Tech MCH builds this set point controller function into the drive.

One of the most common types of set-point controller uses a PID algorithm.

This stands for the three types of adjustments (referred to as "gains") that are used to correct for the error: Proportional, Integral and Derivative adjustments.

The Proportional gain is the most basic adjustment, where the speed command is directly proportional to the error.

If Proportional gain is used alone, however, there will always be an error in the system - if Proportional gain is set too low, system response will be quite sluggish, if it's set too high, the system will oscillate or grow unstable.

To eliminate that error, the Integral adjustment will continue to increase the output speed command based upon the accumulated error over time, or decrease the speed in the event of a negative error.

The Derivative gain, finally, is used to enhance performance.

It basically looks at the rate of change in the error and forces a more dramatic change to the speed command than the one achieved with just Proportional and Integral (PI) alone.

Most set-point controllers are "direct" acting, that is, an increase in the motor speed causes an increase in the process variable you want to change.

But in some systems an increase in motor speed creates a decrease in the process variable you want to control.

Take the case of a fan blowing air over a heat exchanger, and the temperature of the fluid within the heat exchanger is the process variable you are trying to change.

As the motor speed increases, the temperature of the fluid will decrease.

In this case, you would need to use a "reverse-acting" controller in order to achieve the desired change.

Back to top