Motor Drives: Doing More with Less

Cutting energy costs for motors usually means installing an electronic drive. The savings from electronic drives go far beyond just energy costs. In many cases, you'll see a drop in maintenance and operations costs, too.With infrastructures strained to the gills and deregulation making a palpable impact, we must do more with less. We want less maintenance, less energy consumption, and less expense.

Cutting energy costs for motors usually means installing an electronic drive. The savings from electronic drives go far beyond just energy costs. In many cases, you'll see a drop in maintenance and operations costs, too.

With infrastructures strained to the gills and deregulation making a palpable impact, we must do more with less. We want less maintenance, less energy consumption, and less expense. While spending less, we want more uptime and more output. Can we make motors do more with less? Here are two case histories of how some people have done exactly that.

Case 1: Oklahoma oil field. The facility is a pipeline collection point for crude oil pumped from wells spread over several square miles. A large reciprocating pump discharges the oil at pressures high enough to reach a refinery located nearly ten miles away.

The booster pumping station originally operated across-the-line, using a PLC to maintain pressure limits through on/off switching. Sustaining pressure levels through frequent starts and stops of the pump produced severe demand spikes. With these spikes came significant demand charges by the local electrical utility. The existing switching system also produced broad pressure swings, making process control difficult. Additionally, the abrupt starts and stops produced pressure spikes. These spikes raised maintenance and repair costs by stressing pumps, pipelines, and valves.

After evaluating the installation and contemplating alternatives, Gary Dundee, owner of Advanced Industrial Devices, Tulsa (AID) worked with his drive supplier and the pipeline company's engineers to solve the energy and mechanical problems.

Their solution was to install an adjustable speed AC drive and matched 150 hp motor. This arrangement eliminated the demand spikes and pressure swings. As a result, all the associated costs of those events evaporated.

The story gets better. This particular drive has complex control capabilities. AID set up the drive in Proportional Integral Derivative (PID) mode, using its built-in set point control to maintain optimum operating pressure based on an input signal from an in-line pressure transducer. Set-point control permitted significant reductions in the number of pump starts and stops required to maintain stable pressure levels. AID reduced utility demand penalties through the drive's built-in soft start acceleration ramp. A further boost to savings: the drive's ability to produce a 98% power factor. Inrush current for the system is now less than 150%; a maintenance-saving reduction from the 600% to 800% normal when the system operated across-the-line.

With PID set point control, the motor no longer has to run at full speed to maintain desired line pressure. In fact, operating current dropped from over 160A to an average of 116A; a reduction of nearly 30%.

How did the drive's built-in set point control lower maintenance costs and increase system reliability? Previously, operators had to use a separate set point controller for process control installations. Now, they enter a target pressure into the drive. The PLC provides an error signal to the drive. The drive automatically compares this signal to the set point, then speeds up or slows down the motor to keep the pressure at the right level.

And better, still. Because of high potential pressures in the pipeline, the company must use a pressure transducer rated at 102 the system's operating pressure. As a result, the operating range available for PID control of the drive is less than 1.5VDC out of a possible range of 1-10VDC. Before installing the drive, system pressure fluctuated between 40 psi and 400 psi (depending on the viscosity of the pumped fluid). With the reciprocating pump being controlled by an ON/OFF type pressure switch, unstable pressures caused severe mechanical stress on the pipeline, valves and related components. With the drive installed, the system's operating pressure dropped to one half the previous levels. And the oscillations are gone. The company is now applying the same technology to a 500 hp motor.

Case #2: Adjustable speed drives solve blast furnace feed problems. Reliability problems on the main feed conveyor to the blast furnace at a major Midwestern steel mill were interrupting production and incurring repair expenses. The massive conveyor is critical to operation of the main blast furnace. It travels some 150 ft horizontally and 60 ft vertically to feed extremely heavy loads of taconite pellets, lime, coke, slag, and recycled steel to the four-story tall furnace.

A 100 hp AC gear motor with a mechanical clutch originally provided power to the feed conveyor. The motor/clutch assembly, designed to absorb the shock of across-the-line motor starting, presented costly limitations in conveyor operation and loading. Accompanying mechanical stress caused significant downtime for conveyor maintenance and repair. Inability to control delivery speeds of feed conveyors emptying into the main belt caused frequent overloading of the main conveyor. This overloading made starting difficult and stressed the clutch assembly even further. Overloading also caused excessive wear and stretching, which meant frequently replacing the large, expensive belt. The mill engineers worked with a drive manufacturer (Magnetek drives and systems) and its distributor to arrive at a solution. Here is what they did.

Mechanical clutch eliminated. The drives include a 100 hp unit on the main conveyor motor and a 10 hp unit on each of the feed conveyors. Installing the drive on the main conveyor immediately solved one of the major problems by eliminating the mechanical clutch assembly.

The drives provide soft-start/soft-stop capabilities, as well as variable speed operation. This permits the operator to maintain a constant weight on the main conveyor and vary feed rates on the two feed conveyors to avoid overloading. This, in turn, allows the operator to optimize conveyor loading; getting more work for less.

This optimization is interesting. Frost Electric worked with the mill engineers to tie an analog output from the main conveyor to a PLC. The PLC automatically adjusts the feed conveyors to accommodate the main conveyor's load requirements.

The rest of the story. These are just two examples of how to apply drives for energy savings and cost reduction. They illustrate an important point: the cost savings aren't just from lower energy consumption to do the same work. When you are doing a cost justification analysis for applying an electronic drive, you need to look at operational differences. This could be uncharted territory for you; if so, simply contact your drive manufacturer or distributor, and ask for a case history or two that can help you bolster the case for installing a drive.

Sidebar: Those Days Are Gone

Objections to installing a drive include: excess harmonics generation, complex setup and maintenance, units are too big, units are too expensive, and it's too hard to select the right drive. Those objections are, for the most part, invalid today, because manufacturers have successfully been working to eliminate each of these problems. If you made a decision not to upgrade from a mechanical drive to an electronic one, it may be time for you to revisit that decision. The advantage of mechanical drives over electronic is their ability to multiply torque. Other than that, electronic drives have mechanical ones beat.

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