For many outside of a small circle of professional electrical engineers, AC and DC are all-capped letters that combine most famously in the name of a heavy-metal band. But for those who seek to run ski area lifts as smoothly, safely, and efficiently as possible, those letters are at the core of a fundamental operational choice. Now, throwing VFD into the alphabet soup, the choice between running lifts on AC motors or DC motors—one that in recent years has tended to tilt toward the latter—deserves more consideration than ever before.
“VFD” is shorthand for Variable Frequency Drive, a technology some ski-area operators have been applying to older AC motors (and snowmaking pump motors) since the early 1990s. Advocates claim that VFDs can give a single-speed AC motor speed and acceleration control comparable to that of a DC motor. VFDs can also cut electricity costs for those AC motors, too.
More to the point, retrofitting an older AC motor with a new VFD can be done at a fraction of the cost of replacing the motor entirely with a DC version. According to Peter Kiggan, general manager for Emerson Control Techniques, a retrofit will typically pay for itself, primarily through energy savings and lower maintenance costs, within five years. But the benefits extend beyond cost savings. VFD-driven AC motors, advocates say, are easy to operate, require minimal maintenance, and are less exposed than DC motors to fluctuations on the power grid.
“The torque and speed characteristics of new AC drives can now compare to DC drives,” says Bob Paul of RPE services. So if the performance of retrofitted AC motors can compare with DC motors, it would be logical to think that area operators are lining up to install AC motors, or at least keep those old AC motors running with VFD upgrades. But life is not that simple.
Over the last 20 years, the vast majority of new lifts were built with DC motors. AC motors of the ’60s and ’70s, says Dan Etman of Green Mountain Control Systems, “didn’t give you very good speed control.” For decades, area operators have preferred DC motors because of “better torque,” says Etman.
Some operators have resisted the idea of VFD retrofits because early versions of the drives developed a reputation for operational problems. Etman concedes that “VFDs in the early ’90s could be unreliable.” Kiggan agrees. “Ten to 15 years ago, there were some issues [with VFDs] with torque control,” he says. However, he notes that the technology has since made significant strides. But with doubts still entrenched among operators, says Etman, he spends more time these days on sales pitches about VFDs than actually making sales.
One believer is Doug Lux, electrical manager at Snoqualmie Summit, Wash. Lux has been in the process of VFD-retrofitting a lift system that was originally installed in the ’60s and ’70s with AC motors using wound-rotor and slip-clutch drives. He did the first retrofit about 15 years ago, although most have been done in the last five years. The area plans to complete the overhaul this summer by installing VFDs on the last two lifts that currently don’t have them.
Lux is more than happy with the results to date. “In 15 years, I haven’t lost a drive yet,” he says, addressing the reliability issue.
But the greater advantages, he adds, have been on the operational and maintenance side of the equation. The retrofits have allowed him to standardize all of the lift controls, so that once the lifties know how to operate one lift, they know how to operate them all. With that, he says, comes an improvement in safety, because there is “a lot less risk of an operator making a mistake.” Maintenance is relatively easy—e.g., no need to clean brushes as in a DC or wound-rotor drive—so that a general mechanic rather than a trained electrician can do the job, he adds.
Another believer is Jim VanderSpoel, director of the ski area management program at Gogebic Community College in Michigan and director of the college-run Mt. Zion ski area. He retrofitted the ski area’s one AC-powered chairlift with a VFD six years ago. “I don’t regret making the change at all,” says VanderSpoel. “The new drive paid for itself in electrical savings within 4 1/2 years.”
WHICH LIFTS ARE BEST CANDIDATES?
Not all lifts, even those already powered by AC motors, are necessarily suitable for VFD drives. On a lift that is usually run at a single speed for most of its operating hours, a single-speed drive might be perfectly adequate, and the expense of a VFD retrofit is probably not justified. “The place where it makes the least sense is an expert lift where you’re not going to stop very often or vary the speed much,” says Paul. In addition, VanderSpoel calculates that if the electrical savings don’t pay for the upgrade within five years, installing a VFD might not be financially sensible.
But the more a lift is used in variable-speed modes, and the more frequent the stops and starts—beginner lifts or summer applications are prime examples, according to Paul—the more sense a VFD makes.
Meanwhile, the suitability of VFDs is increasing for more powerful motors. Originally, VFD retrofitting was limited to relatively small motors, but “the threshold has been creeping up,” says VanderSpoel. VFDs, he says, can now work well for lift motors of up to 500 horsepower.
VanderSpoel says he foresees the move toward VFD-drive AC motors “picking up steam,” especially in the Midwest, where many older, single-speed AC motors are still in service. He estimates that, within the next five to 10 years, 80 percent of the AC motors in the Midwest will be fitted with VFDs, and that VFD retrofits will eventually pick up nationwide.
While others (e.g., Etman) aren’t quite so optimistic, trust in the viability of VFDs should grow as more are installed, dispelling lingering concerns about reliability. And it might not just be in the retrofitting of older lifts; lift manufacturers are often encouraging—and operators are accepting—a shift to VFD-driven AC motors rather than DC motors in new lift installations. That’s beautiful music to guys like Etman and Kiggan, who have been touting the technology for years. As more operators face issues of energy costs and power quality, VFDs and AC motors sound like a solution whose time has arrived.
CALCULATING ENERGY SAVINGS
Retrofitting your existing wound rotor lift drive with a variable frequency drive (VFD) can result in significant energy savings for applications with frequent operation below full speed. As an example we’ll look at a typical 460V, 125A, 100HP system. A few assumptions will simplify the calculations:
Existing motor power factor: 0.85
Existing motor efficiency: 88%
Y connected stator winding
Load at 75% capacity (75% full load current)
Three phase input power
= SQRT(3) x VLL x IL x Load % x Power Factor
= (1.732) x 460 x 125 x (0.75) x (0.85)
= 63.5 KW @ full speed
For the wound rotor system, input power is essentially constant for any speed, since the current demand remains unchanged. The shaft power required for the constant torque load (as with ski lifts and conveyors, etc.) decreases with speed. The difference in Pin vs. Pout is energy lost though heat in the rotor resistor grid. Figure A illustrates the increase in wasted energy with a decrease in speed—as happens anytime you start, stop or slow down your lift.
With a VFD system, the output power again decreases with speed, due to the constant torque nature of the load. However, due to the reduced applied stator voltage, the input power demand decreases with a decrease in speed. The difference in Pin vs. Pout is relatively constant throughout the speed range, as illustrated in Figure B.
For lifts that require frequent stop/start cycles or periods of reduced speed operation, VFDs can provide substantial energy savings, as Figure C illustrates. Coupled with their inherent capacity to provide better speed regulation, as well as built-in ANSI code safety functions, they are the best choice for retrofitting existing wound rotor or single speed AC lift drives. In many cases, the existing wound rotor motor can be modified and used in the new system. The addition of a high-efficiency inverter motor will extend the energy savings to include application at full speed. Typical efficiency for a complete system with new motor is 93 to 95 percent, vs. 85 to 88 percent for existing wound rotor systems (at full speed).
— Robert Paul