Throughout the world ski lifts are powered by AC and DC motors and drives up to about 1,000 horsepower. In more and more applications, AC motors are replacing older DC motors because they draw less current (at least when operated at less than full speed). And, because they don’t use commutator brushes, which must be replaced periodically, they save a few hundred dollars a year on maintenance. This has sparked a debate recently between devotees of proven DC motors and AC-struck futurists.
In lifts, where motors run near their top speed most of the time, the power consumption difference between AC and DC motors is minimal. AC may have a cost advantage in smaller installations, under about 125hp, which is why a lot of fixed grip and surface lifts now run AC motors. Because AC motors can run more efficiently when throttled back, this makes sense for beginner lifts. But until recently, DC motors have been the only real choice for bigger lifts.
Before venturing into the pros and cons of AC vs. DC, it’s important to understand that a drive and a motor are two different things. The motor is the big round machine that turns the lift’s transmission. The drive is a box (or a series of boxes) full of control circuits, switches, capacitors, heat sinks, filters, fuses, and black boxes, all designed to convert, clean up and control the electric current to (and from) the motor. AC and DC motors require different types of drives, and, as discussed below, the AC motor often needs a bigger, more complex and more expensive drive.
The Case for AC
One of the chief cheerleaders for AC is Jon Mauch, lift director at Keystone and Breckenridge. “Soon you won’t be able to buy a DC motor,” he says. “AC maintenance costs will beat DC costs hands down. Every other industry has gone to AC, except ski areas.
“We started installing AC drives in 1991, and now have 18 of them, including the smaller motors in our surface conveyors. They’ve been very reliable.”
Mauch still has DC motors running most of his big lifts. But, his newest lift, a six-pack, has a 900hp AC motor controlled by a variable frequency drive (a VFD), and he claims it pulls a lot less current than a comparably-sized DC motor.
Power consumption is a complex issue for a non-electrician. It concerns a concept called the power factor, which is how much current the motor appears to draw. A big AC motor seems to pull current for half its cycle and push current back the other half; if it’s a three-phase motor the pulses felt by the electrical utility grid can be complex and noisy. One of the chief functions of a VFD is to filter out this noise, to make the AC motor look like a simple and predictable resistance load. For this reason, many rural power companies would like to see their ski resort customers switch to AC VFD drives, Mauch says.
Add reduced power consumption to the equation and power companies have plenty of reasons to push the industry toward AC, says Mauch. “There are two kinds of power companies,” he points out. “Generating companies with their own power plants just want to sell you power. Rural electric associations (REAs) simply transmit it, and they tend to be more concerned about brownouts.”
Mark Boone, western region ski area manager for Eurotherm Drives, explains, “An AC drive with an active front end virtually eliminates the harmonics. It can exceed the IEEE 519 standard, feeding a unity power factor into the grid. This will make the utility company happy and eliminate the need for a power filter.”
Sugarbush is on Vermont’s Green Mountain Power grid, a power generator, but according to Sugarbush’s lift director, Rob Swift, the state encourages use of the more efficient AC motors. “Through Efficiency Vermont, the state offers a really good energy incentive,” Swift says.
That’s just one reason Sugarbush put in a 700hp AC/VFD in 2002. The lift has a 6,244-foot run and a 1,500-foot vertical. “The big cost saving is in periodic maintenance,” Swift says. “We run a 770hp DC motor on our Slide Brook transport lift. It needs a full set of brushes every two years, at about $1,000 a set and two hours labor.” Sugarbush runs six smaller AC motors on fixed-grip lifts and 12 DC drives on large lifts.
And, the AC installation isn’t necessarily more expensive. According to Mauch, a new AC drive costs about as much as a comparable DC installation, when you count in the power filter system required by the DC motor. According to Troy Miller, national account manager at ABB Drives, a typical 300hp detachable quad could be retrofitted with a new VFD and AC motor for about $60,000.
The Case for DC
Standing foursquare for DC drives is Bob McDonald, electrical supervisor at Beaver Creek. The resort chose a 900hp DC motor for a new 2,159-foot vertical quad largely for reasons of standardization. “Between Vail and Beaver Creek we have 24 DC motors from 700 to 1,000 hp, all running off the same Eurotherm 590 and 590+ drives,” McDonald explains. “It’s just simpler for the techs to service them all the same way, and we have the spares in stock. We’ve had really good reliability with these systems.”
Space allocation is another issue. The variable frequency control circuits, with their banks of capacitors, need plenty of room. “Have you seen the underground vault they built at Breckenridge to house those big cabinets?” McDonald asks. “Building that is another expense.” DC motors can have their power filtration located elsewhere.
And McDonald doesn’t believe Vail will ever run short of supply on DC motors. “We’ll always be able to rebuild them,” he says.
Peter Kiggen, vp/general manager at Saftronics, sits in McDonald’s corner. He disputes the idea that DC motors are obsolescent. Many industrial processes require DC, he says. “DC motors can run multiple motors from a common drive with individual trim capabilities,” he says. “AC systems can’t do that. So there’s no feeling in the industrial market that DC motors are on the way out.” He suspects that the state-granted energy rebate is designed mainly to promote better efficiency in plants that run big pump motors in continually varying power cycles—entirely unlike a ski lift.
“And if DC motors are rated for ski lift duty, the brushes can last five to eight years,” he notes. Compared to the difference in the cost of the drives, the added maintenance cost with a DC motor may be insignificant.
It’s a Tie?
Mike Beeley, vp/marketing at Doppelmayr CTEC, views the AC-DC decision as a wash. “A ski lift is a constant-torque machine, which means it takes as much power to start as it takes to run full speed,” he said. “This is an ideal application for a DC motor. There are cost savings to be had with AC if you’re looking at the long haul, but DC is still cost-effective in the short term.”
The AC motor is actually cheaper than a DC motor of equivalent power. What makes the installation more expensive is the drive—the circuitry that delivers the controlled current to the motor. Indeed, Kiggen suggests, a whole new level of training is needed to deal with AC VFD maintenance.
Kiggen isn’t against AC—his company sells AC motors and variable frequency drives alongside DC motors and drives. But, he adds, “In 2003, we shipped 32 ski lift duty drive systems, and only 12.5 percent were VFDs.” And most of the AC systems were smaller motors; only two percent of the newly installed horsepower ran AC.
But price is becoming less of an issue. “A Eurotherm Drives AC system and motor is close to the same price as a DC system through 500hp,” Boone says.
Trending Toward AC
In the long run, the trend is slowly favoring AC. “Let me put this in historical perspective,” says Mike Miller, business development manager of Rockwell Automation. “The reason lift operators went to DC drives is that they could hold steady at zero rpm. But the new digital AC VFDs can emulate anything a DC motor can do—hold at zero speed, with a soft ramp up and down. They’re cheaper to operate in the long run, especially in a harsh environment. And the millionaires won’t complain about noise on their big-screen home theatres.
“The DC motor is just fine for the mindset that says, ‘If it ain’t broke, don’t fix it.’ But slowly, AC will replace DC.”
