By Robert C. Herguth
Date of Publication: May 16, 2003
Source: Chicago Sun-Times
Soon, AC/DC will have L trains rocking and rolling.
For decades, direct current electricity has coursed through the third rail to power L trains. While the third rail won't change, the CTA plans to start buying trains with alternating current technology, which is increasingly used at transit systems.
The conversion--which will come incrementally as rail cars are replaced, starting perhaps with an order next year--may sound like something of interest just to science geeks. But CTA officials said the switch-over will have practical benefits for riders.
"What the passenger will see . . . is a very smooth acceleration," said Jack Hruby, the CTA's vice president of rail operations. Nowadays, when a motorman accelerates "you'll feel a little jerk--a little jerk, then it continues."
Braking also should be "smoother," he said. "Everything beyond that is transparent, hopefully, to the passenger because that's all under-floor equipment."
Dr. Mark Ehsani, a professor of electrical engineering at Texas A&M University, described DC motors as "old 19th century technology" while "AC motors, along with modern controls that they use, are very much up-to-date and in every way better . . . higher efficiency . . . more reliable, better controls and operation, lower noise."
In an AC power system, the current "alternates" in an electrical conductor in forward and backward directions. In a DC power system, the current moves a single direction. The third rail's DC power will continue to work fine with the new motors, which are paired with equipment that converts DC to AC electricity.
So why does the CTA want the change?
"To keep the authority moving forward with the best technology available for the system, in that the technology for DC propulsion is at the end of its road," Hruby said.
The CTA soon will be replacing 336 of its 2200 and 2400 Series rail cars, which were bought primarily in the 1970s, so the time was right to make the jump, CTA officials said.
They insist the conversion makes financial sense as well, because AC motors and parts are more prevalent, less maintenance is required of them and they will help the CTA cut down on the cost of electricity because AC equipment can harness energy created by braking and redirect it into the third rail.
But CTA officials are vague about the upfront costs, which generally are higher for AC units. "From what we have gathered in talking to various potential manufacturers it could be cost-neutral," Hruby said, referring to the cost difference between AC and DC units.
But in CTA-speak, that probably means more upfront expense. The agency also plans to hire a consultant--for wages not yet known--because "we don't have the expertise to handle that so we're looking for assistance to handle that," Hruby said.
At least part of the rail fleets in New York, Washington, Boston, Atlanta, Paris, London and Berlin operate with AC propulsion, officials said.