150-hp regenerative crane system drive retrofits on the 10-ton overhead cranes
at the Covanta Plymouth Renewable Energy plant in Conshohocken, PA, are
yielding savings of more than $160,000 per year, including about $34,000 of
energy savings alone.
Known locally as the "trash-to-steam"
plant, it controls the waste flow for 24 municipalities, a total of more than 1,200
tons per day, and generates more than 32 MW of renewable energy. The facility
is equipped with a water-cooled turbine generator condenser to convert steam to
water for continuous cycling of water through the boilers. Two overhead cranes
are used around-the-clock for handling the trash and moving it to the infeed on
two reciprocating grate and water wall furnaces that process 608 tons per day.
These cranes were proving to be a maintenance nightmare, with
considerable downtime and energy costs spiralling out of control. The drive
systems on the cranes wasted the excess energy created during braking of the
hoist in the form of heat generated by a wound rotor resistor. But the heat
also created other problems, requiring air conditioning to bring the control
environment within the required 104F. In addition, the control system created
excessive wear and tear on all components with brake pad maintenance running at
some $18,000 per year (per crane) and quarterly motor rebuilds at a maintenance
cost of $48,000 each year. Almost continuous maintenance created other cost as
well because of considerable downtime for both cranes.
The company at that time, Montenay Energy Resources, brought
in local distributor Bearings and Drives Ltd., who in turn put the Control
Techniques Drive Center in York, PA on the project. After surveying the cranes,
their recommendation for improving the situation was the installation of two
regenerative crane control systems.
Each crane now regenerates the excess energy using a Control
Techniques' Unidrive SPM modular ac drive that operates in regenerative
mode to feed power back into the plant's electrical supply. Another Unidrive
SPM provides hoist control.
Each 150-hp, four-quadrant drive system gives substantial
energy savings of $10,000-12,000 per year with very low harmonic distortion
(less than 4 percent) and maintains a near unity power factor throughout the
entire speed range. In addition, because of the virtual elimination of heat
generation, the need for air conditioning is eliminated, which produces an
additional energy savings of another $5,000 annually per crane.
Substantially reducing mechanical wear and tear and
controlling the cranes' acceleration and deceleration means no more direct online
starting, and a substantial reduction in the need for maintenance. The cost
savings across the board has provided very fast payback.
The Unidrive SP Modular drives range offers flexible power
from 60 to 2,900 hp on a common dc bus system with or without an active front
end (regenerative, four-quadrant operation). Very high current motors may be
controlled using a multi-drive modular system. The drives are configurable into
five operating modes including open and closed loop, vector, servo and
regenerative modes. This range gives connectivity for most industry-standard
networks and accepts 14 position feedback protocols. With a range of plug-in
module options, its onboard PLC can be supplemented with a variety of programmable