Comment

1 December 2001

Variable-frequency drives live large

The objective is to use only the energy required—not an electron more!

As energy deregulation continues to roil the California market and cause prices to skyrocket, the recent retrofit of two 1,000-ton chillers with variable-frequency drives (VFDs) at the downtown One America Plaza office tower in San Diego verges on sheer genius, amazing timing, or both.

With prices now ranging to 30 cents per kilowatt hour (vs. 9 cents in 1999 and early 2000), the payback period on the two 1,000-horsepower VFDs has decreased from the original projection of 21/2 years to 11/2 years after only nine months of production. This number may go even lower.

"On this kind of project, your savings are there," said Paul Wolf, chief engineer of the building. "It would be a very good project for any building, even if energy prices were not increasing."

OWNERS NOT BUYING IN

Wolf began pitching ownership early on as to the energy savings benefits and availability of utility incentives for a drives retrofit of the chillers. The building owners concentrated expenditures on attracting occupants to the building.

In the meantime, Wolf implemented other retrofit projects to conserve energy and costs. These included full lighting retrofits of the tower and underground garages, which dropped wattage consumption in half per fixture and reduced light replacement costs dramatically; addition of LEDs on all exit signs; installation of window filming to resist radiant heat gains; and addition of VFDs to the cooling tower fans.

These changes alone saved $500,000 in energy costs annually before the retrofit of the chiller. The calculated payback periods for each of these improvements are shorter due to the energy crunch as well. Additionally, these changes reduced the cooling demands made on the chillers by a third, even on the hottest days.

Adding VFD control and, in this specific case, direct torque control to the chillers advanced Wolf's objective of using only the energy required based on the actual cooling demands of the building. At one time, the chillers accounted for 50% of the building's total energy draw.

Motor drives make it possible to operate compressor pump motors at the speed needed for the capacity required. Without the ability to turn down or up the speed and horsepower of the compressor in parallel with the variances in outside temperatures, the chiller runs at a constant, often excessive speed. It wastes a lot of expensive energy.

READ MOTOR CURRENT AND VOLTAGE

Direct torque control (DTC) is an optimized AC drives control principle where inverter switching directly controls the motor variables: flux and torque. The measured input values to the DTC control are motor current and voltage.

The DC-bus voltage and inverter switch positions define the voltage. The voltage and current signals are inputs to an accurate motor model, which produces an exact actual value of stator flux and torque every 25 microseconds.

Motor torque and flux two-level comparators compare the actual values with the reference values produced by torque and flux reference controllers. The outputs from the two-level controllers update, also every 25 microseconds, and indicate whether the torque or flux has to change.

Depending on the outputs from the two-level controllers, the switching logic directly determines the optimum inverter switch position. Therefore, every single voltage pulse is determined separately at the atomic level. The inverter switch positions again determine the motor voltage and current, which in turn influence the motor torque and flux, and the control loop is closed. IT

Behind the byline

Bob Stojanovic has degrees in mechanical engineering and business administration. He works for ABB Inc.

Sidebar

Embedded processor runs this VFD pump