Focus on Drives: How serial comms can save thousands

By adopting serial communications, a typical installation in say, a paper mill, can save more than $23,000 over a hard-wired solution. This example explains how (see Fig. 3).

Assume that Smith Manufacturing has a plant using 30 ACS 600 frequency converters from ABB, controlled by an automation system.

Before changing over to a serial communications solution, Smith Manufacturing set specific requirements, including the need for galvanic isolation between the drive and the automation system. Versatile fault diagnostics were required, as well as a standardized interface to the drive and compatibility with other ABB drive products.

Prior to the use of serial communications, Smith Manufacturing’s galvanic isolation approach used analogue I/O signals equipped with signal transducers. Digital inputs were connected via 220 V AC interface relays, with the digital outputs wired to the automation system.

Using serial communications and ABB adapter modules, galvanic isolation can be implemented on RS-485 interface as standard. All digital and analogue signals are transferred via the RS-485 interface. Additional signal availability enables future expansion.

Here is a summary of the savings potential on the total system.

Savings on drive sections: $28,590-$17,610=$10,980

Savings on automation system: $11,716-$504=$11,212

Savings on cabinet assembly: $1,212

Total savings: $23,404.

Chiller running time reduced

In another example, a 60% reduction in the running time of chillers and a four-month payback have been achieved following the installation of a serial communications link used with AC drives.

ABB installed 84 of its AC drives in a 42-storey building. Its estimated energy savings program calculated a 2-1/2-year payback on the 42 supply fan drives and 42 return fan drives. These drives are applied to a system that was previously using inlet guide vanes. This facility also used a thermal storage system.

The AC drive per cent load is continuously monitored by the Direct Digital Control (DDC) system. If any single air handler exceeds 40% load, the chilled water supply temperature is reset to a lower value. With the resulting cooler water to the coils, the space is satisfied faster and the AC drives decrease speed to save energy.

This chilled water reset is accomplished automatically and triggered by monitoring the AC drive load over the serial communications link.

The chillers are now required to operate at an average of five hours per night. Before the AC drive retrofit, the chillers ran an average of 12 hours per night. The payback on the 84 drives (counting the decreased chiller run time) was four months.

ABB calculated savings from reduced fan power consumption only. But by reducing the volume of air moved using the AC drives, the amount of heat that the chillers had to reject was significantly reduced.

A typical HVAC application often requires an AC drive with at least four I/O points. However, by connecting the AC drive through a serial interface connection, only one point needs to be used within the DDC system. If a typical point costs $600, then a total of $1,800 can be saved per drive installed.

Additional savings can be made in wiring costs. Using a serial connection, only one, two-conductor shielded cable needs to be run. With hard wiring, eight or more individual conductors must be run.

These savings are especially large when considering high-rise buildings. Also, in some cases, the Terminal Equipment Controller hardware is no longer required. This can mean up to $800 additional hardware savings.

Other energy savings include peak demand avoidance, night set back, and building warm-up.

Finally, the energy savings verification and validation report, including the kilowatt hours and operating hours, can now be generated remotely via modem. Previously, such data would be gathered by a monthly site visit. Traditionally, the data would have been collected using current transducers and potential transducers and/or wattmeters. Current sensing relays are also often used to accumulate operating hours.

All of this hardware and the associated wiring can be eliminated with a serial connection. The hardware alone typically represents an $800 savings.