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Simultaneous control of three devices

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article image Figure 2.

THE new Multimaster PFC from Vacon, represented by CMG , provides an effective solution to the simultaneous control of up to three pumps, fans or compressors, which cuts investment and start-up costs, reduces system vulnerability, and improves cost-effectiveness.

In conventional systems, a single frequency converter is connected to the pump, fan or compressor motors via a series of contactors and relays.

By contrast, the Multimaster PFC system is built on a one motor/one frequency converter principle with each motor being individually controlled by a Vacon NXS frequency converter (see Figure 1).

Each system can consist of up to three drives which intercommunicate via a high-speed communications line (see Figure 2).

The required intelligence for all three drives resides in the frequency converters, eliminating the need for separate contactors, additional contactor units and their associated logic.

With the Multimaster PFC, each drive is directly connected to the control system and also to the system by its own sensors. A malfunction in any part of the system disconnects the faulty drive.

Continuous communication between the drives enables the system to handle special situations. For example, should the master drive be disconnected from the process, pre-programming allows its functions to be automatically transferred to one of the other drives thereby minimising or eliminating disturbances in the process.

The characteristics of ac drives also protect the grid and the process against mechanical shocks at starts and stops.

An additional feature of new system is the IP54 enclosure which makes it possible to install Vacon NX frequency converters in pumping stations and other demanding environments without providing any extra protection.

The stepless auto-change function of the Multimaster PFC increases the range of operational options because it provides continuous and full control of the system.

Preprogrammable transfer of the master function between drives means, for example, that Drive 1 can be programmed to log a pre-selected number of running hours after which the master function is automatically transferred to Drive 2.

Such controlled operations distribute mechanical wear uniformly between the drives thereby lengthening the interval between services. Drives that would normally have long downtimes can also be regularly used in order to verify faultless operation.

In pumping installations, when consumption grows, Drive 1 of the Multimaster PFC runs as a master drive. When the maximum flow rate for Drive 1 is reached but more capacity is needed, it locks itself to the maximum flow rate and the master function is transferred to Drive 2.

Drive 2 and subsequently Drive 3 take over control from their zero flow rate in the same manner as Drive 1.

When consumption decreases, Drives 1 and 2 continue to run at their maximum flow rate and Drive 3 controls the flow. When consumption decreases to the level at which Drive 3 reaches zero flow, it stops and transfers the master function back to Drive 2.

The same procedure is repeated when Drive 2 reaches zero flow. When the third drive reaches zero flow, the system stops and transfers to 'sleep' mode while continuing to monitor consumption, ready for automatic re-start when required.

Vacon's Multimaster PFC can be controlled manually by operating a single switch. Compensation for pressure loss at the end of the line is achieved by automatically increasing the output pressure.

The output pressure is also automatically reduced when the input pressure fails below a set value, eliminating the risk of running wells dry, for example.

The same system can also be used for controlling a number of compressors, maintaining the pressure and sharing the load between units.

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