APPLICATION BULLETIN - AUGUST 1993 | BACK TO INDEX  

Variable speed AC drives in Water Treatment

Most technical advances in water treatment today are ultimately dependent on the variable speed capability of the AC motor. PDL Electronics Ltd of Napier originally pioneered the concept of giving infinite speed regulation to a standard AC machine, and have been among world leaders in the field ever since. They have now produced the MICRODRIVE-3 (UD3) a computer based fully intelligent drive with built in process control capability.

In this article engineering writer Donald Briggs reports on his visit to Palmerston North City Council, to get a first hand view of the benefits these drives are bringing to the water treatment industry.

Development of Water Treatment services and plant by Palmerston North City Council, like most other expanding cities, has been extensive in recent years. Ongoing policies of pursuing energy saving through the installation of high efficiency plant and systems, combined with maximised customer convenience, have resulted in wide use of the latest variable speed AC drive technology. Pressure/flow control and chemical dosing are typical applications where both of these aims are now being realised.

The latest technology PDL Microdrive-3 (UD3) drive has been installed at Turitea Valley water treatment plant on the air scour blower. It was fitted in unusual haste, following the sudden demise of an elderly overseas supplied drive. Mechanical Engineer, Mr Chris Neilsen reports that the PDL drive was ordered, received and installed, all on the same day as a result of co-operation, quick acting and some fast driving by the PDL team from Napier. This drive (70 amps output) controls the 37kW air scour blower in conjunction with a fixed speed water pump for gravity sand filtering by combined air/water back washing. The process is controlled by a Yokogawa distribution control system.

WHY MICRODRIVE-3?

Apart from the fact that service from PDL Electronics and delivery of the drive unit was so prompt, Chris Neilsen recognised numerous real advantages in the latest Microdrive-3 of particular significance to the water treatment industry. First, the technology is held to be some six to eight years ahead of others, depending on which you look at. In practical terms, from the user's point of view, this means it is simpler to operate; in spite of having more than a hundred possible adjustments. Setting is now by keypad, rather than trim pots, making for better accuracy. It is also said to have enhanced reliability. Size is markedly reduced also. In line with Palmerston North CIty Council policy of energy saving the Dynaflux dynamic flux optimisation system was seen as a very attractive feature. The Dynaflux system is unique to PDL Electronics design and was first introduced on their ASDI series of drives. It has the effect of ensuring the motor always runs at its highest efficiency point, at any speed and any load. Such improvements in efficiency and savings in energy are cumulative and are beneficial for any process. Chris Neilsen, on behalf of Palmerston North City Council, saw the savings as being particularly attractive for a pumping situation.

SIMPLIFIED PROCESS CONTROL

A particularly attractive feature of the new Microdrive-3 (UD3), is the ability to provide a simple and economic pressure control system without employing any external controller. By the addition of a process feedback sensor, the UD3 can be configured to provide complete process control. For such as constant pressure pumping a pressure sensor is connected directly to the UD3 feedback input, with the speed input used to set the reference pressure. This feature represents considerable savings and operational convenience in pumping (and general) process systems.

Plant Schematic of Turitea Water Treatment Plant.

NEW WHISPERWAVE MODULATION

In earlier drive design it was a question of balancing tolerable levels of noise against power loss. Drives of this type suffer high levels of noise simply because it is not practical with pulse width modulation (PWM), to reduce noise at the cost of massive increase in power loss. The Microdrive-3 eliminates the characteristic "whine" associated with drives employing PWM.

The Microdrive-3 achieves its high switching rate by utilising two new technologies - Space Vector Modulation, an entirely new and improved waveform generation system; and a new fast switching power device - the IGBT.

Space Vector Modulation is the PDL Electronics method of generating three phase current waveforms in a vectorially co-ordinated manner. It considers not just each output phase independently but eliminates redundant power device switchings by cleverly calculating vectorially the requirements of the motor flux with the necessary minimum number of switchings. Inverter efficiency is increased; waveform ripple is reduced so that an almost pure current waveform is produced and consequently motor noise is reduced to almost nothing at all. Insulated Gate Bipolar Transistors (IGBT), bring a new power device technology, combining the voltage control of a MOSFET device with the power ruggedness of bipolar transistors. In practical terms, it switches rapidly and is able to handle increased switching frequencies.

It all seems to add up to a new level in AC drive design which should bring many advantages to the field of Water Treatment.

Other PDL drives at Turitea include an ASDi-2085 (85 amp) for raw water boosting and three Microdrives arranged for flow proportional chemical dosing, plus one shortly to be installed to pass concentrate sludge into the City sewage system. This is to have flow pressure control over a distance of 6km.

At Ashhurst Pumping Station more Microdrives have been installed, two for chemical dosing and one for main pump flow control. At Aokautere Pumping Station, PDL engineers designed a system for five pumps under constant pressure control.