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Converting from DC control to AC variable speed drives

Converting from DC control to AC variable speed drives

Converting spinning frames from inefficient direct current (DC) control to alternating current (AC) control has helped increase production capacity of spun yarn at Brintons Carpets, the UK's leading premier carpet maker, while eliminating motor spare parts, lowering maintenance costs and saving energy.

All of the spinning frames were DC controlled. As the existing DC motors were obsolete, spares were not readily available. The 75kW motors needed regular maintenance with each one requiring twelve carbon brushes to be exchanged every year. "This totalled about £5,000, just on replacement carbon brushes across all the DC motors and excluding the manpower needed to carry out the maintenance work," explains Dave Evans, electrical coordinator at Brintons Carpets. "AC induction motors, however, can be repaired or rewound easily, thereby removing maintenance and spares issues faced with the DC solution."

Brintons worked with Sentridge Control, with whom it has a long term relationship, and asked about the feasibility of the DC to AC conversion. "We have provided Brintons with variable-speed drives and technical back up for many years," explains Phil Tomkinson, Sentridge's area manager. "The DC to AC conversion is something we have done before and when Brintons suggested this as a solution to help increase capacity we immediately saw the potential. The biggest challenge was the increase in power needed by bringing in the additional machines, with each motor rated at 75kW, bringing the demand close to 1MW."

At first it was thought that the factory's transformer capacity was not sufficient to handle the increased loading from the additional carding machines and spinning frames. This was because the DC solution gave a poor power factor of about 0.4, which meant heavy current usage. However the AC solution vastly improves the power factor, reducing current and subsequently active power.

Sentridge undertook analysis on the transformers, logging all loading and calculating various 'what if...' scenarios over two weeks. The results confirmed that the 400A busbars did not need to be upgraded, nor did the transformers have to be increased in capacity. In fact, Sentridge calculated a significant saving in current across the Canalis busbar system, thereby avoiding an increase in busbar copper and any change to the installed transformers, substation and switchgear.

Although energy saving was not the motivation for the conversion, Sentridge's calculations showed that a saving could be achieved across all spinning frames of £40,000 per year. The trial was undertaken with an early generation ABB industrial drive and revealed 19% savings, equating to 0.5MW per year.

Since the trial, ABB has introduced a more efficient drive, the ACS880, that features fourth generation motor control platform, DTC, and the energy savings are even greater, totalling some 504,000kWh saved. Together with the installation of IE3 ABB motors, return on investment was expected within three years, but when the Government's Enhanced Capital Allowance is factored in payback drops to 2.6 years.

"Energy saving was not the purpose of this project," says Evans. "It is a bonus. Our intention is not to slow down the process; in fact we are giving the production team exactly what they had before."

He adds: "I have long been an advocate of AC variable-speed drives, with some 50 installed across the site. I particularly favour the ABB drive, primarily because of the ease of use and programming provided by the control keypad. We have, on several occasions, programmed one keypad and then been able to transfer the data from it across several drives."

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