A dc to ac conversion avoids costly plant overhaul
10 October 2015
Converting spinning frames from inefficient direct current control to alternating current control has helped increase production capacity of spun yarn at the UK’s leading premier carpet maker, while eliminating motor spare parts, reducing maintenance costs and saving energy.
All of the spinning frames were dc controlled, and as the existing dc motors were obsolete, spares were not readily available. The 75kW motors also needed regular maintenance, with each one requiring the replacement of twelve carbon brushes every year, as Dave Evans, the electrical coordinator at Brintons Carpets explains.
“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. The ac induction motors, however, can be repaired or rewound easily, thereby removing maintenance and spares issues we faced with the dc system.”
Brintons’ worked with Sentridge Control, with whom they have had a long term relationship, and Sentridge area manager Phil Tomkinson was quizzed about the feasibility of the dc to ac conversion. “This is something we have done before and when Brintons suggested it as a solution to help increase capacity we immediately saw the potential,” says Mr Tomkinson. “The biggest challenge was the increase in power needed by bringing in 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 poor factor of about 0.4, which meant heavy current usage. However the ac system vastly improves the power factor, reducing current and subsequently active power.
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Sentridge undertook an analysis of the transformers, logging all loadings and calculating various “what if…” scenarios over a two week period. The results confirmed that the 400A busbars did not need to be upgraded, nor was there a need to increase the capacity of the transformers. 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 also showed that a saving could be achieved across all spinning frames amounting to a significant £40,000 per year.
The trial was undertaken with an early generation ABB industrial drive and revealed potential for around 19 percent energy savings. Since the trial, ABB has introduced a more efficient drive - the acS880 with fourth generation DTC motor control – which has resulted in even greater energy savings for Brintons. Together with the installation of ABB IE3 rated 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.
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