Transformer Soft Starter spins a good yarn

Edited by the Engineeringtalk editorial team Jun 19, 2000

At Rieter, the TSE soft starter performs the joint task of avoiding switching noise and replacing expensive, large, heavy transformers significantly reducing the cost of production.

The production of synthetic carpet yarn is a complex process, starting with chips of nylon, rayon, etc, which are melted, compressed and homogenized.

Sealing additives and dyes are added and the molten material is measured and fed by pumps to spinning 'packs'.

After emerging from the spin packs, the polymer chain molecules solidify in an air-cooled chamber, creating filaments that are then bunched together into threads and coated with a finishing liquid that reduces friction, optimising the yarn's processing ability.

The yarn is then ready to go into the texturing unit, the part of the process that really determines the texture, quality and consistency of the end product.

This step is critical to the yarn's stability in transport and during any subsequent forming processes within carpet manufacture.

The process involves carefully heating the filaments and drawing them several times over twin heated rolls rotating at different speeds, the difference in the speeds determining the degree of stretch in the yarn.

This is a highly delicate process, involving extremely high processing speeds - up to 5,000 metres per minute - at ultimate precision in order to cost effectively produce high quality yarn.

The most crucial factor in this process is precise temperature distribution.

If the filament bundles, up to four of them running over the same roll, are not heated with absolute uniformity, the result will be noticeably different fibre characteristics.

To achieve this uniformity, temperature must be accurately measured at a roll face that is spinning at up to 8,400 rpm and transmitted to the control unit regulating the heat.

The required degree of control involves high heat outputs with rapid warm-up times and rapid response to changes in load.

To achieve this, heating has to be done by induction, the roller being, essentially, a transformer with a short-circuited secondary winding.

The mass of the roller is very low so high switching rates are needed.

When switching occurs on such transformers, however, large inrush currents are created, causing premature contact wear and reflected interference on the power network.

With values as high as 20 times nominal, optimisation of transformers under these conditions was just not possible without outside influence.

Initially, to overcome the problem, a specially designed control unit was used to check the polarity at the switch-off moment and the switching on was performed with reversed polarity.

This was both an expensive and unsophisticated solution, however, and it has only recently been improved upon by installing a Carlo Gavazzi TSE soft starter.

The TSE Transformer Soft Starter is semiconductor-based, with a microprocessor controlled patented switch-on procedure.

A smooth power-on, without spikes, is achieved by a short pre-magnetisation of the transformer core.

When operated for more than two seconds, the internal thyristors are shunted by the built-in bypass relays, ensuring the integrated heat sink in the Transformer Soft Starter is kept as compact as possible.

The protection required for the TSE is the readily available and widely used type- b circuit breaker and with no transformer secondary fuse required, simplicity is the result.

The general configuration for the Transformer Soft Starter is as a compact device for nominal current of 16A, but it can be combined above that rating for up to 63A using an external thyristor module.

The TSE not only optimises the output of a transformer, but reduces power losses thanks to the use of small diameter cables.

At Rieter, the TSE performed the joint task of avoiding switching noise and replacing expensive, large, heavy transformers significantly reducing the cost of production.