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Cable-wound HV motor eliminates transformers

01 March, 2001

Cable-wound HV motor eliminates transformers

ABB has revamped the design of high-voltage synchronous motors with a new form of stator, based on windings made from insulated cable rather than rectangular copper conductors. The motors will be able to operate directly from an HV supply at up to 150kV without an intervening transformer.

ABB plans to produce the four- and six-pole motors in a range of power ratings from 5-45MW and in voltage ratings from 20-150kV. Initially it will focus on machines with operating voltages up to 66kV and expects to make its first commercial deliveries by the end of this year.

The Motorformer technology is based on a cable-winding technique that ABB has already used to produce large two-pole generators and wind turbines. The use of a copper or aluminium cable insulated with XPLE (cross-linked polyethylene) allows a substantial increase in winding voltages. Whereas conventional rectangular conductors are limited to voltages of around 15kV and produce non-uniform fields with high field concentrations at their corners, the round cables can withstand 150kV and deliver uniform fields.

The cable is laid in specially-designed slots in the stator which also carry a cooling system. At lower ratings, both the stator and rotor will be air-cooled; for larger machines, the stator will be water-cooled.

Although the Motorformers are larger than their conventional counterparts, the elimination of the transformer saves overall weight and space. It also eliminates the losses in the transformer, resulting in a system with an efficiency 0.5-1% higher than a conventional motor-transformer combination, according Göran Eriksson, the Motorformer project manager.

The Motorformer will cost a similar amount to a combination of a conventional motor, transformer and switchgear, but ABB argues that there will be savings both in installation and running costs.

By doing away with the transformer, the new motor also avoids the need for insulating oils. Also eliminated are medium voltage components such as switchgear, current and voltage transformers, and surge arrestors.

One possible drawback of connecting motors directly to the HV grid is that they draw large amounts ofsubstantial energy on start-up. In a normal 5-10s start-up, the grid voltage can dip by up to 10%. For installations where this could affect neighbouring equipment, ABB has developed delayed starting techniques, based on reactors, which could extend the start-up time to around 20s.

ABB has also developed software to optimise the motor design and the starting method for each installation.

ABB stresses that the MotorFormer is based on proven technology. It uses conventional solid-pole rotors fabricated from a single forging. Its protection equipment is similar to conventional systems and a brushless, rectifier-based system is used to control the excitation current in the rotor. According to ABB, the cable-based coils have operated without problems in its Powerformer generators, the first of which was installed in 1998.

The first Motorformer - a 42kV, 10MVA model with an operating speed of 1,500 rpm and a maximum shaft power of 6.5MW - is due to start driving an air separator at a Swedish gas plant in September. The stator winding for each of its three phases will consist of 1.5km of cable.

Eriksson expects the Motorformer to be used in applications such as compressors, blowers, pumps, mills and paper refiners. He says that there are already several potential applications in the UK.




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