The Vacon CXR unit is the first capable of returning "clean power" back into the network.
Vacon, based in Vaasa, Finland, designs and manufactures frequency converters which are electronic drive devices for smoothly controlling the speed of squirrel cage induction motors, com-monly found in industrial equipment, fans, pumps, cranes, winches and so on. Such motors are inherently simple, durable and economical. However, before frequency converters were introduced, they relied on a grossly inefficient throttling process to control their speed of rotation. Frequency converter technology saves energy, protects the power generation systems from the direct start-up stress produced by squirrel cage motors, and guards machinery from the strain of direct startup.
The company claims to be the most rapidly growing supplier of these devices in the world - last year its turnover was around 41 million, up 72% on the previous year. Over 80% of production is exported. Frequency converters from 0.55kW to 1.5MW are available, although the company is hoping to extend this range upwards in the future.
Around 1,500 of the company's units have so far found their way into the offshore and marine sector with bow thrusters, cranes, cargo pumps, compressors, fans, and rotating deck machinery - especially winches and cable laying drums - featuring strongly.
Vacon has been working with Kamewa Finland, now part of Rolls Royce, since 1997, when Kamewa began to actively market winches with frequency converter drives. The two companies collaborated closely to develop customized drives to suit the particular demands of the winch business. So far, Kamewa winches with Vacon frequency converters have been supplied to 23 ships.
One of the features of this kind of application is that the load on the rotating machinery can often end up pulling or driving the motor - for example, during the deployment of anchors and mooring systems. Under these circumstances, the power generated is normally dissipated using so-called "breaking resistors," which essentially waste the energy as heat. To give some idea of the scale of the problem, two 140-ton Kamewa winches installed in 1998 on the Maersk Boulder - a supply vessel operating in the South Atlantic - and rated at 500kW each, have been fitted with breaking resistors capable of handling 750kW of continuous power generated by the winch during periods when it is under "negative load." While the resistors are over-sized to reduce wear and tear, this is a great deal of power that is not being used.
In order to exploit this effect, Vacon has developed a regenerative frequency converter, the Vacon CXR. The unit, available from 2.2kW to 1.5 MW, is equipped with an active input bridge. This gives it the ability to feed back power into the supply network. Most converters sold so far have been fitted with a diode bridge rectifier on the power supply side. While these are robust, the diode bridge causes high harmonic distortion of the supply current and the output voltage is often lower than the supply voltage. Hence, energy can only be transferred in one direction from the supplying network to the motor. AC inductance coils are needed in conjunction with the diode bridge rectifier to control the current distortion within reasonable limits - generally about 30%. By comparison, the Vacon CXR units have total harmonic current distortion typically as low as 4%.
Inside Vacon's plant at Vaasa in Finland.
The low harmonic distortion of the supply power also means that the supply current is significantly lower than for a diode bridge rectifier. Consequently, fuses, supply cable, and transformers can be kept small, also yielding cost savings. There is also the possibility of raising the output voltage in relation to the supply voltage. With a 400V supply, for example, it is possible to create a 500V output to the motor. As a result, output currents can be reduced, cable voltage losses are compensated and the motor power can be increased - Vacon claims nominal power increases typically of up to 25%.
Kimmo Weissenberg, responsible for Vacon's offshore business, believes this constitutes the biggest breakthrough in frequency conversion technology since it was introduced. "It allows clean, harmonic-free current to be passed back into the supply grid to be used for other electrical applications. The unit will also filter the power returned from other devices. The power savings, which can be directly related to fuel savings, onboard vessels fitted with this technology are very significant," said Weissenberg.
As an example, the company was recently awarded the contract to supply CXR units to control the cable-laying machinery on eight ships owned by the Norwegian company, ODIM. The ships will have, in total, over 400 motors for cable handling, 36 of them rated at 250kW each. Calculations suggest that the savings on fuel will be of the order of 50% compared to a solution relying on brake resistors.
Currently 5% of the squirrel cage motors used by industry are equipped with frequency converters of any type. Vacon believes that the advantages of the technology are only just beginning to be appreciated and that there will be rapid developments in this area over the next few years.
