The container transpor tation industr y has witnessed a remarkable growth over the last decade. More cranes are being purchased annually then ever before and crane sizes have increased dramatically.
The myriad of crane types required in terms of form and function have always been evident. Thus, more efficient AC converter solutions were developed to cater the demand for ‘power hungry’ cranes yet with the availability of different power configurations, ease of maintenance and reliability in mind to meet the demands of the ever challenging container transportation industry.
Siemens as the leading supplier for container crane drive systems now introduces an innovative drive and control system solution that offers crane builders, system integrators and end-users for the first time standardized yet flexible hardware and software solutions.
Crane drive systems
The different types of cranes available in the market have one thing in common: a need for power. Since on cranes, energy is stored during hoisting and released during lowering, the AC drive systems were designed to deliver power with the capability to regenerate.
Over the last 10 years AC drive technology has been further improved by gradually introducing self-commutated line-side converters that offer the end-user an almost perfect power factor, low harmonic injection and a high tolerance with respect to line voltage fluctuations. Also, the line commutated converters have become more robust and efficient compared to their predecessors.
Line side converters with common DC bus configurations deliver the torque required for all motions on the crane apart from being efficient and reliable to the end-user. Nevertheless, safety should not be compromised for performance hence the relevant standards and norms have to be fulfilled.
The principle of infeeds
The AC converter system converts the AC line supply of 50 Hz or 60 Hz to variable voltage/frequency required for all motions on acrane. The diode bridge rectifier or IGBT types are the two most commonly found infeed configurations in the market today. The former is used for 2 Quadrant non-regenerative operations, for example on a RTG and straddle carrier. This type of unregulated infeed is usually robust and compact yet in principle, fairly priced. However, braking modules and resistors have to be implemented to dissipate the regenerative energy which is mostly generated by the hoisting system, leading to high energy cost.
Modern RTG’s nowadays are equipped with hybrid solutions, such as Siemens CO RTG’s, which are more fuel efficient then conventional RTG’s. These hybrid solutions utilize 4 Quadrant infeed systems. Self commutated IGBT infeed is the preferred solution for regenerative operation in STS and RMG’s. The IGBT’s are actively pulsed using a pulse width modulation method and are not susceptible to short time line voltage fluctuations (within limits), thus maintaining a constant DC bus voltage at all times.
Also, the power factor of near unity (during loaded operation) is attainable, making it the preferred solution for end-users with ever increasing electricity bills from power utilities. Furthermore, the latest development in state-of-the-art IGBT’s, have a higher power density, hence they are more compact in size with lower conduction losses.
Due to lower thermal losses, the heat sinks are also smaller. An optimised modulation scheme allows lower PWM carrier switching frequency which results in reduced losses within the drive system.