One of the largest escalator networks in Germany transports over half a billion people through Munich’s subway system every year, and engineers have taken steps to ensure continued fault-free and safe service by opting for YASKAWA’s U1000 Matrix Converters
Münchner Verkehrsgesellschaft (MVG) is a subsidiary of Stadtwerke Munchen (SWM) and deploys trains and buses in subway and tram stations, with 770 escalators (around half as many again as the London Underground) in operation. An escalator has a typical operational life of 30 years and the network has an ongoing process of modernisation. At any one time, multiple generations and models from different manufacturers are in use across the network.
Exploring the benefits of inverter drives
MVG has been using inverter drives since before the development of the new control systems in place, where inverter control enables variable start-up and stopping. However the company had been searching for a better solution for downward escalators. While many systems work with conventional braking resistors, the resulting waste heat has to be dissipated safely, not least for fire safety reasons. The motor windings can also serve as ‘braking resistors’, converting energy into heat, but only to a limited extent and not continually. It would also be possible to supplement an inverter drive with an additional regenerative unit, but this was not deemed feasible due to limited space in the escalator control cabinets.
Eventually a solution was found using technology that is still rarely used in low voltage applications: a matrix converter that combines the inverter and regenerative unit in a single device. In the spring of 2016 MVG tested the capability of the few market-ready options available using their own simulator. Among the specific escalator requirements was a flying start function to ensure synchronised start-up after automatic deactivation. Further, all electrical components have to operate year round at the ambient year-round outside temperatures found in Munich. YASKAWA’S newly developed U1000 was one of the units tested and MVG says it ticked all their boxes. First introduced by Yaskawa in 2014 the U1000 matrix converter is an efficient, regenerative direct inverter for powering induction and permanent magnet machines, with or without speed sensors. Today’s range covers from 2.2kW to 500kW and doesn’t require any DC bus capacitors. Also, no space is needed for an additional regenerative unit, and like all Yaskawa 1000 Series drives, the U1000 is designed for 10 years of maintenance-free, continuous operation.
Application specific benefits
The regenerative energy that can be fed back into the system to meet other needs in the building such as lighting can be a significant benefit, says Yaskawa, but this was not the main reason that prompted MVG’s selection of the U1000 for its escalator application. The U1000 removed the need for regenerative resistors, greatly simplifying the cooling and ventilation of the system. Another stated advantage of the Yaskawa system was the upkeep of the grid quality during regeneration. The line currents during operation of the U1000 are nearly sinusoidal in both motor and regenerative mode and harmonics are reduced to a minimum. Whilst this reduces losses in network components, such as transformers, cables and lines, it also improves the overall efficiency of the entire system. Further, it reduces potential interference with other system components which in turn helps to prevent failures with downtime.
The U1000 features an integrated EMC filter, removing the need for external components such as DC reactors or LC filters says Yaskawa. Nevertheless, the space requirements are still up to 50% with conventional integrated solutions with sinusoidal input and regeneration. It also features a built-in SIL3 STO input to allow for high machine safety requirements. Yaskawa also offers options for all common fieldbuses making connection to EtherCAT, Powerlink, Profinet, Profibus, Ethernet IP and other systems straightforward.
Following testing of the latest U1000 matrix converters at new subway station locations where space is even more limited, the converters have been duct mounted on the side wall of control cabinets. This mean that the cooling module is on the outside. The device is connected to the MVG control system so that current operation status can be monitored in real-time, providing reliable test data at all times.