January 2021

32 January 2021 www.drivesncontrols.com n VARIABLE-SPEED DRIVES Matrix converters give Munich’s subway escalators a lift M unich’s public transport operator Münchner Verkehrsgesellschaft (MVG) runs an extensive network of underground trains, buses and trams that criss-cross the Bavarian capital. In 2019, the system carried more than 600 million people. The subway network alone includes 95km of track as well as 770 escalators – compared to around 450 in London’s Tube system. The escalators need to be reliable, not only for the convenience of passengers. “Without the escalators, the passengers cannot enter and exit the stations at the required speeds,” explains MVG’s head of service development, Mario Princip. With the typical service life of an escalator being 30 years, MVG has multiple generations and models from different manufacturers in use across its network. The space available for the installations varies widely from one station to another. “Every system is unique”, says Daniel Mayer, who is responsible for the escalators’ electrical technology. One way that MVG has tackled this diversity has been to develop its own escalator control system. This means that all of its escalators can be maintained by in-house personnel, with faults being rectified faster, and costs for spare parts and conversions being reduced. MVG has used inverter drives for many years. On escalators that are carrying passengers upwards, inverter control enables variable start-up and stopping, not only improving comfort for the users, but also protecting the mechanisms. However, the escalators that carry passengers downwards have an extra complication – energy recovery. If conventional braking resistors are used, 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 continuously. Although it would be possible to add regenerative devices to the drives, in many locations this would not be practical due to the limited space inside the control cabinets. MVG therefore started to search for an alternative way of controlling its descending escalators. Eventually it found a technology that that combines inverter and regeneration functions in one device – the matrix converter. These are AC/AC devices incorporating an array of bidirectional power semiconductor switches arranged in a matrix. They convert three-phase AC supplies directly into the required voltage and frequency, without needing the DC bus found in conventional VSDs. Although matrix converters have been around for a long time, their practical applications have been limited, especially in low-voltage applications, partly because the cost of the many power semiconductor devices needed. Munich is using matrix converters to control escalators on its extensive subway network. Sometimes regarded as a niche technology, the converters’ built-in regenerative capabilities makes them ideal for this application, because they save space and solve potential heating problems. Matrix converters are helping to recover energy from escalators on Munich’s subway system